Evolution, by Stephen Baxter (2002)

Published in 2002, Evolution is an ambitious attempt by British science-fiction writer Stephen Baxter to chart the whole of mankind’s career, from earliest primate origins to final extinction, 500 million years from now. Inevitably there are those who will draw comparisons with Olaf Stapledon’s Last and First Men, but these are misplaced. Evolution concerns itself primarily with the anatomical and social development of humanity rather than the cultural and philosophical considerations of Stapledon’s classic work. The book is – in common with all Baxter’s work – firmly based on hard science and the pre-human and human primates featured are described in some detail, though Baxter points out that it is not intended as a textbook.

Evolution draws extensively on current theories about primate social dynamics and the origins of modern human behaviour, for example Steven Mithen’s “cognitive fluidity” (Mithen, 1996) and Robin Dunbar’s compelling theory connecting grooming with the origins of language (Dunbar, 1996). The book also appears to be influenced by works as diverse as Richard Fortey’s 1997 highly acclaimed Life: An Unauthorised Biography and Brian Aldiss’s 1962 Hugo-winning novel Hothouse.

Evolution has three parts, covering pre-human primates, humans (Homo ergaster through to H. sapiens), and post-humans (following the fall of modern man). Through this runs a narrative thread following 34-year old palaeontologist Joan Useb and her companions in the year 2031, as civilization unravels following a massive eruption of the Rabaul caldera in Papua New Guinea.

Prologue
The book opens with heavily-pregnant Joan en route by private jet to a biodiversity conference in Darwin, Australia. She is accompanied by primatologist Alyce Sigurdardottir, genetic programmer Alison Scott and the latter’s two genetically-enhanced daughters. The skies around the aircraft are full of smoke from seasonal forest fires in Indonesia and the eastern coast of Australia, which now burn for months each year. There is also concern aboard about Rabaul, which has been causing earthquakes for the last two weeks.

Joan recalls a childhood field-trip to Hell Creek, Montana with her mother (also a palaeontologist) and her discovery of a tooth of an early primate known as Purgatorius. The book then jumps back 65 million years to follow the story of the tooth’s owner, a mouse-sized female called Purga. She is living in the last days of Cretaceous Period, shortly before the Chixulub impact event that brought about the extinction of the dinosaurs.

Part One: Ancestors
Chapter 1: Dinosaur Dreams
The night sky is dominated by a comet, which is becoming steadily brighter as it approaches Earth on a collision course. I assume this is done for dramatic effect, as the object responsible for the Chixulub impact crater was an asteroid, not a comet. A comet would not have produced the anomalously high levels of iridium associated with the impact that led Luis and Walter Alvarez their 1980 theory of a meteorite impact being responsible for the dinosaurs’ demise. Recent work (Bottke, Vokrouhlicky and Nesvorny, 2008) suggests the impactor was part of a much larger body that was broken up by a collision 160 million years ago, again ruling out a comet.

NOTE: 24/03/2013 – Baxter vindicated – http://www.bbc.co.uk/news/science-environment-21709229

Like most of the characters that feature in the first part of the book, Purga’s life is largely about the Three Fs – feeding, fighting and reproduction. She has more than her fair share of problems with the last of these – three of her mates are killed (only one as a result of the impact) and she loses two of her three litters. Fortunately for her – and indeed every subsequent species of primate, including Homo sapiens – one of Purga’s offspring survives to maturity.

This episode differs from subsequent tales in that the story is presented from multiple points of view, ranging from that of a moth eaten by Purga to an “air whale” – a pterosaur with a hundred-metre wingspan living in the stratosphere and feeding on the aerial “plankton” of small insects swept up into the upper atmosphere. The “air whale” is Baxter’s dramatic conceptualisation of the hypothetical creature capable of exploit this niche, originally proposed by Richard Fortey (Fortey, 1997).

Chapter 2: The Hunters of Pangaea
Purga’s story is also broken by an interlude entitled The Hunters of Pangaea, set eighty million years earlier and featuring the orniths, a species of intelligent dinosaur that existed for only a few thousand years and, like the air whale, left no trace in the fossil record. “Hunters” also appeared separately as a stand-alone short story.

Chapter 3: The Devil’s Tail
Purga is described as being nocturnal, in line with the then-current thinking that ancestral primates were nocturnal. However a recent study (Tan, Yoder, Yamasita & Li, 2005) rejects nocturnality – or at least exclusive nocturnality. The nocturnal view rests largely on the fact that the majority of living prosimians are nocturnal, and the large orbits of many fossil forms, suggesting that they were also.

The study considered the gene sequences of opsins in primates. Opsins are light-sensitive proteins found in retinal photoreceptor cells. Trichromatic or colour vision requires three types of opsin, sensitive to short, medium and long wavelengths. However colour vision is not is particularly useful for nocturnal animals, and has been found that in nocturnal animals either the genes coding for short wavelengths or those coding for medium/long-wave opsins rapidly pick up deleterious mutations, rendering the opsins themselves non-functional and giving the animal only monochromatic (“black-and-white”) vision. Because the “bad” opsin genes do not in such cases affect the organism’s survival, there is no Darwinian natural selection acting to eliminate them.

For any species, this mutation would be expected to occur at the same rate across successive generations, and on the nocturnal picture the opsin genes in all nocturnal primates would be expected to have undergone similar degrees of deleterious mutations, reflecting similar times of divergence from the last common diurnal ancestor (presumed not to be a primate).

However this prediction was not borne out by the study, which showed considerable variation in the degree of genetic defects found across a range of prosimians, indicating different time periods of deleterious mutation for different lineages, and suggesting different diurnal ancestry for each.
This in turn implies that the common primate ancestor of all of these lineages must have been diurnal, unless each lineage independently went through a phase of diurnality, before reverting to nocturnality, which seems unlikely

The assertion that every human alive today is a direct descendant of Purga is a good story but bad science. Even if we assume that Purgatorius is on the direct line of evolution leading to Homo sapiens (and while it is sufficiently generalised in its anatomy to have given rise to later Eocene primates, there is no strong evidence suggesting it actually did so), speciation events require founder populations, not founder individuals. The evolution of Homo sapiens (and indeed all other species) was never contingent on the success or failure of a particular individual to breed.

Chapter 4: The Empty Forest
The story moves on to that of another prosimian, a squirrel-sized female plesiadapid called Plesi, living in Texas two million years after the Chixulub impact, before picking up the engaging tale of a young male notharctus called – wait for it – Noth.

Chapter 5: The Time of Long Shadows
The lemur-like notharctus live on Ellesmere Island, in the Canadian Arctic Archipelago. It is 51 million years before the present. Unlike earlier primates, the notharctus have taken to group living. Noth and his younger sister get cut off from their troop and after several days wondering, are adopted by another. They hibernate through the winter with their new companions, then the mating season gets underway and young Noth has just 48 hours to lose his virginity. He defeats his arch-rival (called simply Rival), then bites off a bit more than he can chew when he challenges a solitary male called Solo, who has just bitten off a testicle from the troop’s alpha male, the Emperor. Fortunately notharctus seem have much a lower pain threshold in that particular part of their anatomy than humans – the Emperor rapidly recovers and together with Noth and several other males, puts Solo to flight.

At the – um – climax of the story, Noth gets to mate with Big, one of the troop’s senior females, but not before Rival has mated with Noth’s sister. Noth, however, bears Rival no ill will. It has to be said that Noth and his companions are the most likable characters in this entire saga, and notably their story provides the one conventionally happy ending in an otherwise bleak tale. The science is not however absent, and we learn much about the more powerful brains and complex social dynamics of the notharctus. Nor is the ending entirely positive – we learn that the notharctus are eventually driven out by competition with what will eventually prove to be mankind’s nemesis – the rodents.

Chapter 6: The Crossing
The scene now shifts to West Africa, 32 million years ago, to tell the tale of a group of anthros – monkey-like simians, which are swept out to sea by a flash flood. They survive the immediate peril by clinging to a raft of matted vegetation, but then have to endure weeks of thirst and starvation, during which many of them die. At length, however, the raft drifts ashore and the survivors find themselves in South America, the progenitors of the New World monkeys that live there to this day.

As implausible as it sounds, on a timescale of many million years it would only need to happen once, and there really is no other way of explaining the presence of the New World monkeys in South America without any fossil evidence of them ever having lived in North America.

Chapter 7: The Last Burrow
The next chapter is a short speculative piece set 15 million year ago in a small shrinking strip of tundra in Antarctica where small lemming-like primates compete with cold-adapted dinosaurs which survived the Cretaceous-Tertiary extinction event, and are now buried below many miles of sheet ice.

Chapter 8: Fragments
The final chapter of the book’s first part centres on an alpha male ape, with the appropriate name of Capo, who belongs to an unnamed (and as yet undiscovered) species of ape, living in a forest near the coast of North Africa 5 million years ago. They resemble chimpanzees, though the latter have yet to evolve. Their society is male dominated, like that of chimpanzees, but they copulate using the missionary position, a trait shared with bonobos and humans, whereas chimps use the doggy position (de Waal & Lanting, 1997).

Capo – the progenitor of mankind, the ancestor of Socrates, Newton and Napoleon – is the troop’s capo di tutti capi and lets nobody forget it. He has a habit of beginning his day by shitting on his subordinates, thereby starting a management practice that is still widespread five million years later. Unfortunately for Capo his territory is shrinking. As the Earth continues its long-term cooling, so the forest patch occupied by his troop is slowly dying off. The change has become significant over Capo’s 40 year life, and now the forest patch has become too small to support the troop. By a leap of instinct, Capo realises he must lead the troop to a new territory.

