Did the modern brain shape only evolve recently?

Study claims that brain did not reach present-day range of variation until between 100,000 and 35,000 years ago.

A new study (Neubauer, et al., 2018) has suggested that globular form of the human cranial vault did not reach its present-day range of variation until between 100,000 and 35,000 years ago, and that this was linked to continuing evolutionary change affecting the shape and proportions of the brain. Fully modern human behaviour, it is claimed, did not emerge until that time.

Present-day humans are distinguished from archaic humans such as Neanderthals by a globular as opposed to a long, low cranial vault. The earliest representatives of our species (‘archaic Homo sapiens’), who lived around 300,000 years ago, retained the archaic brain shape; but by 200,000 years ago this had given way to the modern, globular form – or had it?

Paleoanthropologists at the Max Planck Institute for Evolutionary Anthropology in Germany used CT scans to generate virtual endocasts of modern human skulls from 315,000 to 195,000 years ago, 120,000 years ago, 35,000 to 8,000 years ago, along with skulls of Neanderthals, Homo heidelbergensis, and Homo erectus. They applied statistical methods to these, and they concluded that globularity within the present-day range of variation did not appear until between 100,000 and 35,000 years ago.

The transition from the long, low to globular condition has been long attributed to changes in the proportions of rather than the size of the brain. However, the Max Planck report suggested that this happened in two stages. In the first stage, the cerebellum, parietal, and temporal areas increased in size. This was followed by a second stage in which the cerebellum continued to increase in size, but this was accompanied by size increases in the occipital lobes. This second stage was not completed until between 100,000 and 35,000 years ago. The report suggested that the most important changes were the expansion of the parietal areas and the cerebellum.

The parietal areas are associated with orientation, attention, perception of stimuli, sensorimotor transformations underlying planning, visuospatial integration, imagery, self-awareness, working and long-term memory, numerical processing, and tool use. The cerebellum is associated not only with motor-related functions including coordination of movements and balance but also with spatial processing, working memory, language, social cognition, and affective processing.

The report links these changes with evidence for the emergence of modern human behaviour in the archaeological record. It notes that, firstly, the onset of the Middle Stone Age in Africa 300,000 years ago corresponds closely in time the earliest known fossils of Homo sapiens (the Jebel Irhoud remains from Morocco). Secondly, behavioural modernity gradually developed over time in concert with increasing globularity. Thirdly, the point at which the modern condition was achieved corresponds to the transition from the Middle to the Later Stone Age in Africa and from the Middle to the Upper Palaeolithic in Europe around 50,000 to 40,000 years ago.

The idea that anatomically modern humans were not behaviourally modern in the first instance is an old one, based on the idea changes in the archaeological records of Europe and Africa 50,000 years ago were linked to a cognitive ‘Great Leap Forward’. This, it was argued, was the result of a favourable genetic mutation that somehow ‘rewired’ the human brain, enabling it to function more efficiently. The Max Planck report rejects this conclusion, suggesting that the Great Leap Forward simply represented the end-point of the globularization process.

The problem is that the notion that changes in the archaeological record could be linked to a cognitive advance 50,000 years ago was thoroughly debunked by anthropologists Sally McBrearty and Alison Brooks almost two decades ago – ironically in a paper cited by the authors of the Max Planck report. (McBrearty & Brooks, 2000) In Europe, there is no doubt that a dramatic change is seen with the onset of the Upper Palaeolithic. Cave paintings, carved figurines, and other art appears for the first time. Nobody doubts that these artefacts are products of wholly modern human minds – but they simply herald the arrival of modern humans in Europe, not a cognitive advance by people already living there. Similarly, the transition from Middle to Later Stone Age in Africa is more parsimoniously explained by the need of growing populations for better tools and more sophisticated hunting techniques. Many supposed innovations can be found tens of thousands of years earlier at African Middle Stone Age sites. These include:

  • 60,000-year-old ostrich eggshells engraved with graphic patterns from Diepkloof Rock Shelter, South Africa.
  • Evidence for a well-developed catfish harvesting industry at Katanda on the Upper Semliki River in the Democratic Republic of the Congo, 90,000 years ago.
  • Ochre pieces engraved with abstract patterns from Blombos Cave, South Africa, in some cases over 100,000 years old.
  • Microliths from Pinnacle Point, South Africa, dating to 164,000 years ago. Microliths are used in multi-component tools, and they are associated with the most advanced (mode 5) stone tool technologies.

Furthermore, many traits once considered to be markers of late emerging modern human behaviour have now been identified much further back in the archaeological record, and indeed are not restricted to modern humans. These include fowling and use of seafood, both of which have since also been attributed to Neanderthals.

This evidence suggests that modern human behaviour had certainly emerged by 100,000 years ago, and probably by 164,000 years ago. While a link between globularity and modern human behaviour is likely, the associated cognitive changes probably only occurred during the first phase of globularization between 315,000 to 195,000 years ago. Subsequent increases in globularity might be linked to factors other than changes in brain shape. Early modern humans were far more powerfully built than present-day people, and the more gracile, fully-modern form did not appear until after 35,000 years ago. Brains actually show a slight decrease in average size during this period.

References:

McBrearty, S. & Brooks, A., 2000. The revolution that wasn’t: a new interpretation of the origin of modern human behaviour. Journal of Human Evolution, Volume 39, pp. 453-563.

Neubauer, S., Hublin, J. & Gunz, P., 2018. The evolution of modern human brain shape. Science Advances, 24 January, Volume 4, p. eaao5961.

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Modern humans left Africa almost 200,000 years ago

But should we be surprised?

With an age range of 120,000 to 90,000 years old, the fossils from the Levantine sites of Skhul and Qafzeh have long been the oldest known anatomically modern human remains from outside Africa. The recent find of an upper jawbone and associated dentition at Misliya Cave in Israel has now been independently dated by uranium series (U-Th), combined uranium and electron spin resonance (U-ESR), and thermoluminescence (TL) methods to yield an age range of 194,000 to 177,000 years old. The jawbone and teeth are associated with the Homo sapiens clade, meaning that they predate the Skhul and Qafzeh remains by more than 50,000 years. (Hershkovitz, et al., 2018)

The Misliya Cave remains were associated with large numbers of Levallois (mode 3) stone tools, characteristic of the Middle Palaeolithic.

