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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Toba Catastrophe Theory

74,000 years ago, the Earth experienced the largest volcanic event of the last two million years when a supervolcano beneath Lake Toba in northern Sumatra erupted with a Volcanic Explosivity Index intensity of 8 (“Ultra-Plinean”), ejecting 2,800 km3 of magma, with around 800 km3 of this falling as ash. In comparison, Krakatoa, Tambora and Mount Pinatubo would have seemed like firecrackers. Much of the ash was blown north-west by the wind and the Indian subcontinent and part of the Malay Peninsula were blanketed in ash. Now known as Youngest Toba Tuff (YTT), these deposits ranged in depth from 15cm to as much as 6m at one site in Central India.

The effects of the explosion would have been to cause a “volcanic winter”, during which temperatures worldwide may have fallen by 3-5 degrees Celsius for several years. In 1998, Stanley H Ambrose suggested that the eruption caused a bottleneck in human populations (Ambrose, 1998). Geneticists Lynn Jorde and Henry Harpending believe the world population of anatomically modern humans fell to as few as 5,000-10,000 individuals.

Could this really have happened; could Homo sapiens really have teetered on the brink of extinction?

In the aftermath of the eruption, conditions for life would have become very harsh and the Indian subcontinent would certainly not have been the healthiest of places to be! But were any anatomically modern humans there? The first evidence of Homo sapiens outside of Africa are the 110-90,000 year old fossil remains found in the Levant. This migration seems to have petered out; the migrants either died without issue or (less likely) returned to Africa. The migration(s) believed to have given rise to the world’s present-day non-African population occurred much later. Estimates vary as to the timing, with some authorities claiming it was as long ago as 80,000 years ago but most opting for between 50-60,000 years ago, long after the Toba eruption.

Stephen Oppenheimer is among those who do believe that modern humans were already in India and Malaysia when Toba erupted. He believes the migrants left the Horn of Africa 80,000 years ago.

Oppenheimer claims the Toba eruption resulted in the extinction of human life in India, leaving a “genetic furrow” that is visible in Asia’s genetic record to this day. This arose as the subcontinent was repopulated by settlers from both East and West Asia. Although descended from the same root lines of the single exodus, Indian maternal branch genetic (mtDNA) lines are completely different from those of the Far East and mostly different from those in the West.

Oppenheimer also draws on archaeological evidence from Kota Tampan in the Lenggong Valley, Malaysia where stone tools were found in the 1960s. These were covered by volcanic ash, now known to be from the Toba eruption. The large pebble-tools, fashioned on one side only were thought to be the work of an earlier human species but in the absence of fossil remains the matter could not be settled one way or the other.

Work by Prof. Zuraina Majid, of the University of Science in Penang suggests that the local pebble-tool culture may have persisted continuously right up until only 7,000 years ago. If so, the possible implication is that the earlier pebble-tools were actually made by modern humans. Support for this view came in 1990 with the discovery of Perak Man, a 10,000 year old anatomically-modern human found in the same context as the pebble tools (Oppenheimer, 2003).

But not everybody accepts the bottleneck theory. Gathorne-Hardy & Harcourt-Smith (2003) point out that if Toba has caused a bottleneck in the human population, it would have also affected other species, especially other, more environmentally sensitive taxa with more specialised ecological requirements. These would have been expected to suffer at least a similar population crash leading to many becoming extinct. But there is no evidence for mammal extinction associated with Toba.

In 2007 dramatic evidence was presented that suggested that not only were modern humans in India at the time of the Toba eruption, but that they survived the catastrophe. A team led by Michael Petraglia of the University of Cambridge recovered stone artefacts from both above and below the 2.55m thick ash deposit near Jwalapuram, in the Jurreru River valley of southern India.

There appears to be a strong element of technological continuity between the two sets of artefacts and together with the presence of faceted unidirectional and bidirectional bladelike core technology, they suggest closer affinities to African Middle Stone Age traditions such as Howieson’s Poort than to those of the contemporaneous Eurasian Middle Paleolithic. The latter are typically based on discoidal and Levallois techniques. This, together with the behavioural flexibility needed to survive the catastrophe, suggests that modern humans were already in India at the time of the eruption (Petraglia et al, 2007).

In addition, there are recent genetic studies that do support the presence of modern humans in India 74,000 years ago (Kivisild et al, 2003; Metspalu et al, 2004).

Petraglia’s interpretations are also a problem for the Kota Tampan pebble tools being made by modern humans. If the Indian settlers were using a technology derived from the African MSA, why were their counterparts in Malaysia using far more primitive tools?

Human remains found in the context of ash from Toba are the one thing missing; the matter cannot be considered to be settled until such time as these come to light; but on the balance of probabilities it does seem likely that the effects of the Toba eruption were not as deleterious as some have supposed.

References:

Ambrose S H (1998): Late Pleistocene human population bottlenecks, volcanic winter, and differentiation of modern humans, Journal of Human Evolution 34 (6): 623–651

Gathorne-Hardy F.J. & Harcourt-Smith W.E.H. (2003): The super-eruption of Toba, did it cause a human bottleneck? Journal of Human Evolution 45 (2003) 227–230

Kivisild T, Rootsi S, Metspalu M, Mastani S, Kaldma K, Parik J, Metspalu E, Adojaan M, Tolk H-V, Stepanov V, Golge M, Usanga E, Papiha S S, Cinnioglu C, King R, Cavalli-Sforza L, Underhill P A & Villems R (2003): The Genetic Heritage of the Earliest Settlers Persists Both in Indian Tribal and Caste Populations, Am. J. Hum. Genet. 72:313-332.