Roughly half the troop elects to remain behind, though Capo is relieved when his favourite female Leaf joins the migration. As Capo leads his diminished band to the edge of the forest, those remaining behind waste no time in battling to establish the new hierarchy. The apes make their way across open savannah and a salt pan. Overnight they are attacked by hyenas, and one of the younger males is taken. Next day, they reach the apparent safety of a new patch of forest, but by now Capo’s authority over the group is breaking down, with two young males – Finger and Frond – looking to depose him.

Worse is to follow. It turns out that the forest is already occupied by apes of the same species as Capo, who are in no mood to welcome immigrants. Heavily outnumbered, Capo’s troop begin posturing and displaying but the locals do not back down. Capo realises that he has no choice but to retreat and lead the troop onwards, though he now realises that any other forest they come across is likely to be similarly occupied.

Finger refuses to accept the retreat and after attacking Capo he launches himself at the other group. He is rapidly overpowered and killed. Capo’s band retreats, but only when Frond signals a retreat. One of the younger females defects – she will be grudgingly accepted by the others, provided she becomes pregnant quickly. The group are not pursued, but Capo realises his days as boss are over. He climbs into a tree, and is comforted by Leaf.

Meanwhile Frond cracks open a thigh bone from the corpse of a gomphothere (an extinct relative of present-day elephants) and finds he can eat the bone marrow. To make a living out on the open savannah is very difficult, but enough of the troop and their descendants will survive to lead to the evolution of the first humans….

The main problem with this story is that it is difficult to believe that an ape, confined all his life to a single patch of forest, would be able to formulate the concept of the existence of other patches of forest beyond his own. It is one thing to be aware of the resources needed to stay alive, and where they may be found within a particular environment; entirely another to postulate the existence of similar environments elsewhere.

Interlude
The conclusion of this first part of the novel is followed by a brief interlude in Joan Useb and her companions arrive at Darwin Airport. Earthquakes from Rabaul are making themselves felt as is the presence of anti-globalization protestors. The group are bottled up in the airport, waiting for the authorities to disperse the protestors. A delegate named Ian Maughan introduces himself to Joan. They discuss a self-replicating probe nicknamed “Johnnie” (after mathematician and computer scientist John von Neumann) that has landed on Mars. Meanwhile, Alison Scott has unveiled her latest genetic creation – a recreated australopithecine.

Part 2: Humans
Chapter 9: The Walkers
The story proper then resumes with an episode set 1.5 million years ago, in Kenya. “Far” (this is the nearest thing she has to a name) is a pre-pubescent female hominid of a type that will one day be labelled Homo ergaster, though Baxter suggests that what we have found is merely the tip of the iceberg and there were many different human species living all across the Old World at this time. (Until recently, the term Homo erectus was used for humans of this era; however the current tendency is to reserve this term for Eurasian populations [the first to be discovered] and class the African populations as Homo ergaster. Whether the physical differences, albeit greater than those found in modern humans, warrant the use of two species is debatable, and some authorities (e.g. Conroy, 1997) continue to class both populations as H. erectus.)

Far enjoys running, in fact as both a sprint and middle-distance runner she could outstrip any athlete living today, male or female. She can run 100 metres in 6 or 7 seconds and a mile in three minutes! Her performance in long-distance events isn’t given, but I suspect she’d have left Paula Radcliffe trailing in her wake.

Although horses, greyhounds and indeed many other mammals could comfortably outperform even Olympic athletes in track events, could earlier types of human? The pelvic region in Homo sapiens is basically a design compromise between two conflicting needs – bipedal motion versus a birth-canal capable of allowing the passage of a large-brained infant. A part-way solution is that H. sapiens births are to all intents and purposes premature births, taking place while the head is still of a just about manageable size. The down side of this, of course, is that a new-born H. sapiens requires more postnatal care than the infants of any other species.

Even then, the locomotion capabilities of H. sapiens are still compromised, and it is likely that the earliest fully-bipedal humans such as H. ergaster, which were smaller-brained than ourselves, were probably more efficient bipeds. Enhanced middle and long-distance running abilities would have given them an edge when hunting; conversely when the tables were turned, sprinting abilities would have helped them to escape predators.

Far also packs a punch that would have had Mohammad Ali on the ropes. While sheltering from a bushfire, she is attacked from behind, stunned and dragged partially conscious into forest by a hungry australopithecine, but as she is about to be carved up for dinner she revives and punches him hard enough to do considerable facial damage.

A modern-day chimp, though weighing in at around half the size of a modern-day human, possesses far greater upper body strength. There is nothing mysterious about this – compare the thickness of the upper arm with that of the thigh in a human, and it is obvious that strength has been concentrated in the lower body at the expense of the upper. In a tree-climber such as a chimp, strength is more evenly distributed. The same would have applied, albeit to a lesser extent, to australopithecines and the earliest humans such as Homo habilis, which retained vestiges of the arboreal habit. But would the same apply to a fully-evolved biped like H. ergaster? The answer, probably, is yes. H. ergaster/erectus, while less robust than the stocky Neanderthals, still had a more powerful all-round physique than a present-day human.

We don’t know about their hand-to-eye co-ordination, but there is no reason to suppose it was inferior to ours. So given a good tennis coach, and combined with her enhanced speed and strength, we can speculate that Far would probably run rings round Roger Federer or Rafael Nadal. But with her smaller brain, she’d probably be less effective at team sports and a Homo ergaster football team probably wouldn’t beat anybody, apart of course from the current England side.

Far’s story contains an interesting take on the two major issues concerning Mode II or Acheulian tool technology. Mode II tools superseded the more primitive Mode I or Oldowan tools made by the earliest humans and possibly by some of the later australopithecines. Mode II tools are characterised by teardrop-shaped hand-axes, the first examples of which were found at Saint-Acheul in Northern France in the mid-19th Century (hence “Acheulian”). The oldest known Acheulian tools are dated to 1.65 million years ago and come from West Turkana in northern Kenya (Scarre, 2005). The Acheulian hand-axe tradition endured with little change for over a million years.

But one major puzzle about these often beautifully-crafted hand-axes is that they are often too large to be useful (see, for example, the fine example in the Natural History Museum in Kensington). Also, they often appear to have been discarded soon after manufacture, with no sign of wear, suggesting that they were never used.

One theory (Kohn & Mithen, 1999) proposes that the axes were made to impress prospective mates. When a female saw a large, symmetrical axe, she might conclude that its maker possessed the right attributes to father successful offspring. The axe, having served its purpose (or not) would then be discarded. This – like the elaborate bower of the male bower bird – would be an example of the extended phenotype of a species playing a role in sexual selection (Dawkins, 1982).

Another issue with the hand-axes is that while they are ubiquitous in Africa and western Eurasia, they are not found east of Northern India. This was first noted by American archaeologist Hallam Movius in 1948. The “Movius Line” has stood the test of time and two theories have been proposed to explain it. One is that the ancestors of those living east of the Movius Line left Africa before the hand-axes were invented. The other possibility is that the migrants from Africa passed through a region lacking suitable materials to make the axes, and by the time they emerged from it, the tradition had been forgotten.

Baxter describes Far’s ancestors a few generations back as having originated from east of the Movius Line, but having migrated back to Africa. Far, cut off from her own people, is adopted by another group and when a male suitor named Axe presents her with a hand-axe, she does not understand its significance, although she is attracted to its maker.

In the chapter’s most speculative development, Far deceives Axe into thinking she is older than she actually is by using a piece of ochre to simulate menstruation. The “sham menstruation” hypothesis (Knight, Power & Watts, 1995; Power & Aiello, 1997) proposes the use of ochre to feign menstruation by early modern humans (Homo sapiens). But while pigment usage by early modern humans is well-attested and is taken as evidence of symbolic behaviour, such usage by earlier human species is less so. Recent work (Soressi & d’Errico, 2007) does however suggest Neanderthals may have made use of pigment for symbolic purposes, but the Neanderthal brain was comparable in size to that of a modern human, much larger than those of Far’s people.

Chapter 10: The Crowded Land
We then fast-forward to 127,000 years ago, remaining in Kenya, and pick up the story of Pebble (which still isn’t really a name), a male Neanderthal. That Neanderthals lived in Africa as well as Europe and western Asia will never become known to science. As a boy, Pebble was forced to flee when outsiders invaded their settlement, killing most of the inhabitants, including Pebble’s father. Kin groups are identified by ochre makings scrawled on their faces, hands and arms. Pebble’s group wear vertical lines, the invaders wear a cross-hatch design. These body markings are the beginning of art, but also of nations and of war.

Chimps have a keener sense of smell than humans; Capo and his band were repelled by locals who could pick up subtle differences in their scent. If early humans had a poorer sense of smell, they would need something else to establish a group identity. The use of pigment by Neanderthals, as noted above, is a possibility though its use in this particular context is pure speculation.

Pebble’s people then establish friendly trading relations with a group of anatomically modern humans and one perennial question is answered in the affirmative when Pebble starts having sex with Harpoon, one of the moderns, and in due course she falls pregnant and produces fertile offspring. Later, the Neanderthals and moderns find a way of crossing to an offshore island, using logs as swimming aids, and they exterminate the local population of late Homo erectus people, stranded there millennia earlier as sea levels rose.

There is little doubt in my mind that modern humans did on occasions have sex with Neanderthals. Even in the wild, closely-related species will on occasion mate, for example horses and donkeys, lions and tigers, and whales and dolphins. While the fruit of such unions are generally infertile, they are usually viable. Given that modern humans will have sex with sheep, it seems inconceivable that at some stage they did not have sex with Neanderthals. Whether this resulted in fertile offspring, and whether any Neanderthal DNA exists in the current human genome remains contentious, though genetic studies have failed to find evidence, and it seems that Neanderthals diverged from modern humans as long as 800,000 years ago.

No evidence has yet come to light of the widespread warfare and genocide described in this chapter. Some claim such behaviour has always been a part of the human condition, for example Nicholas Wade, who bases his claim on the behaviour of chimpanzees and some contemporary hunter-gatherer tribes (Wade, 2007). But while some hunter-gatherer societies are warlike, such as the Yanomamo of the Amazon rainforest, other indigenous people are not.