While the findings have understandably generated a good deal of excitement, should we be unduly surprised? The Sahara and Sinai deserts can only be crossed during interglacials, when warm, wet climatic conditions cause these normally inhospitable regions to green, and the Levant effectively becomes a northeasterly extension of Africa. The date range of the Skhul and Qafzeh remains suggest that these people left Africa during the Eemian interglacial (Marine Isotope Stage 5e) 126,000 to 110,000 years ago. Similarly, the upper end of the age range of Misliya Cave remains lies within the warm, wet Marine Isotope Stage 7 which lasted from 245,000 to 186,000 years ago.

Until recently, the earliest anatomically modern humans were believed to be those from Omo, Kenya, now thought to be 195,000 years old (though originally thought to be more recent). Accordingly, it was not thought that modern humans could have left Africa prior to the Eemian. Recent discoveries from China and the Arabian Peninsula have overturned the longstanding view that the Levant was the extent of our species’ excursions beyond Africa prior to around 65,000 years ago. However, the Eemian was still thought to represent the upper limit.

The re-dating of the Jebel Irhoud remains from Morocco last year has changed the picture. The remains were found at a cave site 100 km (60 miles) from Marrakech in the early 1960s and were originally thought to be no more than 40,000 years old. The puzzle was that while the facial features are modern, the brain case is still long and low, a characteristic of archaic humans and suggesting that they really belonged to a much earlier lineage of Homo sapiens. This eventually turned out to be the case. In 2007, the remains were found to be much older at 160,000 years old with US-ESR methods – but even this turned out to be a gross underestimate. Excavations carried out between 2004 and 2011 enabled radiation dosages to be estimated more accurately, yielding a TL date of 286,000 ± 32,000 years old – making the Jebel Irhoud the earliest representatives of our species by some considerable way.

With modern humans having existed throughout Marine Isotope Stage 7, it is unsurprising that some of them reached the Levant, and entirely possible that some went further. This raises the possibility that some of these pioneers encountered and interbred with Neanderthals, which would explain a 2017 genetic study which suggested that Neanderthals and modern humans were interbreeding as long ago as the period between 460,000 and 219,000 years ago (Posth, et al., 2017). The upper end is clearly an overestimate, but the lower end could point to interbreeding in the Levant, where Neanderthals are known to have been present. While there is no suggestion at this stage that modern humans reached Europe prior to 46,000 years ago, such a discovery would call into question the attribution of recent discoveries, such as the stone circle Bruniquel Cave in southwest France reported in 2016 to be 176,500 years old, and accordingly assumed to be the work of Neanderthals. (Jaubert, et al., 2016)

References:
Hershkovitz, I. et al., 2018. The earliest modern humans outside Africa. Science, Volume 359, pp. 456-459.
Jaubert, J. et al., 2016. Early Neanderthal constructions deep in Bruniquel Cave in southwestern France. Nature, 2 June, Volume 534, pp. 111-114.
Posth, C. et al., 2017. Deeply divergent archaic mitochondrial genome provides lower time boundary for African gene flow into Neanderthals. Nature Communications, 4 July, Volume 8, p. 16046.

Meat-eating and food processing were major drivers of human evolution

Study shows how dietary changes and stone tools enabled reductions in size of teeth, jaws and gut

In comparison to earlier hominins, Homo erectus was bigger both in stature and brain size. As such, its energy requirements would have increased – but paradoxically the teeth and chewing muscles were smaller, maximum bite forces weaker and the gut size was reduced. It has long been assumed that this was made possible by increased meat consumption, slicing and pounding food with stone tools, and by cooking. However, the latter was uncommon until around 500,000 years ago. By these means, it is believed that Homo erectus and later humans reduced the both amount of chewing required for their food and workload of the gut in digesting it.

In a newly-published study, Zink and Lieberman report on a series of experiments intended to test these hypotheses. They measured chewing performance in adult human subjects fed size-standardized portions of meat and underground storage organs (roots, tubers, etc.) which are thought to have formed a major component of hominin diet. Goat meat, yams, carrots and beets were chosen for the test; goat is tougher than beef and therefore more similar to the wild game eaten by early hominins. The food was either unprocessed, processed by simple mechanical methods available in Lower Palaeolithic times (slicing and pounding), or roasted (the simplest form of cooking).

They found that the subjects were unable to chew the raw meat effectively, but slicing it resulted in substantial reductions in both the amount of chewing and bite forces required, and in smaller and more digestible meat particles were swallowed. Roasted meat required a greater chewing effort, but even smaller meat particles resulted. However, even unprocessed meat required considerably less masticatory effort than the raw USOs.

Although the advent cooking brought considerable benefits in terms of hygiene and increased energy yields, Zink and Lieberman believe that the reductions in dental size and jaw musculature observed in Homo erectus would have been made possible by the combined effects of eating more meat and mechanically processing both it and USOs. By eating a diet of one-third meat and two-thirds USOs, and slicing the meat and pounding the USOs with stone tools prior to eating, early humans would have reduced chewing by 17 percent and enabled a 26 percent reduction in bite forces.

Although it is possible that food processing and meat eating favoured evolutionary selection for smaller teeth and jaws, Zink and Lieberman believe that it is more likely that these relaxed the selective pressures maintaining robust masticatory anatomy, thus enabling selection to decrease facial and dental size for other functions such as speech production, locomotion, thermoregulation, and possibly even changes in the size and shape of the brain, so leading eventually to the modern condition of Homo. Regardless of what evolutionary factors favoured these changes, they would not have been possible without increased meat eating combined with food processing technology.

Reference:
Zink, K. & Lieberman, D., Impact of meat and Lower Palaeolithic food processing techniques on chewing in humans. Nature (Published online) (2016).

Human evolution favoured brain over brawn

Metabolite study demonstrates human muscle and brain tissue underwent disproportionate evolutionary change

A new study, published in the open access journal PLoS One Biology, has used metabolites to track evolutionary changes in brain and skeletal muscle tissues. Metabolites are metabolic products or intermediates of low molecular weight (1,500 amu or less), which are associated with the physiological processes that maintain the functionality of body tissues. Changes in the concentrations of these metabolites are thought to be closely related to evolutionary changes in the associated tissues.