Though more mentally-adept than the Neanderthals, Harpoon’s people are described as not yet being behaviourally modern. Anthropologists define modern human behaviour as the use of abstract thought, symbolic behaviour (such as art and creative expression), use of syntactically-complex language and the ability to plan ahead.

The following are generally accepted as evidence of modern human behaviour:

Finely made tools.
Fishing.
Evidence of long-distance trade among groups.
Use of pigment and jewellery for decoration or self-ornamentation.
Figurative art, such as cave paintings, petroglyphs and figurines.
Burial of the dead.
Systematic use of space in living-areas, with particular areas reserved for particular functions, e.g. food storage.

The first anatomically-modern humans may have lived as long as 195,000 years ago (Omo Kibish 1 and 2, Ethiopia) or at least 154,000-160,000 years ago (Herto Bouri, Ethiopia), rather earlier than the 132,000 years suggested by Baxter. According to many authorities (e.g. Mithen, 1996; Klein & Edgar, 2002), modern human behaviour did not arise until much later in a “big bang” of human consciousness, but this is disputed by others (e.g. Oppenheimer, 2003), who claim there was no “big bang” and knowledge, skills and culture were gradually acquired over hundreds of millennia.

Baxter takes a middle view, with the use of ochre going back 1.5 million years, and anatomically but not-quite-behaviourally modern humans engaging in trade. The final cognitive breakthrough occurs in the next chapter, set 60,000 years ago in the Sahara.

Chapter 11: Mother’s People
The protagonist in this chapter is a 30-year-old woman referred to as Mother though she still doesn’t really yet have a name. As a result of a chance mutation, Mother has the mental organisation of a modern human, or what Steven Mithen, Professor of Archaeology at Reading University has described as cognitive fluidity. Mithen believes that the human brain originally had separate cognitive “domains” for different functions, such as social interaction, tool-making, food and resource gathering (“natural history”), etc, drawing on the work of Jerry Fodor, Annette Karmiloff-Smith, Michael Tomasello, Howard Gardiner, Leda Cosmides and John Tooby. Modern human behaviour came about when the barriers between these domains broke down, allowing them to interact with each other. Art, religion and language all arise from the synergistic interactions between the various domains. The idea of initially separate domains interacting may have been inspired in part by Julian Jaynes’ controversial theory about “bicameral minds”, proposed in 1976. (See Mithen, 1996; Fodor, 1983; Karmiloff-Smith, 1992; Tomasello, 1999; Gardiner, 1983 & 1999; Jaynes, 1976).

Mother understands the concept of cause and effect, and is capable of abstract thought. This enables her to invent the spear-thrower (atlatl) – a crucial invention because her people are starving to death. But Mother’s enhanced mental powers come at a cost; many of her ideas come to her when she is having crippling migraine attacks. These do, however, form the basis of shamanistic rituals and hence the world’s first religion. That cave art may be associated with such practices was first proposed by eminent French prehistorian Jean Clottes and South African anthropologist David Lewis-Williams. The latter also suggested that some African rock art may be derived from migraine aura (Lewis-Williams, 2002).

Unfortunately Mother begins to suffer from paranoid schizophrenia following the death of her son, and after murdering her aunt in the irrational belief that she killed her son, she instigates the practice of human sacrifice to bring rain. Fortunately only two sacrificial victims are required before the rains come; she was quite prepared to work her way through the entire tribe. Other influences are more benign: her cognitive skills gradually work their way through the tribe and the first true syntax-rich language develops.

Some years later, Mother develops cancer. Her condition rapidly worsens and she is eventually smothered by one of her acolytes, Sapling, in the world’s first mercy killing. Sapling calls her Ja-ahn – “Mother” in the new language; thus Mother becomes the first person in human history to have an actual name.

Chapter 12: The Raft Continent
In the next few stories, set in Australia 52-47,000 years ago, we meet a series of Ja-ahn’s descendants, all bearing mutated versions of her name. (The ultimate implication, though, that this is the origin of the name “Joan” is a little suspect. “Joan” is actually of Hebrew origin, meaning “the Lord is gracious”). Baxter does not describe the first migration of modern humans from Africa, skipping on eight thousand years, to the story of one of Ja-ahn’s descendants, a young man called Ejan. Following a failed attempt by three of his brothers, Ejan and his sister Rocha make the first voyage to Australia, crossing the then narrow straits from Indonesia. Over the next five thousand years, humans colonise Australia, but their depredations soon kill off all the continent’s megafauna, such as giant kangaroos, which survive only as cave paintings. Painted over with later images, they are dismissed as childish doodling by people who have already forgotten what has been lost.

Chapter 13: Last Contact
Jahna is another of Ja-ahn’s descendants, living in Western France at the height of the last Ice Age, 31,000 years ago. Her people co-exist with Neanderthals, but despise them and have reduced them to slavery. Only one Neanderthal, known as the Old Man, continues to live in freedom. He looks after Jana and her brother when they are cut off from a hunting party in a snowstorm – but when Jahna’s father eventually finds them, he kills the sleeping Neanderthal by repeatedly smashing him over the head with a rock.

Chapter 14: The Swarming People
The action of final chapter to be set in prehistoric times takes place in Anatolia, Turkey, 9,600 years ago and describes the interaction between Mesolithic hunter-gatherers and Neolithic farmers. The story is based around Colin Renfrew’s theory that the Indo-European languages (Latin, Greek, Sanskrit and their descendants) were spread by farmers, originally living in Anatolia, who spread across Eurasia taking their language with them (Renfrew, 1987). See also this article on Indo-European origins.

Juna – the latest incumbent of the “Ja-ahn” name – is a young woman living with a group of hunter-gatherers. She is pregnant and concerned that her child will be killed on birth. Her tribe – in common with others described earlier – practice infanticide when times are hard. A possible way out presents itself when Juna’s people begin trading with a man called Cahl, who brings them beer. Nobody knows how to make beer: Cahl’s mysterious people keep the secret to themselves.

But Cahl has a fetish about pregnant women and Juna persuades him to take her with him back to his people, where – it is said – no babies have to be killed. Cahl’s people live in a town – one of the first in the world – called Keer. They practice a primitive form of agriculture – but it is enough to feed everybody, including Juna’s soon-to-be-born baby. But conditions in Keer are squalid and disease is rife. Juna is put to work in the fields where she befriends a woman called Gwerei and learns the language of her people, the language now known as proto-Indo-European. By night, she is used sexually by the repulsive Cahl.

As the months go by she learns that Keer is but a satellite of a larger town, Cata Huuk [sic], whose ruler is known as the Potus. While a passable cognate for “potentate”, I assume this is a humorous play on the acronym POTUS for President of the United States. The reason for Baxter’s spelling of Catal Hoyuk – the Neolithic settlement on which the story is based – is unclear; I have not seen it spelled that way elsewhere. Possibly the idea is that Cata Huuk was the original name (similar to Londinium for London), but this is certainly incorrect. “Catal Hoyuk” means “Fork Mound” in present-day Turkish, and Turkish is not an Indo-European language.

The Potus’ youngest son Keram is tasked with collecting tribute from Keer and other outlier towns. On one such visit, while Cahl is trying to ingratiate himself, Juna emerges from Cahl’s hut and begs Keram to take her and her unborn baby to Cata Huuk where, she claims, she was originally born but abducted as a child by the people of Keer. Although dubious about her story, and over the enraged protests of Cahl, Keram takes Juna with him, together with the tribute. Juna is puzzled that the people of Keer – who are hardly well off – don’t get beer in return. (I could comfortably retire on the money I’ve handed over to HM Revenue & Customs over the last few decades, and I’m still waiting for them to buy me a pint!)

At Cata Huuk, the Potus takes a liking to Juna, and allows Keram to marry her. Her son is born and she has another child with him. The society of Cata Huuk is rigid and hierarchical. The Potus, his family and the priesthood are the first people ever to live without having to work for food. This new way of life has more in common with the chimpanzee colonies of the forest than it does with the hunter-gather lifestyle of the Upper Palaeolithic.

For four years, all is well, but then Cata Huuk is sacked by outsiders, and Juna, Keram and their children are forced to flee. As they make their way to the coast, they pass through Juna’s old home, now a rough shanty town. Juna has a brief re-union with her sister. Most of the inhabitants have died from measles – one of many diseases that have flourished in the new urban societies, to which the hunter-gatherer people have no immunity.

Catal Hoyuk, though often described as one of the world’s first cities, is probably better described as a large village (see, for example, Hodder, 2006). Contrary to Baxter’s description, society appears to have been fairly egalitarian. There is no evidence of a civic centre or the type of organization one would expect in the kind of state-level society described. At the time of the events described, states with the stratified societies we know so well today were still some millennia in the future.

Regardless of whether the Anatolian farmers spoke proto-Indo-European, they spread out in what Italian geneticist Luigi Luca Cavalli-Sforza described in the 1980s as a “wave of advance”, leaving a genetic imprint in present day European populations (Cavalli-Sforza, 2000). However later work by Bryan Sykes at Oxford suggested that a strong Palaeolithic/Mesolithic component remained (Sykes, 2001). This can be explained by the farmers gradually moving out generation by generation from their homeland, but with significant intermarriage with local Mesolithic hunter-gatherers (Bellwood & Renfrew, 2002). In some places almost certainly the Mesolithic people took up farming on their own. This probably explains the existence today of isolated languages such as Basque, which may have been related to the languages originally spoken by Mesolithic hunter-gatherers. Though there were undoubtedly exceptions, it seems likely that relations between hunter-gatherers and incoming farmers were amicable, as each would have had something the other needed. The hunter-gatherers would know the lie of the land, having lived there for generations. In exchange for this knowledge, the farmers would be able to offer them food – and possibly even beer!

Chapter 15: The Dying Light
The next chapter, set in Rome in AD 482, deals with intrigue in the years after the fall of the Roman Empire in the West. Athalaric and his mentor, the elderly Honorius, are what would later become known as antiquarians. They are shown dinosaur bones and the skulls of Homo erectus and a Neanderthal. They speculate quite accurately on what these remains might mean. But Honorius is murdered after turning down the opportunity to become Pope, and such investigations would have to wait for another 1300 years.

Chapter 16: An Entangled Bank
The second section ends as we once again pick up the story of Joan Useb and her companions. The conference goes ahead, but is hijacked by terrorists led by a young man named Elisha, who releases smallpox into the air and is about to rape one of Alison Scott’s genetically-enhanced daughters when the police storm the building and kill most of the terrorists. The remainder, including Elisha, commit suicide. Joan goes into labour and simultaneously Rabaul blows up.

Joan delivers her baby safely and the conference delegates are vaccinated against smallpox, but the Rabaul eruption is sufficient to push Earth’s already-stressed eco-system over the edge, although it isn’t even the biggest eruption in human history. Wars break out across the globe. Mankind’s complex civilization collapses completely and utterly.

Part 3: Descendants
Chapter 17: A Long Shadow
The final part of the book is set in the distant future, long after the fall of Mankind. Homo sapiens is almost – but in the first chapter – not quite extinct. Royal Navy flyer Lt. Robert Wayne Snow – “Snowy” – awakes from suspended animation to see the face of senior pilot Ahmed supervising his revivification. There is no sign of his CO, Robert “Barking” Madd, telling Snowy at once that something has gone wrong. When asked how he is, he nevertheless jokes that any landing you walk away from is a good one.

Snowy and his colleagues have been placed in a suspended animation chamber known as the Pit, and buried at an undisclosed location. They are part of a UN Protection Force, the idea being that if the UK or its allies are invaded, they will be thawed out and spring out of the ground, ready to fight. But the Pit appears to be leaning at an angle from the vertical, and much of the instrumentation is dead. Worse still, it soon turns out that all but five of the twenty-strong contingent is dead. Other than Snowy and Ahmed, the only survivors are the group’s resident genius Sidewise, a young pilot called Bonner and the only surviving woman, Moon (whose actual name is June – the final descendant of Ja-ahn???). There has been no “tally” or wake-up call, no orders, no clue as to what is going on. The Pit’s clock only goes up to fifty years, and its hands have jammed against the end of their dials…

As the senior ranking survivor, Ahmed takes charge. They emerge from the Pit to find themselves in the middle of a forest. Maps, supposedly stored outside the Pit, are nowhere to be found. Taking weapons and equipment from the Pit, they strike off north. After some hours, they get clear of the forest, only to discover the last pitiful remnants of human civilization – the crumbled remains of a dam, a ruined church, the dimly-recognisable street layout of a town, with nothing surviving above waist height. We never learn how long the group were in suspended animation, or even where they are, but Sidewise guesses that at least a thousand years have passed.

Looking at the night sky, Sidewise locates Jupiter, Saturn and Venus, but he can’t find Mars and speculates something has happened to it. He is correct – it has been dismantled by the von Neumann machines, the robotic descendants of humanity.

As the weeks pass, the morale of the group deteriorates. The fauna appears drastically changed, with rodents the size of wolves in the ascendant. Escaped budgerigars seem to be thriving, but not cats. Sidewise – obviously no cat-lover – claims that cats weren’t so tough, just a pain in the arse. Finally Snowy encounters a human female, but she lacks the power of speech. Sidewise dubs the female Weena (the “old literary reference” is actually from “The Time Machine” by H.G. Wells). The pair discovers a colony of small, hairy ape-like people, descended in all probability from feral children who lived in sewers during the collapse of civilization. Without culture and learning, the power of speech was soon lost. With no need for energy-expensive big brains, these too were lost.

Ahmed still dreams of rebuilding civilization, despite the group containing – as Sidewise puts it – just one womb. This remark infuriates Moon, who feels increasingly threatened, especially by the sex-starved Bonner. Things finally unravel as Ahmed falls ill, Moon disappears, and the now barely rational Bonner goes after her. Snowy and Sidewise decide to leave and go their separate ways. Sidewise admits he was having sex with Moon. Snowy spends the remainder of his life following the ape-people.

It seems doubtful that language would be lost, even if civilization collapsed. If Noam Chomsky is correct, the capacity for language is intrinsic to the human brain. This is borne out by the rapid emergence of creoles – fully-featured languages – from the linga franca known as pidgins that develop when people speaking different languages come into contact. Any group of humans capable of surviving for 1000 years would need language, and if they had only a rudimentary pidgin to begin with, it would soon expand into a proper language – as indeed described by Baxter in Chapter 12.

Long before the rise of civilization, primates gained a survival advantage by having ever bigger brains. Primates’ key strategy was to be smarter than the competition, and this would not change in a world after civilization had collapsed. Humans living in a post-apocalyptic world would need all their wits to survive. Possibly Baxter intends a metaphor in this and subsequent chapters – a social commentary on “dumbing down” (The effete Eloi and brutish Morlochs from “The Time Machine” were also intended as social commentary).

This chapter does have some dubious plot-devices. Putting a group of military personnel into suspended animation is a good way of getting them into the future, but makes little sense from a military point of view. In the absence of command-and-control facilities, scattered groups of twenty men and women armed only with Walther PPK semi-automatic pistols would be a pretty ineffective deterrent against invasion.

That the Pit’s clock would only be good for fifty years seems implausible. A Casio G-Shock’s calendar will run up to AD 2100 and it would have been trivial to provide a digital calendar that could record the passage of time for millennia. And why on earth leave the maps outside?

When Sidewise cannot find Mars in the night sky, he assumes it has been destroyed. But at any given time it is unusual for more than one or two of the customarily naked-eye planets to be on view. Typically at least one will be below the horizon, or will only be up during the daytime. Even to be able to see Venus, Jupiter and Saturn at the same time is actually quite uncommon.

Chapter 18: The Kingdom of the Rats
In east Africa, 30 million years from now, the rodents have consolidated their grip on Earth. Elephant-sized post-humans, with Big Brother contestant-sized brains, are farmed for their meat by rodents. Other monkey-sized post-humans live in the trees, as their pre-human ancestors once did. Detritus from Mankind’s tenure of Earth still litters the ground – glass from car windscreens, bottles, etc. At the end of this chapter, the asteroid Eros collides with Earth, but the light of its approach does not register as a threat in the dim consciousness of any of the planet’s current denizens.

Chapter 19: A Far Distant Futurity
The book’s final chapter, like its first, is set in Montana, now part of the supercontinent of New Pangaea, some 500 million years from now. The Sun is beginning to leave the main sequence and Earth is now a desert of salt and sandstone. Small monkey-like people now live in a symbiotic relationship with borametz trees, rather like the Fisher folk of Brian Aldiss’ Hothouse. Earth is visited by descendants of the von Neumann machines that dismantled Mars. Eventually, as the Sun heats up, bacteria inhabiting rock hurled into space by meteorite impacts is all that remains of life on Earth. Some of these bacteria eventually reach other planets, where life begins anew.

Epilogue
18 years after the Rabaul eruption, Joan Useb and her daughter Lucy are living on Bartolome Island in the Galopagos, looking after feral children left behind when the islands were evacuated during the post-Rabaul wars. She realises, though, that Homo sapiens day is done….

References:

Bellwood, P & Renfrew, C. (eds.) 2002: Examining the farming/language dispersal hypothesis, McDonald Institute, Cambridge.

Bottke W, Vokrouhlicky D and Nesvorny D (2008): “An asteroid breakup 160 Myr ago as the probable source of the K/T impactor”, Nature 449, 48-53 (6 September 2007).

Cavalli-Sforza L L (2000): “Genes, Peoples and Languages”, Penguin.

Conroy G (1997): “Reconstructing Human Origins: A Modern Synthesis”, W.W. Norton & Co. Inc, New York, NY & London.

Dawkins R (1982): “The Extended Phenotype”, Oxford University Press.

Dunbar R (1996): “Grooming, Gossip and the Evolution of Language”, Faber and Faber.

Fodor J (1983): “The Modularity of Mind”, MIT Press, Cambridge, MA.

Fortey R (1997): “Life: An Unauthorised Biography”, Flamingo.

Gardiner H (1983): “Frames of Mind”, Basic Books.

Gardiner H (1999): “Intelligence Reframed”, Basic Books.

Hodder I (2006): “Catalhoyuk: The Leopard’s Tale”, Thames & Hudson.

Jaynes J (1976): “The Origin of Consciousness in the Breakdown of the Bicameral Mind”, Mariner Books, USA.

Karmiloff-Smith A (1992): “Beyond Modularity”, MIT Press, Cambridge, MA.

Klein R & Edgar B (2002): “The Dawn of Human Culture”, John Wiley & Sons Inc., New York.

Knight C, Power C & Watts I, 1995): “The human symbolic revolution: A Darwinian account”, Cambridge Archaeological Journal. 5(1): 75-114.

Kohn M & Mithen S (1999): “Handaxes: products of sexual selection?” Antiquity 73: 518-526.

Lewis-Williams D (2002): “The Mind in the Cave”, Thames & Hudson.

Mithen S (1996): “The Prehistory of the Mind”, Thames & Hudson.

Oppenheimer S (2002): “Out of Eden”, Constable.

Power C & Aiello L (1997): “Female Proto-symbolic Strategies”, in Lori D Hager (ed), Women in Human Evolution. Routledge: London & New York. ISBN 0-415-10834-9.

Renfrew C (1987): “Archaeology and Language”, Cambridge University Press.

Scarre C (2005) (Ed): “The human past”, Thames & Hudson.