Researchers measured the concentrations of more than 10,000 metabolites in the prefrontal cortex, primary visual cortex, cerebellar cortex, skeletal muscles and kidneys of humans, chimpanzees, macaque monkeys and mice using mass spectrometry-based techniques. They found that in most cases the differences reflected genetic distances between the species rather than environmental differences.

The striking exception was found in the human lineage. The concentration profiles of metabolites associated with the human prefrontal cortex, cerebellar cortex and skeletal tissues showed far greater changes than could be accounted for by genetic difference: by a factor of four for the brain tissue, and eight for the muscle tissue. In fact the muscle tissue is implied to have undergone more evolutionary change in the 6 to 7 million years since the divergence from chimpanzees than it did during the 130 or so million years separating mice from the common ancestor of the apes and Old World monkeys. No comparable differences were noted for the primary visual cortex or kidneys. Nor were significant differences to any of these results found after controlling for differences in diet and levels of physical activity.

It is well known that humans are physically quite weak in comparison to chimpanzees, despite weighing in at around twice the size. Surprisingly, this is largely based on anecdotal observations mostly predating the 1950s. Accordingly, the researchers set macaque, chimpanzee and human subjects a ‘pulling task’, which tested both upper and lower body strength. These tests confirmed the anecdotal observations.

The researchers concluded that the metabolic changes in human muscle tissue were associated with a drastic reduction in muscle strength; and that these changes might be linked to the changes in brain metabolism and enhanced cognitive abilities.

The findings are an extension of Aiello and Wheeler’s ‘expensive tissue’ hypothesis, which proposed that the considerable energy requirements of the human brain (around 20 percent of the total energy budget) could only be met by making savings elsewhere. Aiello and Wheeler (1995) proposed these savings were made by downsizing other energetically-expensive organs, principally the gut. Apparently, though, this was insufficient and further savings were required in the form of a decrease in the energy expenditure of skeletal muscle.

References:
1.  Bozek, K. et al., Exceptional Evolutionary Divergence of Human Muscle and Brain Metabolomes Parallels Human Cognitive and Physical Uniqueness. PLoS One Biology 12(5), e1001871 (2014).
2.  Aiello, L. & Wheeler, P., The expensive tissue hypothesis: the brain and the digestive system in human and primate evolution. Current Anthropology 36, 199-221 (1995).

Link (open access):
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001871.

Were multiple early human species living in Georgia, 1.85 million years ago?

New skull with ‘enigmatic’ jawbone and differing tool technologies suggests that two different hominin groups are represented by Dmanisi remains.

The former Soviet republic of Georgia is located at the crossroads of Europe and Asia. Lying on the eastern shores of the Black Sea, it was the destination of Jason and the Argonauts in their quest for the Golden Fleece, but long before this it was a stopping point for the earliest-known hominin migration out of Africa. In 1984, stone tools were discovered at the small medieval town of Dmanisi in the southeast of the country, 93 km (58 miles) southwest of the capital, Tbilisi. Archaeologists broke through the foundations of a medieval building into an ancient river deposit, where simple stone tools resembling those made by the earliest humans were found with the bones of extinct mammals.

During the 1990s, the remains of early humans were recovered, including two partial skulls and a lower jawbone. The fossils were dated by palaeomagnetic, potassium-argon and argon-argon methods, giving an age for the remains of 1.77 million years old (Gabunia, et al., 2000). Subsequent dating of the stone tools indicated that the site was first occupied 1.85 million years ago, and that repeated occupations continued over a period of 80,000 years. There was evidently a long-term human presence in the Caucasus at around or even before the time of the earliest evidence for Homo erectus in Africa (Ferring, et al., 2011).

There have been a number of subsequent discoveries of human remains at the site. These include the skull, lower jawbone and partial skeleton of an adolescent (Vekua, et al., 2002; Lordkipanidze, et al., 2007); the skulls and lower jawbones of two adults (Lordkipanidze, et al., 2006; Lordkipanidze, et al., 2013); and postcranial bones from three other individuals, all adults (Lordkipanidze, et al., 2007). One of the skulls belonged to an elderly male who had lost all but one of his teeth some years prior to his death. He could not have survived unaided and must have been cared for by his companions throughout those last years of his life (Lordkipanidze, et al., 2005; Lordkipanidze, et al., 2006). The other skull, the fifth to be discovered at the site and hence known as Skull 5, is characterised by a large face and thick browridges. Skull 5 is complete and undeformed; it is the only known fully-preserved adult hominin skull from the early Pleistocene (Lordkipanidze, et al., 2013).

From the various remains, body size metrics have been estimated for the Dmanisi hominins. They were 1.45 to 1.66 m (4 ft. 9 in. to 5 ft. 5 in.) tall and weighed 40.0 to 50.0 kg (88 to 110 lb.). The cranial capacities of the five skulls range from 546 to 730 cc, about half that of a modern human. The encephalization quotient (a measure of brain size in relation to body size) lies in the range from 2.4 to 3.13; a figure that is at the lower end of the estimates for African Homo erectus, and is more comparable to that of Homo habilis or Australopithecus (Lordkipanidze, et al., 2007; Lordkipanidze, et al., 2013).

The Dmanisi hominins display a mosaic of primitive and derived (more modern) features. Their limb proportions were similar to those of a modern human. The lower limbs and feet were essentially modern, although the feet turned slightly inwards. On the other hand, the forearm lacked what is known as humeral torsion. In modern humans, the elbow joint is typically rotated relative to the shoulder joint, so that the forearm naturally hangs with the palms facing inwards; but the Dmanisi forearm lacked this rotation, so their palms were oriented more forwards. The inward-turning feet, lack of humeral torsion, small body size and small brain size may be seen as primitive traits, sharing more in common with Homo habilis than with Homo erectus (Lieberman, 2007; Lordkipanidze, et al., 2007).