Soressi M & d’Errico F (2007): “Pigments , gravures , parures :les comportements symboliques controversies des Néandertaliens”, Les Néandertaliens. Biologie et cultures. Paris, Éditions du CTHS, 2007 (Documents préhistoriques ; 23), p. 297-309.

Sykes B (2001): “The Seven Daughters of Eve”, Bantam Press.

Tan Y, Yoder A, Yamasita N & Li W (2005): “Evidence from opsin genes rejects nocturnality in ancestral primates”, PNAS October 11, 2005 vol. 102 no. 41.

Tomasello (1999): “The Cultural Origins of Human Cognition”, Harvard University Press, Cambridge, MA & London.

de Waal F & Lanting F (1997): “Bonobo: the Forgotten Ape”, University of California Press.

Wade N (2007): “Before the Dawn”, Duckworth.

© Christopher Seddon 2008

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A Brief Guide to Evolution

Before Darwin

We have seen how Linnaeus laid the foundations of modern taxonomy, but he did not himself believe that species changed and was an adherent to the then-prevalent view of creationism, claiming that “God creates and Linnaeus arranges” (it has to the said that the self-proclaimed “Prince of Botany” was not the most modest of men!). Linnaeus died in 1778. At that time it was widely believed that Earth was less than 6000 years old, having been created in 4004 BC according to Archbishop Ussher, who put forward this date in 1650.

But the existence of extinct organisms in the fossil record represented a serious problem for creationism (about which creationists are still in denial – get over it!). Fossils had been known for centuries and it was becoming clear that they represented in many cases life forms that no longer existed. William Smith (1769-1839), a canal engineer, observed rocks of different ages preserved different assemblages of fossils and that these succeeded each other in a regular and determinable order. Rocks from different locations could be correlated on the basis of fossil content; a principle now known as the law of faunal succession. Unfortunately Smith was plagued by financial worries, even spending time in a debtor’s prison. Only towards the end of his life were his achievements recognised.

Georges Curvier (1769-1832) studied extinct animals and proposed catastrophism which is modified creationism. Extinctions were caused by periodic catastrophes and new species took their place, created ex nihil by God, though his view that more than one catastrophe might have occurred was contrary to Christian doctrine. But all species, past and present, remained immutable and created by God. Curvier rejected evolution because one highly complex form transitioning into another struck him as unlikely. The main problem with evolution was that if the Earth was only 6,000 years old, there would not be enough time for evolutionary changes to occur.

The French nobleman Compte de Buffon (1707-88) suggested that planets were formed by comets colliding with the Sun and that the Earth was much older than 6,000 years. He calculated a value of 75,000 years from cooling rate of iron – much to the annoyance of the Catholic Church. Fortunately the days of the Inquisition had passed; only Buffon’s books were burned! Buffon rejected Noah’s Flood; noted animals retain non-functional vestigial parts (suggesting that they evolved rather than were created); most significantly he noted the similarities between humans and apes and speculated on a common origin for the two. Although his views were decidedly at odds with the religious orthodoxy of the time, Buffon maintained that he did believe in God. In this respect, he was no different to Galileo, who remained a faithful Catholic.

Catastrophism was first challenged by James Hutton (1726-97), a Scottish geologist who first formulated the principles uniformitarianism. He argued that geological principles do not change with time and have remained the same throughout Earth’s history. Changes in Earth’s geology have occurred gradually, driven by the planet’s hot interior, creating new rock. The changes are plutonic (caused by volcanic action) in nature rather than diluvian (caused by floods). It was clear that the Earth must be much older than 6000 years for these changes to have occurred.

Hutton’s Investigation of the Principles of Knowledge was published in 1794 and The Theory of the Earth the following year. In the latter work he advocated evolution and natural selection. “…if an organised body is not in the situation and circumstances best adapted to its sustenance and propagation, then, in conceiving an indefinite variety among the individuals of that species, we must be assured, that, on the one hand, those which depart most from the best adapted constitution, will be the most liable to perish, while, on the other hand, those organised bodies, which most approach to the best constitution for the present circumstances, will be best adapted to continue, in preserving themselves and multiplying the individuals of their race.” Unfortunately this work was so poorly-written that not only was it largely ignored; it even hindered acceptance of Hutton’s geological theories, which did not gain general acceptance until the 1830s when they were popularised by fellow Scot Sir Charles Lyell (1797-1875), who also coined the word Uniformitarianism. However, it is now accepted that the catastrophists were not entirely wrong and events such as meteorite impacts and plate tectonics also have shaped Earth’s history.

The best-known pre-Darwinian theory of evolution is that of Jean-Baptiste de Lamarck (1744-1829). Lamarck proposed that individuals adapt during their lifetime and transmit acquired traits to their offspring. Offspring carry on where they left off, enabling evolution to advance. The classic example of this is the giraffe stretching its neck to reach leaves on high branches, and passing on a longer neck to its offspring. Some characteristics are advanced by use; others fall into disuse and are discarded. Lamarck’s two laws were:

1. In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.

2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.

Lamarck was not the only proponent of this point of view, but it is now known as Lamarckism. There is little doubt that confronted with the huge body of evidence assembled by Darwin, Lamarck would have abandoned his theory. However the theory remained popular with Marxists and its advocates continued to seek proof until well into the 20th Century. Most notable among these were Paul Kammerer (1880-1926), who committed suicide in the wake of the notorious “Midwife Toad” scandal; and Trofim Lysenko (1898-1976). With Stalin’s backing, Lysenko spearheaded an evil campaign against geneticists, sending many to their deaths in the gulags for pursuing “bourgeois pseudoscience”. Lamarckism continued to enjoy official backing in the USSR until the after fall of Khruschev in 1964, when Lysenko was finally exposed as a charlatan.

Natural selection.

Not until the middle of the 19th Century did Charles Darwin (1809-1882) and Alfred Russel Wallace (1823-1913) put forward a coherent theory of how evolution could work.

Darwin was appointed Naturalist and gentleman companion to Captain Robert Fitzroy of the barque HMS Beagle, joining the ship on her second voyage, initially against his father’s wishes. Fitzroy, serving as a lieutenant in Beagle, had succeeded to the captaincy when her original skipper Captain Pringle-Stokes committed suicide on the first voyage. Fitzroy was a Creationist and objected to Darwin’s theories. Darwin sailed round the world in Beagle between 1831 and 1836. He studied finches and turtles on the Galapagos Islands – different turtles had originated from one type, but had adapted to life on different islands in different ways. These changes and developments in species were in accord with Lyell’s Principles of Geology. Darwin was also influenced by the work of economist Thomas Malfus (1766-1834), author of a 1798 essay stating populations are limited by availability of food resources.

Darwin developed the theory natural selection between 1844 and 1858. The theory was as the same time being independently developed by Alfred Russel Wallace and in 1858 Darwin presented The Origin of Species by means of Natural Selection to the Linnaean Society of London, jointly with Wallace’s paper. Wallace’s independent endorsement of Darwin’s work leant much weight to it. Happily there were none of the unseemly squabbles over priority that have bedevilled so many joint discoveries down the centuries of which Newton and Leibnitz’s spat over differential calculus and strain placed on Anglo-French relations in the 1840s by John Couch Adams and Jean Urbain Le Verrier’s independent discovery of the planet Neptune are but two examples.

Darwin’s pivotal On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (usually simply referred to as The Origin of Species) was published in 1859 and promptly sold out. The book caused uproar and a debate was held at Oxford where Darwin and Thomas Huxley (grandfather of Aldous Huxley, author of Brave New World) were opposed by Bishop Samuel Wilberforce (son of the anti-slavery campaigner William Wilberforce), the clergy and Darwin’s erstwhile commanding officer, Captain Fitzroy of the Beagle. Darwin was by now in poor health due to an amoebal infection contracted during the Beagle voyage, but Huxley defended his theories vigorously.

The theory of natural selection states that evolutionary mechanisms are based on four conditions – 1) organisms reproduce; 2) there has to be a mode of inheritance whereby parents transmit characteristics to offspring; 3) there must be variation in the population and finally 4) there must be competition for limited resources. With some organisms in a population able to compete more effectively than others, these are the ones more likely to go on to reproduce and transmit their advantageous traits to their offspring, which in turn are more likely to reproduce themselves.

Evolution is the consequence of natural selection – the two are not the same thing, as evolution could in principle proceed by other means. Natural selection is a mechanism of change in species and takes various forms depends on certain conditions.

If for example existing forms are favoured then stabilising selection will maintain the status quo; conversely if a new form is favoured then directional selection will lead to evolutionary change. Divergent selection occurs when two extremes are favoured in a population.

Adaptation is a key concept in evolutionary theory. This is the “goodness of fit between an organism and its environment”. An adaptive trait is one that helps an individual survive, e.g. an elephant’s trunk, which enables it to forage in trees, eat grass, etc; colour vision helps animals to identify particular fruits, etc (and bright distinctive colour schemes are plants’ adaptations to help them to be located).

Sexual selection, proposed by Darwin in his second work The Descent of Man and Selection in Relation to Sex (1871), refers to adaptations in an organism specifically for the needs of obtaining a mate. In birds, this often leads to males having brightly coloured plumage, which they show off to prospective mates in spectacular displays. In many mammal species, males fight for access to females, leading to larger size (sexual dimorphism) and enhanced fighting equipment, e.g. large antlers.

The Descent of Man also put forward the theory that Man was descended from apes. Darwin was characterised as “the monkey man” and caricatured as having a monkey’s body. But after his death in 1882, he was given a state funeral and is buried in Westminster Abbey near Sir Isaac Newton. A dubious BBC poll ranked Charles Darwin as the 4th greatest Brit of all time, behind Churchill, I.K. Brunel and (inevitably) Princess Diana, but ahead of Shakespeare, Newton and (thankfully) David Beckham!

The main problem with Darwin’s theory was that by itself, it failed to provide a mechanism by which changes were transmitted from one generation to the next. Most believed that traits were “blended” in offspring than particulate – the latter being the view now known to be correct.

Mendelian inheritance

Ironically at the very time Darwin was achieving world fame, the missing link in his theory was being discovered by an Augustinian abbot named Gregor Mendel (1822-1884), whose work was practically ignored in his own lifetime. Between 1856 and 1863, Mendel studied the inheritance of traits in pea plants and showed that these followed particular laws and in 1865 he published the paper “Experiments in Plant Hybridization”, which showed that organisms have physical traits that correspond to invisible elements within the cell. These invisible elements, now called genes, exist in pairs. Mendel showed that only one member of this genetic pair is passed on to each progeny via gametes (sperm, ova, etc).

The set of genes possessed by an organism is known as its genotype. By contrast, a phenotype is a measurable characteristic of an organism, such as eye or hair colour, blood group, etc (it is sometimes used as a synonym for “trait” but phenotype is the value of the trait, e.g. if the trait is “eye colour” then possible phenotypes are “grey”, “blue”, etc.). Mendel investigated how various phenotypes of peas were transmitted from generation to generation, and whether these transmissions were unchanged or altered when passed on. His studies were based on traits such as shape of the seed, colour of the pea, etc, beginning with a set of pure-breeding pea plants, i.e. the second generation of plants had consistent traits with those of the first. He performed monohybrid crosses, i.e. between two strains of plants that differed only in one characteristic. The parents were denoted by a P, while the offspring – the filial generation – was denoted by F1, the next generation F2, etc. He found that in the first generation of these crosses, all of the F1s were identical to one of the parents. The trait expressed in the offspring he called a dominant trait; the unexpressed trait he called recessive (the Law of Dominance). He also observed that the sex of the parent was irrelevant for the dominant or recessive trait exhibited in the offspring (the Law of Parental Equivalence).

Mendel found that the phenotypes absent in the F1 generation reappeared in approximately a quarter of the F2 offspring. Mendel could not predict what traits would be present in any one individual, but he did deduce that there was a 3:1 ratio in the F2 generation for dominant/recessive phenotypes. In describing his results, Mendel used the term elementen, which he postulated to be hereditary “particles” transmitted unchanged between generations. Even if the traits are not expressed, he surmised that they are still held intact and the elementen passed on. These “particles” are now known as alleles. An allele that can be suppressed during a generation is called a recessive allele, while one that is consistently expressed is a dominant allele. An organism where both alleles for a particular trait are the same is said to be homozygous; where they differ, it is heterozygous.

For example, consider traits X and y, where X is dominant. A homozygous organism will be of phenotype X and genotype XX and a heterozygous organism will still have phenotype X, but the genotype will be Xy. (Note that the recessive allele is written in lower case.) The recessive trait will only be expressed when the genotype is yy, i.e. it receives the y-allele from both parents. There is a 50% chance of receiving the y-allele from either parent; hence only a 25% of receiving it from both; explaining the 3-1 ratio observed. The Law of Segregation states that each member of a pair of alleles maintains its own integrity, regardless of which is dominant. At reproduction, only one allele of a pair is transmitted, entirely at random.

Mendel next did a series of dihybrid crosses, i.e. crosses between strains identical except for two characteristics. He observed that each of the traits he was following sorted themselves independently. Mendel’s Law of Independent Assortment states that characteristics which are controlled by different genes will assort independent of all others. Whether or not an organism will be Ab or AA has nothing to do with whether or not it will be Xy or yy.

Mendel’s experimental results have been criticized for being “suspiciously good” and he seems to have fortunate in that he selected traits that were affected by just one gene. Otherwise the outcome of his crossings would have been too complex to have been understood at the time.

Population genetics

Mendel’s work remained virtually unknown until 1900, when it was independently rediscovered by Hugo de Vries, Carl Correns, and Erich von Tschermak and vigorously promoted in Europe by William Bateson, who coined the terms “genetics”, “gene” and “allele”. The theory was doubted by many because it suggested heredity was discontinuous in contrast to the continuous variety actually observed. R.A. Fisher and others used statistical methods to show that if multiple genes were involved with individual traits, they could account for the variety observed in nature. This work forms the basis of modern population genetics.

The discovery of DNA

By the 1930s, it was recognised that genetic variation in populations arises by chance through mutation, leading to species change. Chromosomes had been known since 1842, but their role in biological inheritance was not discovered until 1902, when Theodor Boveri (1862-1915) and Walter Sutton (1877-1916) independently showed a connection. The Boveri-Sutton Chromosome Theory, as it became known, remained controversial until 1915 when the initially sceptical Thomas Hunt Morgan (1866-1945) carried out studies on the eye colours of Drosophila melanogaster (the fruit fly) which confirmed the theory (and has since made these insects virtually synonymous with genetic studies).

The role of DNA as the agent of variation and heredity was not discovered until 1941, by Oswald Theodore Avery (1877-1955), Colin McLeod (1909-1972) and Maclyn McCarty (1911-2005). The double-helix structure of DNA was elucidated in 1953 by Francis Crick (1916-2005) and James Watson (b 1928) at Cambridge and Maurice Wilkins (1916-2004) and Rosalind Franklin (1920-1958) at King’s College London. The DNA/RNA replication mechanism was confirmed in 1958. Crick, Watson and Wilkins received the Nobel Prize for Medicine in 1962. Franklin, who died in 1958, missed out (Nobel Prizes are not normally awarded posthumously), but her substantial contribution to the discovery is commemorated by the Royal Society’s Rosalind Franklin Award, established in 2003.

How DNA works

With these discoveries, the picture was now complete, and it could now be seen how the genome is built up at a molecular level and how it is responsible for both variation and inheritance which are – as we have seen – fundamental to natural selection.

The genome of an organism contains the whole hereditary information of that organism and comprises the complete DNA sequence of one set of chromosomes. It is often thought of as a blueprint for the organism, but it is better thought of as a set of digital instructions that completely specify the organism.

The fundamental building blocks of life are a group of molecules known as the amino acids. An amino acid is any molecule containing both amino (-NH2) and carboxylic acid (-COOH) functional groups. In an alpha-amino acid, both groups are attached to the same carbon. Amino acids are the basic structural building blocks of proteins, complex organic materials that are essential to the structure of all living organisms. Amino acids form small polymer chains called peptides or larger ones called polypeptides, from which proteins are formed. The distinction between peptides and proteins is that the former are short and the latter are long. Some twenty amino acids are proteinogenic, i.e. they occur in proteins and are coded for in the genetic code. They are given 1 and 3 letter abbreviations, e.g. A Ala Alanine. Not all amino acids can be synthesised by a particular organism and must be included in the diet; these are known as essential amino acids. An amino acid residue is what is left of an amino acid once a molecule of water has been lost (an H+ from the nitrogenous side and an OH- from the carboxylic side) in the formation of a peptide bond.

Proteins are created by polymerization of amino acids by peptide bonds in a process called translation, a complex process occurring in living cells, etc. The blueprint – or to take a better analogy – the recipe or computer program for each protein used by an organism is held in its genome. The genome is comprised of nucleic acid, a complex macromolecule composed of nucleotide chains that convey genetic information. The most common nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). For nearly all organisms, the genome is comprised of DNA, which usually occurs as a double helix.

Nucleotides comprise a heterocyclic base (an aromatic ring containing at least one non-carbon atom, such as sulphur or nitrogen, in which the nitrogen atom’s lone pair is not part of the aromatic system); a sugar; and one or more phosphate (-PO3) groups. In the most common nucleotides the sugar is pentose – either deoxyribose (in DNA) or ribose (in RNA) and the base is a derivative of purine or pyrimidine. In nucleic acids the five most important bases are Adenine (A), Guanine (G), Thymine (T), Cytosine (C) and Uracil (U). A and G are purine derivatives and are large double-ringed molecules. T, C and U are pyrimidine derivatives and are smaller single-ringed molecules. T occurs only in DNA; U replaces T in RNA. These five bases are known as nucleobases.

In nucleic acids, nucleotides pair up by hydrogen bonding in various combinations known as base pairs. Purines only pair with pyrimidines. Purine-purine pairing is does not occur because the large molecules are far apart for hydrogen bonding to occur; conversely pyrimidine-pyrimidine pairing does not occur because the smaller molecules are too close and electrostatic repulsion overwhelms hydrogen bonding. G pairs only with C and A pairs only with T (in DNA) or U (in RNA). One might also expect GT and AC pairings, but these do not occur because the hydrogen donor and acceptor patterns do not match. Thus one can always construct a complimentary strand for any strand of nucleotides.

e.g. ATCGAT
TAGCTA.

Such a nucleotide sequence would normally be written as ATCGAT. Any succession of nucleotides greater than four is liable to be called a sequence.

DNA encodes the sequence of amino acid residues in proteins using the genetic code, which is a set of rules that map DNA sequences to proteins. The genome is inscribed in one or more DNA molecules. Each functional portion is known a gene, though there are a number of definitions of what constitutes a functional portion, of which the cistron is one of the most common. The gene sequence is composed of tri-nucleotide units called codons, each coding for a single amino acid. There are 4 x 4 x 4 codons (= 64), but only 20 amino acids, so most amino acids are coded for by more than one codon. There are also “start” and “stop” codons to define the beginning and end points for translation of a protein sequence.

In the first phase of protein synthesis, a gene is transcribed by an enzyme known as RNA polymerase into a complimentary molecule of messenger RNA (mRNA). (Enzymes are proteins that catalyze chemical reactions.) In eukaryotic cells (nucleated cells – i.e. animals, plants, fungi and protests) the initially-transcribed mRNA is only a precursor, often referred to as pre-mRNA. The pre-RNA is composed of coding sequences known as exons separated by non-coding sequences known as introns. These latter sequences must be removed and the exons joined to produce mature mRNA (often simply referred to as mRNA), in a process is known as splicing. Introns sometimes contain “old code,” sections of a gene that were probably once translated into protein but which are now discarded. Not all intron sequences are junk DNA; some sequences assist the splicing process. In prokaryotes (non-nucleated organisms – i.e. bacteria and archaea), this initial processing of the mRNA is not required.