Initially assigned to African Homo erectus (Vekua, et al., 2002), the Dmanisi hominins were later put forward as a new human species, Homo georgicus (Gabunia, et al., 2002); though this proposal has since been retracted (Lordkipanidze, et al., 2013), and it has been suggested that early African Homo erectus was not only quite widespread, but also unusually variable in both body and brain size, and also less modern than sometimes supposed (Lieberman, 2007).

Two more radical (and diametrically-opposed) possibilities have recently been put forward. The first is that the various species often proposed for early African Homo (Homo habilis, Homo rudolfensis, Homo ergaster and Homo erectus) were all actually variants of the same species, and that early Homo was a single lineage which evolved over time without differentiating into multiple species. This conclusion is based on a claim that shape variation between the five Dmanisi skulls is roughly the same as that seen among the various early Homo skulls from East Africa, even though the former represents a single species and the latter are generally thought to represent several (Lordkipanidze, et al., 2013).

The second proposal (Bermúdez de Castro, et al., 2014) is that Skull 5 represents a different group of early hominins to that of the other Dmanisi remains. The lower jawbone is larger than those of others, and is said to represent a ‘large and somewhat enigmatic individual’. Its shape differs, and the differences cannot be accounted for in terms of body size or sex. It possesses a mosaic of primitive and derived features that are absent from other Dmanisi specimens. Furthermore, patterns of dental wear suggest a higher intake of fibrous and abrasive foods. It has accordingly been suggested that the jawbone is adapted to a different ecological niche to the other Dmanisi hominins, and that it represents a different species.

Although tools document a long-term human presence at Dmanisi, all the actual human remains were found in the same geological layer. This makes the ‘two species’ scenario problematic, as it implies that both species lived at about the same time. However, the stratigraphy of Dmanisi is complex, and it is possible that the fossil remains were re-deposited in the same geological layer after initially occupying sediments of different ages. It has also been claimed that the tools found at Dmanisi are consistent with the existence of two different populations.

More evidence is needed to determine just where the Dmanisi hominins fit into the broader human evolutionary picture, but it is becoming clear that the first hominin dispersal out of Africa was a far more complex process than was at one time supposed.

References:
1. Gabunia, L. et al., Earliest Pleistocene Hominid Cranial Remains from Dmanisi,Republic of Georgia: Taxonomy, Geological Setting, and Age. Science 228, 1019-1025 (2000).

2. Ferring, R. et al., Earliest human occupations at Dmanisi (Georgian Caucasus) dated to 1.85–1.78 Ma. PNAS 108 (26), 10432-10436 (2011).

3. Vekua, A. et al., A New Skull of Early Homo from Dmanisi, Georgia. Science 297, 85-89 (2002).

4. Lordkipanidze, D. et al., Postcranial evidence from early Homo from Dmanisi, Georgia. Nature 449, 305-310 (2007).

5. Lordkipanidze, D. et al., A Fourth Hominin Skull From Dmanisi, Georgia. The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology 288A, 1146–1157 (2006).

6. Lordkipanidze, D. et al., A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo. Science 342, 326-331 (2013).

7. Lordkipanidze, D. et al., The earliest toothless hominin skull. Nature 434, 717-718 (2005).

8. Lieberman, D., Homing in on early Homo. Nature 449, 291-292 (2007).

9. Gabunia, L., de Lumley, M.-A., Vekua, A., Lordkipanidze, D. & de Lumley, H., Découverte d’un nouvel hominidé à Dmanissi (Transcaucasie, Géorgie). C.R. Palévol. 1, 243–253 (2002).

10. Bermúdez de Castro, J., Martinón-Torres, M., Sier, M. & Martín-Francés, L., On the Variability of the Dmanisi Mandibles. PLoS One 9 (2), e88212 (2014).

Human footprints up to one million years old discovered at Happisburgh, Norfolk

Human footprints up to one million years old discovered at Happisburgh, Norfolk

Footprints left by small group of adults and children are oldest discovered outside Africa

Human footprints dating to between one million and 780,000 years old have been reported on the beach of the coastal village of Happisburgh, Norfolk (pronounced ‘Hazeborough’), and are the earliest-known direct evidence for the presence of humans in northern Europe. The footprints briefly emerged at low tide in May 2013, having being exposed by rough seas.

Within a fortnight, they had vanished again – but not before a team led by Nick Ashton from the British Museum had obtained plaster casts and 3d images. A total of 152 footprints were recorded, of which twelve yielded complete outlines suitable for analysis. It is thought that these twelve footprints represented five individuals ranging in height from 0.93 to 1.73 m (3 ft. 0 in. to 5 ft. 8 in.), suggesting the presence of both adults and children. It has been suggested that the Happisburgh hominins are related to Homo antecessor (‘Pioneer man’), a human species known from Sierra de Atapuerca in northern Spain during the period between 1.2 million and 800,000 years ago (Ashton, et al., 2014).

Early humans from this period are broadly categorised as Homo erectus. In Europe, Homo erectus was later replaced by the larger-brained Homo heidelbergensis, which might have been the forerunner of the Neanderthals in Europe and modern humans in Africa.

The Happisburgh footprints are the earliest direct evidence for humans in Britain, but tools used by these first Britons have been coming to light since 2005, when 700,000-year-old flint artefacts were reported from Pakefield in Suffolk (Parfitt, et al., 2005). In 2010 even earlier flint artefacts were reported from Happisburgh, estimated to be at least 780,000 years old, and probably older (Parfitt, et al., 2010). Previously, the earliest uncontested evidence for humans in northern Europe dated to no earlier than around 500,000 years ago (Parfitt, et al., 2005).

Analysis of animal remains suggests the Happisburgh people occupied the edges of forests at what was then an estuary of the River Thames, and lived towards the end of a warm interglacial period. It is not certain when the interglacial occurred, but there were warm periods from 866,000 to 814,000 years ago, and from 970,000 to 936,000 years ago (Parfitt, et al., 2010).

Britain was at this time connected to the mainland and lying on the southern edge of the forests of northwestern Europe. The climate was similar to that of today and while comfortable by British standards, it would have been chilly for those used to a Mediterranean or African climates. It remains unclear whether expansion into northern latitudes with lower winter temperatures required human physical adaptation, seasonal migration or developments in technology such as hunting, clothing, the use of shelters and the control of fire (Parfitt, et al., 2005; Parfitt, et al., 2010; Roberts & Grun, 2010).