The second phase of protein synthesis is known as translation. In eukaryotes the mature mRNA is “read” by ribosomes. Ribosomes are organelles containing ribosomal RNA (rRNA) and proteins. They are the “factories” where amino acids are assembled into proteins. Transport RNAs (tRNAs) are small non-coding RNA chains that transport amino acids to the ribosome. tRNAs have a site for amino acid attachment, and a site called an anticodon. The anticodon is an RNA triplet complementary to the mRNA triplet that codes for their cargo protein. Aminoacyl tRNA synthetase (an enzyme) catalyzes the bonding between specific tRNAs and the amino acids that their anticodons sequences call for. The product of this reaction is an aminoacyl-tRNA molecule. This aminoacyl-tRNA travels inside the ribosome, where mRNA codons are matched through complementary base pairing to specific tRNA anticodons. The amino acids that the tRNAs carry are then used to assemble a protein. Its task completed, the mRNA is broken down into its component nucleotides.

Prokaryotes have no nucleus, so mRNA can be translated while it is still being transcribed. The translation is said to be polyribosomal when there is more than one active ribosome. In this case, the collection of ribosomes working at the same time is referred to as a polysome.

In many species, only a small fraction of the total sequence of the genome appears to encode protein. For example, only about 1.5% of the human genome consists of protein-coding exons. Some DNA sequences play structural roles in chromosomes. Telomeres and centromeres typically contain few (if any) protein-coding genes, but are important for the function and stability of chromosomes. Some genes are RNA genes, coding for rRNA and tRNA, etc. Junk DNA represents sequences that do not yet appear to contain genes or to have a function.

The DNA which carries genetic information in cells (as opposed to mitochondrial DNA, etc) is normally packaged in the form of one or more large macromolecules called chromosomes. A chromosome is a very long, continuous piece of DNA (a single DNA molecule), which contains many genes, regulatory elements and other intervening nucleotide sequences. In the chromosomes of eukaryotes, the uncondensed DNA exists in a quasi-ordered structure inside the nucleus, where it wraps around structural proteins called histones. This composite material is called chromatin.

Histones are the major constituent proteins of chromatin. They act as spools around which DNA winds and they play a role in gene regulation, which is the cellular control of the amount and timing of appearance of the functional product of a gene. Although a functional gene product may be an RNA or a protein, the majority of the known mechanisms regulate the expression of protein coding genes. Any step of gene expression may be modulated, from the DNA-RNA transcription step to post-translational modification of a protein. Gene regulation gives the cell control over structure and function, and is the basis for cellular differentiation – i.e. the large range of cell types found in complex organisms.

Ploidy indicates the number of copies of the basic number of chromosomes in a cell. The number of basic sets of chromosomes in an organism is called the monoploid number (x). The ploidy of cells can vary within an organism. In humans, most cells are diploid (containing one set of chromosomes from each parent), but sex cells (sperm and ova) are haploid. Some plant species are tetraploid (four sets of chromosomes). Any organism with more than two sets of chromosomes is said to be polyploid. A species’ normal number of chromosomes per cell is known as the euploid number, e.g. 46 for humans (2×23).

Haploid cells bear one copy of each chromosome. Most fungi, and a few algae are haploid organisms. Male bees, wasps and ants are also haploid. For organisms that only ever have one set of chromosomes, the term monoploid can be used interchangeably with haploid.

Plants and other algae switch between a haploid and a diploid or polyploid state, with one of the stages emphasized over the other. This is called alternation of generations. Most diploid organisms produce haploid sex cells that can combine to form a diploid zygote, for example animals are primarily diploid but produce haploid gametes. During meiosis, germ cell precursors have their number of chromosomes halved by randomly “choosing” one homologue (copy), resulting in haploid germ cells (sperm and ovum).

Diploid cells have two homologue of each chromosome (both sex- and non-sex determining chromosomes), usually one from the mother and one from the father. Most somatic cells (body cells) of complex organisms are diploid.

A haplodiploid species is one in which one of the sexes has haploid cells and the other has diploid cells. Most commonly, the male is haploid and the female is diploid. In such species, the male develops from unfertilized eggs, while the female develops from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg. Thus males have no father. Haplodiploidy is found in many species of insects from the order Hymenoptera, particularly ants, bees, and wasps.

Cell division is the process by which a cell divides into two daughter cells. Cell division allows an organism to grow, renew and repair itself. Cell division is of course also vital for reproduction. For simple unicellular organisms such as the Amoeba, one cell division reproduces an entire organism. Cell division can also create progeny from multicellular organisms, such as plants that grow from cuttings. Finally, cell division enables sexually reproducing organisms to develop from the one-celled zygote, which itself was produced by cell division from gametes.

Before division can occur, the genomic information which is stored in a cell’s chromosomes must be replicated, and the duplicated genome separated cleanly between cells. Division in Prokaryotic cells involves cytokinesis only. As previously explained, prokaryotic cells are simple in structure. They contain non-membranous organelles, lack a cell nucleus, and have a simplistic genome: only one circular chromosome of limited size. Therefore, prokaryotic cell division, a process known as binary fission, is straightforward. The chromosome is duplicated prior to division. The two copies of the chromosome attach to opposing sides of the cellular membrane. Cytokinesis, the physical separation of the cell, occurs immediately.

Division in Somatic Eukaryotic cells involves mitosis then cytokinesis. Eukaryotic cells are complex. They have many membrane-bound organelles devoted to specialized tasks, a well-defined nucleus with a selectively permeable membrane, and a large number of chromosomes. Therefore, cell division in somatic (i.e. non-germ) eukaryotic cells is more complex than cell division in prokaryotic cells. It is accomplished by a multi-step process: mitosis: the division of the nucleus, separating the duplicated genome into two sets identical to the parent’s; followed by cytokinesis: the division of the cytoplasm, separating the organelles and other cellular components.

Division in Eukaryotic Germ cells involves meiosis, which is the process that transforms one diploid cell into four haploid cells in eukaryotes in order to redistribute the diploid’s cell’s genome. Meiosis forms the basis of sexual reproduction and can only occur in eukaryotes. In meiosis, the diploid cell’s genome is replicated once and separated twice, producing four haploid cells each containing half of the original cell’s chromosomes. These resultant haploid cells will fertilize with other haploid cells of the opposite gender to form a diploid cell again. The cyclical process of separation by meiosis and genetic recombination through fertilization is called the life cycle. The result is that the offspring produced during germination after meiosis will have a slightly different genome contained in the DNA. Meiosis uses many biochemical processes that are similar to those used in mitosis in order to distribute chromosomes among the resulting cells.

Genetic recombination is the process by which the combinations of alleles observed at different loci in two parental individuals become shuffled in offspring individuals. Such shuffling can be the result of inter-chromososomal recombination (independent assortment) and intra-chromososomal recombination (crossing over). Recombination only shuffles already existing genetic variation and does not create new variation at the involved loci. Since the chromosomes separate independently of each other, the gametes can end up with any combination of paternal or maternal chromosomes. In fact, any of the possible combinations of gametes formed from maternal and paternal chromosomes will occur with equal frequency. The number of possible combinations for human cells, with 23 chromosomes, is 2 to the power of 23 or approximately 8.4 million. The gametes will always end up with the standard 23 chromosomes (barring errors), but the origin of any particular one will be randomly selected from paternal or maternal chromosomes.

The other mechanism for genetic recombination is crossover. This occurs when two chromosomes, normally two homologous instances of the same chromosome, break and then reconnect but to the different end piece. If they break at the same place or locus in the sequence of base pairs – which is the normal outcome – the result is an exchange of genes.

An allele is any one of a number of viable DNA codings of the same gene (sometimes the term refers to a non-gene sequence) occupying a given locus (position) on a chromosome. An individual’s genotype for that gene will be the set of alleles it happens to possess. For example, in a diploid organism, two alleles make up the individual’s genotype.

Organisms that are diploid such as humans have paired homologous chromosomes in their somatic cells, and these contain two copies of each gene. An organism in which the two copies of the gene are identical — that is, have the same allele — is said to be homozygous for that gene. An organism which has two different alleles of the gene is said to be heterozygous. Phenotypes associated with a certain allele can sometimes be dominant or recessive, but often they are neither. A dominant phenotype will be expressed when only one allele of its associated type is present, whereas a recessive phenotype will only be expressed when both alleles are of its associated type. This is Mendelian inheritance at a molecular level.

However, there are exceptions to the way heterozygotes express themselves in the phenotype. One exception is incomplete dominance (sometimes called blending inheritance) when alleles blend their traits in the phenotype. An example of this would be seen if, when crossing Antirrhinums — flowers with incompletely dominant “red” and “white” alleles for petal colour — the resulting offspring had pink petals. Another exception is co-dominance, where both alleles are active and both traits are expressed at the same time; for example, both red and white petals in the same bloom or red and white flowers on the same plant. Co-dominance is also apparent in human blood types. A person with one “A” blood type allele and one “B” blood type allele would have a blood type of “AB”.

Recombination shuffles existing variety, but does not add to it. Variety comes from genetic mutation. Mutations are changes to the genetic material of an organism. Mutations can be caused by copying errors in the genetic material during cell division and by exposure to radiation, chemicals, or viruses. In multicellular organisms, mutations can be subdivided into germline mutations, which can be passed on to descendants and somatic mutations. The latter cannot be transmitted to descendants in animals, though plants sometimes can transmit somatic mutations to their descendents. Mutations are considered the driving force of evolution, where less favourable or deleterious mutations are removed from the gene pool by natural selection, while more favourable ones tend to accumulate. Neutral mutations are defined as those that are neither favourable nor unfavourable.