References:
1. Ashton, N. et al., Hominin Footprints from Early Pleistocene Deposits at Happisburgh, UK. PLoS One 9 (2) (2014).
2. Parfitt, S. et al., The earliest record of human activity in northern Europe. Nature 438, 1008-1012 (2005).
3. Parfitt, S. et al., Early Pleistocene human occupation at the edge of the boreal zone in northwest Europe. Nature 466, 229-233 (2010).
4. Roberts, A. & Grun, R., Early human northerners. Nature 466, 189 (2010).

Reassessment of 1950s fossil find provides early evidence for hominins in Central Africa

2.0 to 2.6-million-year-old tooth is from australopithecine or early human.

A reassessment of a fossil tooth from an old archaeological collection suggests that early hominins had extended their range to the western branch of Africa’s Great Rift Valley by no later than two million years ago. Since the late 1950s, large numbers of early hominin fossils have been found in the eastern branch of the Great Rift Valley, which is often described as the Cradle of Humanity. However, up until now, none have been found in the western branch.

Ishango 11 is an archaeological site in the Democratic Republic of Congo; it is located alongside the Semliki River, in the western branch of the Great Rift Valley. In the 1950s, the site was excavated by the Belgian geologist Jean de Heinzelin, who recovered numerous fossil human and animal remains, together with stone and bone artefacts. The assemblage dates mainly to the early part of the African Late Stone Age, from 25,000 to 19,000 years ago. It is housed in the Department of Anthropology and Prehistory at the Royal Belgian Institute of Science, Brussels.

However, the finds also included an upper left first molar that did not appear to be from such a recent period. Known as #Ish25, doubts were cast on its affinities to modern humans as long ago as 1958. A recent study has shown that #Ish25 probably originated from an earlier geological layer than the other fossils and artefacts. Animal remains associated with this layer suggest that it dates to between 2.6 and 2.0 million years ago. These dates make #Ish25 the earliest fossil hominin find from the western branch of the Great Rift Valley (though not the earliest from Central Africa, as much earlier hominins are known from Chad).

Various statistical analyses of the shape and size of #Ish25 suggest closer affinities to hominins from the Late Pliocene/Early Pleistocene than those from the Middle Pleistocene to Recent epochs. The exact hominin species to which the tooth belongs cannot be determined with certainty; Australopithecus africanus, Paranthropus robustus and early Homo are all possibilities.

The western Great Rift Valley underwent episodes of climate change 3.0, 2.6 and 1.8 million years ago; these led to the partial replacement of Congo flora and fauna with those typical of the East Africa; the latter are adapted to more open grassland conditions. The #Ish25 findings suggest that these conditions led to a dispersal of hominins into the region from either East Africa or South Africa.

The study also demonstrates how valuable knowledge can often be gained by applying modern techniques to old anthropological collections.

 Reference:

1. Crevecoeur, I. et al., First Early Hominin from Central Africa (Ishango, Democratic Republic of Congo). PLoS One 9 (1), e84652 (2014).

530,000 years old Spanish hominins were closely related to Denisovans

Mystery of Sima de los Huesos ‘proto-Neanderthal’ mitochondrial genome.

Sima de los Huesos – ‘the Pit of Bones’ –  is a small muddy chamber lying at the bottom of a 13 m (43 ft.) chimney, lying deep within the Cueva Mayor system of caves in the Sierra de Atapuerca of northern Spain. Human remains dating to the Middle Pleistocene were first discovered there in 1976, and systematic excavation has been in progress since 1984. Investigation of the cramped site has proved to be long and difficult – it is located more than 500 m (⅓ mile) from the mouth of the Cueva Mayor and is hard to access, necessitating at times crawling on the stomach. To date, over 2,000 fragmentary hominin fossils have been recovered, including three skulls. In total, the remains are thought to represent at least 32 individuals of both sexes. Many of the remains are of adolescents and young adults, though, the pattern of mortality was probably quite normal for the time, and a similar peak in adolescence has been found at a site at Krapina in Croatia. There is no evidence for violence and the deaths could simply be the result of hunting accidents and childbirth complications. Hunting accidents were probably not uncommon among inexperienced young hunters and women likely fell pregnant soon after commencing menstruation (Pettitt, 2005).
Uranium-series dating suggests that the remains are least 530,000 years old (Bischoff, et al., 2007), and display a mixture of Homo heidelbergensis and Neanderthal features. For this reason, the  Sima de los Huesos hominins are often described as ‘proto-Neanderthal’ (Klein, 2009), although it has also been argued that they were a species distinct from both Neanderthals and Homo heidelbergensis rather than an intermediate between the two (Tattersall, 2002).

In a newly-published study, researchers at the Max Planck Institute for Evolutionary Anthropology have reported the sequencing of the almost-complete mitochondrial genome of one of the Sima de los Huesos hominins. The mitochondrial DNA was extracted from a thigh bone. An estimated age of 400,000 years was obtained by comparison with other, younger ancient DNA sequences dated by direct means. This is rather more recent than the uranium series dates for the site, but still by far the oldest hominin DNA ever recovered. The previous record-holder was no more than 100,000 years old.

Given the geographical location of the Sima de los Huesos and the apparent affinities of the hominins to Neanderthals, it was expected that the material would show affinity to genetic sequences obtained from later Neanderthal remains. Instead, it more closely resembled ancestral Denisovan mitochondrial DNA (Meyer, et al., 2013).

The Denisovan genome, first identified Denisova Cave in the Altai Mountains of southern Siberia, has been found in the modern populations of New Guinea and Island Southeast Asia, implying that the Denisovan range had once extended from the deciduous forests of Siberia to the tropics. This is a wider ecological and geographic region than any other hominin species, with the exception of modern humans (Reich, et al., 2011); but could their range have extended all the way to Europe?