It will be apparent from the above how both variety and inheritance of variety arise at the molecular level.

The so-called central dogma of molecular biology arises from Francis Crick’s statement in 1958 that “Genetic information flows in one direction only from DNA to RNA to protein, and never in reverse.” It follows from this that:

1. Genes determine characters in a straightforward, additive way: one gene-one protein, and by implication, one character. Environmental influence, if any, can be neatly separated from the genetic.

2. Genes and genomes are stable, and except for rare, random mutations, are passed on unchanged to the next generation.

3. Genes and genomes cannot be changed directly in response to the environment.

4. Acquired characters are not inherited.

These assumptions have been challenged and they do not hold under all conditions, e.g. horizontal gene transfer (for example, haemoglobins in leguminous plants).

Modes of evolutionary change

Put together, natural selection, population genetics and molecular biology form the basis of neo-Darwinism, or the Modern Evolutionary Synthesis. The theory encompasses three main tenets:

1. Evolution proceeds in a gradual manner, with the accumulation of small changes in a population over long periods of time, due to changes in frequencies of particular alleles between one generation and another (microevolution).

2. These changes result from natural selection, with differential reproductive success founded on favourable traits.

3. These processes explain not only small-scale changes within species, but also larger-scale processes leading to new species (macroevolution).

On the neo-Darwinian picture, macroevolution is seen simply as the cumulative effects of microevolution.

However the extent and source of variation at the genetic level remained a bone of contention for evolutionary theorists until the mid-1960s. One school of thought favoured little variation, with most mutations being deleterious and selected against; the other school favoured extensive variation, with many mutations offering advantages for survival in different environmental circumstances. Techniques such as gel electrophoreses settled the argument in favour of the second school: genetic variation turned out to be most extensive. By the 1970s the debate had shifted to selectionism versus neutralism. The selectionists view genetic variation as the product of natural selection, which selects favourable new variants. On the other hand the neutralists contend that the great majority of variants are selectively neutral and thus invisible to the forces of natural selection. It is now generally accepted that a significant proportion of variation at the genetic level is neutral.

Consequently certain traits may become common or may even come to predominate in a population by a process known as genetic drift. This is the random changes in the frequencies of neutral alleles over many generations, which may lead to some becoming common and some dying out. Genetic drift, therefore, tends to reduce genetic diversity over time, though for the effect to be significant, a population must be small (to explain by analogy, while a group of ten people could throw a die and all fail to get a six with reasonable probability [tenth power of 0.8 = 0.1], but the probability of one hundred people all failing to get a six is far smaller [hundredth power of 0.8 = 2x10e-10]). There are two ways in which small isolated populations may arise. One is by population bottleneck in which the bulk of a population is killed off; the other is the founder effect which occurs when a small number of individuals carrying a subset of the original population’s genetic diversity move into a new habitat and establishes a new population there. Both these scenarios could lead to a trait that confers no selective advantage coming to predominate in a population. More controversially, they could lead to genetic drift outweighing natural selection as the engine for evolutionary change.

With the foregoing in mind, how do new species arise? There are two ways. Firstly a species changes with time until a point is reached where its members are sufficiently different from the ancestral population to be considered a new species. This form of speciation is known as anagenesis and the species within the lineage are known as chronospecies. Secondly a lineage can split into two different species. This is known as cladogenesis, and usually happens when a population of members of the species becomes isolated.

There are several such modes of speciation, mostly based on the degree of geographical isolation of the populations involved.

1. Allopatric speciation occurs when populations physically isolated by a barrier such as a mountain or river diverge to an extent such that if the barrier between the populations breaks down, individuals of the two populations can no longer interbreed.

2. Peripatric speciation occurs when a small population is isolated at the periphery of a species range. The difference between this and allopatric speciation is that the isolated population is small. Genetic drift comes into play, possibly outweighing natural selection (the founder effect).

3. Parapatric speciation occurs when a population expands its range into a new habitat where the environment favours a different form. The populations diverge as the descendants of those entering the new habitat adapt to the conditions there.

4. Sympatric speciation is where the speciating populations share the same territory. Sympatric speciation is controversial and has been widely rejected, but a number of models have been proposed to account for this mode of speciation. The most popular is disruptive speciation (Smith), which proposes that homozygous individuals may under certain conditions have a greater fitness than those with alleles heterozygous for a certain trait. Under the mechanism of natural selection, therefore, homozygosity would be favoured over heterozygosity, eventually leading to speciation. Rhagoletis pomonella (Apple maggot) may be currently undergoing sympatric speciation. The apple feeders seem to have emerged from hawthorn feeders, after apples were first introduced into North America. The apple feeders do not now normally feed on hawthorns, and the hawthorn feeders do not now normally feed on apples. This may be an early step towards the emergence of a new species.

5. Stasipatric speciation occurs in plants when they double or triple the number of chromosomes, resulting in polyploidy.

Rates of evolution

There are two opposing points of view regarding the rate at which evolutionary change proceeds. The traditional view, known as phyletic gradualism, holds that it occurs gradually, and that speciation is anagenetic. Niles Eldredge and Stephen Jay Gould (1972) criticized this viewpoint, arguing instead for stasis over long stretches of time, with speciation occurring only over relatively brief intervals, a model they called punctuated equilibrium. They pointed out that species arise by cladogenesis rather than by anagenesis. They also highlighted the absence of transitional forms in the fossil record (an old chestnut, often favoured by creationists).

Richard Dawkins has pointed out that no “gradualist” has ever argued for complete uniformity of rate of evolutionary change; conversely even if the “punctuation” events of Eldredge and Gould actually took 100,000 years, they would still show as discontinuities in the fossil record, even though on the scale of the lifetime of an organism, change would be immeasurably small, and invisible at any given time due to variation between individuals; if for example average height increased by 10 cm in 100,000 years, that would be 1/500th of a cm per generation – completely masked by the variation in height of individuals at any one time. It follows that the speciation event – be it anagenetic or cladogenetic – would be very slow in relation to the lifetime of individuals. Reproductive isolation would occur only over hundreds of generations.

On the Dawkins view, then, there is no conflict between gradualism and punctuationism; the latter is no more than the former proceeding at varying tempo.

The Physical Context

Three factors are recognized as influencing the evolution of new species and the extinction of existing ones. The first is the existing properties of a lineage, which places constraints on how it can evolve. The second is the biotic context: how members of a particular species compete both in both an inter-specific and intra-specific context for food, space and other resources; how they interact with other species in respect of predation; mutualist behaviours, etc. The third is the physical context such as geography and climate, which determine the types of species that can thrive and the adaptations that are likely to be favoured.

The relative importance of the last two is a matter of ongoing debate. Darwin held that biotic factors predominate. He did not ignore environmental considerations, but he saw them as merely increasing competition. This view is central to the modern synthesis and it is held that natural selection is necessary and sufficient to drive evolutionary change. For example, adaptations by predators to better their chances of catching prey are the driving force for evolutionary change in the prey, where adaptations to avoiding capture are selected for; thus maintaining the status quo in a kind of evolutionary “arms race”. This is sometimes referred to as the Red Queen effect (van Valen, 1973), from the Red Queen in Alice through the Looking Glass.

However in recent years, it has become clear that the history of life on Earth has been profoundly affected by geological change. The discovery of plate tectonics in the 1960s confirmed that continental landmasses are in a state of constant albeit very slow motion across the Earth’s surface, and when continents meet, previously-isolated biota are brought together. Conversely, as continents drift apart, previously united communities are separated. The first introduces new elements of inter-specific competition; the other to possible isolation of small groups. Both scenarios are likely to lead to evolutionary change.

There is also a school of thought that downplays natural selection and emphasises climate change as the primary cause of evolutionary change. There are two ideas associated with this view – firstly, the habitat theory, which states that species’ response to climate change represents the principle engine of evolutionary change, and that speciation and extinction events will be concentrated in times of climate change, as habitats change and fragment; secondly this pattern of evolutionary change should be synchronous across all taxa, including hominins (the turnover-pulse hypothesis) (Vrba 1996).

Units of selection

In the original theory of Charles Darwin, the unit of selection i.e. the biological entity upon which the forces of natural selection acted was the individual; for example an animal that can run faster than others of its kind, and so avoid predators, will live longer and have more offspring. This simple picture does, however, fail to explain altruism in which an individual acts in a manner that benefits others at its own expense.

One answer is that selection may operate at social group level (group selection) as proposed by V.C. Wynne-Edwards (1962). On this picture, a group in which members behave altruistically towards one another might be more successful than one in which they do not. Kin selection, proposed by W.D. Hamilton (1964), posits reproductive success in terms of passing on one’s genes, and that by helping siblings and other relatives, one is doing this by proxy. This view largely superseded the group selection view. Robert Trivers (1971) extended the theory to non-kin in terms of doing a favour in the expectation of it being returned (“reciprocal altruism”). This behaviour is common in species of large primates, including humans. Kin-selection and reciprocal altruism act at individual and not group level, so the latter fell out of favour; though it has been recently revived.

By contrast, the gene-centric or “selfish gene” view popularised by Richard Dawkins states that selection acts at gene level, with genes that best promote the interests of their host organisms being selected for. On this view, adaptations are phenotypic effects that enable genes to be propagated. A “selfish” gene can be favoured for selection by favouring altruism among organisms containing it, even if individuals performing the altruism do so at the cost of their own chances of reproducing. To be successful, however, a gene must “recognise” kin and degrees of relatedness and favour greater altruism towards closer relatives (by, for example, favouring a sibling [where the chance of the same gene being present is 1/2] over a cousin [where it is only 1/8]). The green beard effect refers to such forms of genetic self-recognition, after Dawkins (1976) considered the possibility of a gene that promoted green beards and altruism to others possessing them.

© Christopher Seddon 2008