It is likelier that the Sima de los Huesos hominins were the common ancestors of both the Neanderthals and the Denisovans. Mitochondrial lineages originally present in both lineages subsequently disappeared from the Neanderthals, but persisted in the Denisovans. They could have been lost from the Neanderthal line as a result of a population bottleneck of the type known to have affected later Neanderthal populations (Dalén, et al., 2012).

References:

1. Pettitt, P., in The Human Past, edited by Scarre, C. (Thames & Hudson, London, 2005), pp. 124-173.

2. Bischoff, J. et al., High-resolution U-series dates from the Sima de los Huesos hominids yields 600 +/-66 kyrs: implications for the evolution of the early Neanderthal lineage. Journal of Archaeological Science 34, 763-770 (2007).

3. Klein, R., The Human Career, 3rd ed. (University of Chicago Press, Chicago, IL, 2009).

4. Tattersall, I., in The Speciation of Modern Homo sapiens, edited by Crow, T. (Oxford University Press, Oxford, 2002), pp. 49-59.

5. Meyer, M. et al., A mitochondrial genome sequence of a hominin from Sima de los Huesos. Nature (Published online) (2013).

6. Reich, D. et al., Denisova Admixture and the First Modern Human Dispersals into Southeast Asia and Oceania. American Journal of Human Genetics 89, 1-13 (2011).

7. Dalén, L. et al., Partial genetic turnover in neandertals: continuity in the east and population replacement in the west. Molecular Biology and Evolution 29 (8), 1893-1897 (2012).

The Denisovans

In 2008, a distal manual phalanx of from a hominin little finger was recovered from Denisova Cave in the Altai Mountains of southern Siberia. The cave is named for a hermit called Dionisij (Denis) who is supposed to have lived there in eighteenth century, but if this is true he was only the latest in a long line of inhabitants. In April 2010, it was reported that the phalanx had belonged to a hitherto-unknown human species (Krause, et al., 2010).

The small bone was dated by stratigraphic methods and found to be in the region of 30,000 to 48,000 years old. It was believed to have belonged to a child aged between five and seven years old, but other than that no morphological classification could be made. Due to the cool, dry climate, it proved to be possible to extract DNA from the bone, isolate mtDNA fragments, and sequence the entire mitochondrial genome. As we inherit our mtDNA solely from our mothers, this led to the find being dubbed X Woman, despite being a juvenile of unknown gender.

At the time in question, Neanderthals, identified as such by their mtDNA, were living less than 100 km (60 miles) away. The presence of an Upper Palaeolithic industry at Siberian sites such as Kara-Bom and Denisova itself has been taken as evidence for the appearance of modern humans in the Altai before 40,000 years ago. The expectation, therefore, was that the mitochondrial DNA from the bone would match that of either Neanderthals or modern humans, but neither turned out to be the case. Instead, sequencing revealed that X Woman had last shared a common ancestor with Neanderthals and modern humans about a million years ago.

X Woman clearly wasn’t a Neanderthal or a modern human, but what was she (if indeed she was a ‘she’)? One possibility was Homo heidelbergensis, the presumptive common ancestor of the Neanderthals and modern humans, but this species probably appeared no earlier than 600,000 years ago, and was too recent to be associated with X Woman’s ancestors. On the other hand, one million years ago was too recent for X Woman to be a late-surviving descendant of the first wave of Homo erectus to reach Southeast Asia and China.

Towards the end of 2010, it was reported that X Woman’s nuclear genome had been sequenced (Reich, et al., 2010). It turned out that X-Woman lacked a Y-chromosome and therefore was indeed female. The discovery of an upper molar tooth from a young adult was also reported. The sequencing of mtDNA from the tooth confirmed that it belonged to a different individual to the phalanx. For this reason, the term ‘X-Woman’ was dropped in favour of ‘Denisovan’.

The nuclear data allowed more detailed estimates to be made regarding the relatedness of Denisovans, Neanderthals and modern humans. It was found that the Denisovans diverged from Neanderthals 640,000 years ago, and from present-day Africans 804,000 years ago. This meant that the Denisovans were more closely related to the Neanderthals than to modern humans, and may thus be considered a sister group of the former. The most remarkable finding was that 4.8 percent of the nuclear genome of present-day New Guineans derives from Denisovans, greater than the Neanderthal contribution of 2.5 percent (Reich, et al., 2010). The implication was that the Denisovan range had once extended from the deciduous forests of Siberia to the tropics. This is a wider ecological and geographic region than any other hominin with the exception of modern humans (Reich, et al., 2011). Overall, the data was consistent with a scenario in which modern humans, on leaving Africa, interbred with Neanderthals and then, at some subsequent point, the ancestors of present-day New Guineans interbred with Denisovans.

Follow-up studies confirmed the presence of Denisovan genetic material in some other modern populations of island Southeast Asia, and also in Aboriginal Australians, Fijians and Polynesians. Significantly, though, it was absent from mainland populations. The only logical explanation is that the present-day population of Mainland Southeast Asia are descended from a second group of migrants that arrived after the Denisovans had become extinct (Reich, et al., 2011; Skoglund & Jakobsson, 2011; Meyer, et al., 2012).

Interbreeding with Denisovans might have boosted the immune systems of some modern populations. The human leucocyte antigen (HLA) helps the immune system to recognise and combat pathogens. There are three genes known as HLA-A, HLA-B and HLA-C, and it believed that a number of variants of these genes are of Denisovan origin. These variants could have conferred immunity to pathogens to which the incoming modern population had not been previously exposed, and given a survival to those acquiring them from the Denisovans. It is possible that the modern immune system has thus been shaped by ‘importing’ advantageous genes from archaic populations throughout Eurasia (Abi-Rached, et al., 2011).

It has been suggested, on the basis of allele comparison, that the Denisovans were dark-skinned, with brown eyes and hair (Meyer, et al., 2012). Other than that, and beyond their genetic impact on modern populations, we still know very little about them. The Middle Pleistocene fossil record of Southeast and East Asia is very sparse and the Denisova tooth, probably a third or possibly second left upper molar, fails to support a connection with any of the few remains that have been found. The tooth is fairly large, lying within the size range of Homo erectus and Homo habilis. It is above the size range typical for Neanderthals, early modern humans, and the very few third upper molars that have been recovered from other late archaic hominins in the region. The tooth shares no derived morphological features with Neanderthals or modern humans, hinting at the distinctiveness of the Denisovans (Reich, et al., 2010). On the other hand, the report failed to note that some early modern human teeth are also very large, such as those associated with the 35,000-year-old lower jawbone from Peştera cu Oase in Romania (Trinkaus, et al., 2003; Trinkaus, et al., 2003). Size alone probably does not tell us very much (Hawks, 2010).

Recently, it has been suggested that the Denisovans interbred with yet another archaic human species. Given that the Denisovans and Neanderthals diverged from one another after they diverged from modern humans, one would expect the two species to be equally genetically distinct from our own species. However, this is not the case; the Denisovans are more genetically distinct than the Neanderthals. It turns out that scattered fragments amounting to around one percent of their genome is much older than the rest of it. This is best explained by the Denisovans interbreeding with an as yet unidentified human species, possibly Homo heidelbergensis or Homo erectus. We do not yet have genetic material from either species, so this cannot be confirmed  (Marshall, 2013).

At all events, it is now clear that the view of modern humans entirely replacing archaic populations is not correct, either in or out of Africa. There is certainly an element of truth to the multiregional model. It is, however, only an element. The range of morphological variation between modern and archaic humans is greater than that in any existing primate species. We should not think of Denisovans and Neanderthals as simply variant forms of Homo sapiens (Stringer, 2012).

References:
Abi-Rached, L. et al., 2011. The Shaping of Modern Human Immune Systems by Multiregional Admixture with Archaic Humans. Science, 25 August.
Hawks, J., 2010. The Denisova genome FAQ. [Online]
Available at: http://johnhawks.net/weblog/reviews/neandertals/neandertal_dna/denisova-nuclear-genome-reich-2010.html
[Accessed 14 November 2011].
Krause, J. et al., 2010. The complete mitochondrial DNA genome of an unknown hominin from southern Siberia. Nature, 8 April, Volume 464, pp. 894-897.
Marshall, M., 2013. Mystery human species emerges from Denisovan genome. [Online]
Available at: http://www.newscientist.com/article/dn24603-mystery-human-species-emerges-from-denisovan-genome.html#.Uo5FQMTk-m5
[Accessed 21 November 2013].
Meyer, M. et al., 2012. A High-Coverage Genome Sequence from an Archaic Denisovan Individual. Science, 30 August.
Reich, D. et al., 2010. Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, 23/30 December, Volume 468, pp. 1053-1060.
Reich, D. et al., 2011. Denisova Admixture and the First Modern Human Dispersals into Southeast Asia and Oceania. The American Journal of Human Genetics, 7 October, Volume 89, pp. 1-13.
Skoglund, P. & Jakobsson, M., 2011. Archaic human ancestry in East Asia. PNAS.
 
Stringer, C., 2012. What makes a modern human. Nature, 3 May, Volume 485, pp. 33-35.
Trinkaus, E. et al., 2003. Early modern human cranial remains from the Pestera cu Oase, Romania. Journal of Human Evolution, Volume 45, p. 245–253.
Trinkaus, E. et al., 2003. An early modern human from Peştera cu Oase, Romania. PNAS, 30 September, 100(20), p. 11231–11236.

Australopithecus sediba: a possible human ancestor

Australopithecus sediba is a possible human ancestor discovered in South Africa in 2010. The discovery was made at Malapa, a fossil-bearing cave located about 15 km (9.3 miles) NE of the well-known South African hominid-bearing sites of Sterkfontein and Swartkrans and about 45 km (28 miles) NNW of Johannesburg  (Berger, et al., 2010). It is situated within the Cradle of Humankind World Heritage Site. The recovery effort was led by Lee Berger, a paleoanthropologist at the University of the Witwatersrand, Johannesburg. The find was made when Matthew, Lee’s 9 year old son, discovered hominin collar bone embedded in a rock (Balter, 2010).

The find comprised two extremely well-preserved partial skeletons that were initially thought be somewhere between 1.78 and 1.95 million years old (Dirks, et al., 2010), later revised to 1.977 million years (Pickering, et al., 2011). These belonged to a juvenile male (MH1) aged 12 to 13 at time of his death and an adult female (MH2) (Berger, et al., 2010). They were found together buried in alluvial sediment, deep within the Malapa cave, part of an eroded cave system. Also found were the remains of wildcats, hyenas and a number of other mammals. On the ground above the cave are a number of ‘death traps’, or long vertical shafts. The smell of damp issuing from the shaft would have attracted animals. The pair – possibly mother and son – may have fallen to their deaths while searching for water. The sediments imply that subsequent high-volume water inflow, perhaps the result of a large storm, caused a debris flow. This carried the still partially articulated bodies deeper into the cave, to deposit them along a subterranean stream (Dirks, et al., 2010).

MH1 and MH2 were assigned to a new australopithecine species, Australopithecus sediba. The word ‘sediba’ means ‘fountain’ or ‘wellspring’ in the Sotho language. The more complete cranium of the juvenile MH1 has a capacity of 420cc, probably at least 95 percent of adult size. The remains share numerous similarities with Australopithecus africanus in the cranial vault, facial skeleton, lower jawbone and teeth, but there are also significant differences in the cranial, dental and postcranial anatomy. Homo-like features include smaller molars and premolars and less pronounced cheekbones. Certain features of the pelvis are similar to those seen in Homo erectus. The lower-to-upper limb bone proportions are also similar to those of later Homo, and unlike the more apelike proportions of Homo habilis. The anatomy of its hip, knees and ankles suggest that Australopithecus sediba was a habitual biped. Overall, it was claimed that Australopithecus sediba shares more derived features with early Homo than it does with other australopithecines. However, Berger was reluctant to place the new discovery within Homo, preferring to classify it as an australopithecine (Berger, et al., 2010)

The initial announcement of Australopithecus sediba attracted extensive news coverage, but not everybody was convinced by the claims made for it. Australian anthropologist Darren Curnoe was reported (MacKnight, 2010) as claiming that Australopithecus sediba is in the wrong place at the wrong time to be a human ancestor. He noted that Homo habilis emerged in East Africa well before the time of Australopithecus sediba. However, his argument does assume that Homo habilis is indeed an early human.  This may not be the case. It is also possible that at least some of Australopithecus sediba’s humanlike features could have evolved independently, and may not necessarily imply shared ancestry (Wood & Harrison, 2011).

Nevertheless, subsequent studies do support Berger’s initial claims. They suggest that aspects of the brain, dental morphology, pelvis, hand and foot of Australopithecus sediba could be interpreted as incipient humanlike features. A virtual endocast of the brain, obtained from synchrotron scanning, revealed an australopithecine-like size and pattern of convolutions. However, the orbitofrontal region showed possible development towards a humanlike frontal lobe. Possibly some neural reorganization of the brain preceded its later size increase in early humans (Carlson, et al., 2011).

The teeth of MH1 and MH2 are a mosaic of primitive and derived traits. Cladistic analysis of 22 dental traits suggest that Australopithecus sediba was a sister species of Australopithecus africanus(i.e. the two shared a common ancestor) and that the two were further evolved in the direction of Homo than were the australopithecines from East Africa (Irish, Guatelli-Steinberg, Legge, de Ruiter, & Berger, 2013). The lower jawbone morphology reduced dentition (especially canines and premolars) confirms that Australopithecus sediba was a distinct species to Australopithecus africanusand not merely a late-surviving form of that species (de Ruiter, et al., 2013).

The upper ribcage of Australopithecus sediba exhibits an apelike funnel shape, unlike the barrel shape associated with Homo. The funnel shape, as noted above, may be an adaptation to under-branch suspensory locomotion. The barrel shape may be associated with the increased chest volume and lung function necessary for endurance walking and running. The lower thorax, however, appears less flared than that of apes and more closely approximates the morphology found in humans (Schmid, et al., 2013). The spine is long and flexible, a form that has more in common with early Homo than with other australopithecines. Curvature of the lower spine is a hallmark of walking upright (Williams, Ostrofsky, Frater, Churchill, Schmid, & Berger, 2013).

The upper limbs were still predominantly apelike, suggesting the retention of substantial climbing and suspensory abilities (Churchill, et al., 2013). The hands show a mixture of australopithecine and human features. They retained adaptations for tree-climbing, but there was also a long thumb and shorter fingers. These suggest precision gripping of the type associated with tool manufacture and use (Kivell, Kibii, Churchill, Schmid, & Berger, 2011).

The pelvis and foot presented a mosaic of apelike and humanlike characteristics. These suggested adaptations to a more efficient (albeit not entirely human) form of bipedalism, at the expense of reduced arboreal efficiency (Kibii, et al., 2011; Zipfel, DeSilva, Kidd, Carlson, Churchill, & Berger, 2011). The bipedal mechanics differed from those reconstructed for other australopithecines, suggesting that there may have been several forms of hominin bipedalism at this time. The adaptations of Australopithecus sediba may have enabled it to both walk and climb reasonably well and thus survive in a dual arboreal/terrestrial world (DeSilva, et al., 2013).

References:
Balter, M. (2010, April 9). Candidate Human Ancestor From South Africa Sparks Praise and Debate. Science, 328, 154-155.
Berger, L., de Ruiter, D., Churchill, S., Schmid, P., Carlson, K., Dirks, P., et al. (2010, April 9). Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa. Science, 328, 195-204.
Carlson, K., Stout, D., Jashashvili, T., de Ruiter, D., Tafforeau, P., Carlson, K., et al. (2011, September 9). The Endocast of MH1, Australopithecus sediba. Science, 333, 1402-1407.
Churchill, S., Holliday, T., Carlson, K., Jashashvili, T., Macias, M., Mathews, S., et al. (2013, April 12). The Upper Limb of Australopithecus sediba. Science, 340.
de Ruiter, D., DeWitt, T., Carlson, K., Brophy, J., Schroeder, L., Ackermann, R., et al. (2013, April 12). Mandibular Remains Support Taxonomic Validity of Australopithecus sediba. Science, 340.
DeSilva, J., Holt, K., Churchill, S., Carlson, K., Walker, C., Zipfel, B., et al. (2013). The Lower Limb and Mechanics of Walking in Australopithecus sediba. Science, 340.
Dirks, P., Kibii, J., Kuhn, B., Steininger, C., Churchill, S., Kramers, J., et al. (2010, April 9). Geological Setting and Age of Australopithecus sediba from Southern Africa. Science, 328, 205-208.
Irish, J., Guatelli-Steinberg, D., Legge, S., de Ruiter, D., & Berger, L. (2013, April 12). Dental Morphology and the Phylogenetic “Place” of Australopithecus sediba. Science(340).
Kibii, J., Churchill, S., Schmid, P., Carlson, K., Reed, M., de Ruiter, D., et al. (2011, September 9). A Partial Pelvis of Australopithecus sediba. Science, 333, 1407-1411.
Kivell, T., Kibii, J., Churchill, S., Schmid, P., & Berger, L. (2011, September 9). Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities. Science, 333, 1411-1417.
MacKnight, H. (2010, April 8). Experts reject new human species theory. Retrieved September 12, 2012, from Independent: http://www.independent.co.uk/news/science/experts-reject-new-human-species-theory-1939512.html
Pickering, R., Dirks, P., Jinnah, Z., de Ruiter, D., Churchil, S., Herries, A., et al. (2011, September 9). Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo. Science, 333, 1421-1423.
Schmid, P., Churchill, S., Nalla, S., Weissen, E., Carlson, K., de Ruiter, D., et al. (2013). Mosaic Morphology in the Thorax of Australopithecus sediba. Science, 340.
Williams, S., Ostrofsky, K., Frater, N., Churchill, S., Schmid, P., & Berger, L. (2013, April 12). The Vertebral Column of Australopithecus sediba. Science, 340.
Wood, B., & Harrison, T. (2011, February 17). The evolutionary context of the first hominins. Nature, 470, 347-352.
Zipfel, B., DeSilva, J., Kidd, R., Carlson, K., Churchill, S., & Berger, L. (2011, September 9). The Foot and Ankle of Australopithecus sediba. Science, 333, 1417-1420.