Metspalu M, Kivisild T, Metspalu E, Parik J, Hudjashov G, Kaldma K, Serk P, Karmin M, Behar D M, Gilbert M T P, Endicott P, Mastana S, Papiha S S, Skorecki K, Torrioni A & Villems R (2004): Most of the extant mtDNA boundaries in South and Southwest Asia were likely shaped during the initial settlement of Eurasia by anatomically modern humans, BMC Genet. 2004; 5:26

Oppenheimer S (2003): Out of Eden, Constable.

Michael Petraglia, Ravi Korisettar, Nicole Boivin, Christopher Clarkson, Peter Ditchfield, Sacha Jones, Jinu Koshy, Marta Mirazón Lahr, Clive Oppenheimer, David Pyle, Richard Roberts, Jean-Luc Schwenninger, Lee Arnold, Kevin White (2007): Middle Paleolithic Assemblages from the Indian Subcontinent Before and After the Toba Super-Eruption, Science 317, 114.

© Christopher Seddon 2009

Modern humans & Neanderthals in the Levant

The first evidence for modern humans leaving Africa comes from the Levant, where the caves of Mugharet es-Skhul and Jebel Qafzeh (Israel) have yielded the remains of over 20 individuals, many of whom appeared to have been intentionally buried. The remains have been dated to between 110,000-90,000 years ago on the basis of ESR and luminescence dating. Though possessing some archaic features such as robustness, they are essentially modern, anatomically lying within the range of Homo sapiens.

However Neanderthal burials are known from much later at Kebara, Amud and Tabun (Israel), dating to between 60,000-50,000 years ago; and Shanidar Cave (Iraq) and Dederiyeh Cave (Syria) dating to as late as 45,000 years ago: this suggests a later re-occupation of the region by Neanderthals.

Modern humans had returned to the region by 35,000 years ago, and possibly as early as 45,000 years ago.

The likeliest explanation is that the ranges of modern humans and Neanderthals fluctuated back and forth in accordance with climate change. Whenever the climate was warm, the region would be occupied by modern humans; when it was cool it would be occupied by Neanderthals. Faunal evidence shows that in cooler times Palearctic fauna spread down from Europe, but when the climate warmed Afrotropical fauna would move in and replace the Palearctic. The two human species were merely “going with the flow” like any other animal species.

The lithic technology employed by both human species was, up until 50,000 years ago, very similar, being fairly typical Middle Palaeolithic/MSA and it has been described as Levalloiso-Mousterian, dominated by the Levallois reduction technique.

Lieberman & Shea (1994) suggest that hunting strategies between the two species did in fact differ. Analysis of seasonally-deposited cementum in the teeth of mountain gazelle and other ungulates indicates that the Neanderthals hunted these animals over both dry and wet seasons, but modern humans only hunted them during the dry season. Analysis of the lithic hunting technology further suggests more extensive hunting by the Neanderthals. Far more points are found at Neanderthal sites; these show greater signs of wear; and raw materials were procured only a short distance away from each site.

Lieberman & Shea interpret this to mean that the Neanderthals practiced a locally-intensive “radiating mobility” strategy. Their activities were primarily organized from large general-purpose sites, with more specialized or seasonal activities being carried out at smaller sites on the periphery of the main one.

By contrast, the modern humans practiced a seasonal-based “circulating mobility” strategy, living at different sites at different times of the year to facilitate exploitation of seasonal resources.

Both strategies have their pros and cons. The “radiating mobility” strategy permits an increased capacity for storage and investment in material culture (site facilities and hard-to-transport items). The main problem is that resources around the central camp become depleted due to year-long exploitation and a law of diminishing returns begins to effect hunting and foraging activities.

The “circulating mobility” strategy avoids these problems by ability to relocate to sites near periodically-abundant resources, which will then have a year to recover after the group have moved on to the next site. The drawbacks are that the group has to do without items that cannot readily be moved from site to site.

These differing strategies may highlight behavioural differences between modern humans and Neanderthals, with the “circulating mobility” strategy arising from modern human behaviour (which it is inferred the Neanderthals were incapable of). Another possibility is that the heavily-built Neanderthals were not physically adapted for a highly-mobile lifestyle.

The final, conclusive re-occupation of the Levant by modern humans, followed by their expansion into Europe has been taken to support the “Big bang” theory of behavioural modernity arising only 50,000 years ago, after which modern humans were able to overcome the “climate barrier” and move into Europe. This theory is however predicated on a modern human migration out of Africa proceeding via the Levant rather than taking a southern route across the Red Sea, a possibility rejected by some authorities such as Stephen Oppenheimer (Oppenheimer, 2003).

References:

Daniel E. Lieberman and John J. Shea (1994): Behavioral Differences between Archaic and Modern Humans in the Levantine Mousterian, American Anthropological Association.

Oppenheimer S (2003): “Out of Eden”, Constable.

© Christopher Seddon 2009

Omo remains (Homo sapiens)

The Omo I (Kibish) and Omo II (Kibish) remains are currently the oldest-known fossils of anatomically-modern humans (Homo sapiens). They were recovered by Richard Leakey in 1967 from the base of Member I of the Kibish Formation near the Omo River in south-western Ethiopia. Both Omo I and Omo II comprise a braincase and some postcranial material. Omo I fully modern in appearance; Omo II is slightly more primitive with a long, low cranium. However both are believed to be about the same age.

Originally believed to be around 130,000 years old, the Omo remains have recently been assigned a date of 195,000 +/- 5,000 years old, based on argon-40/argon-39 dating of volcanic tuffs (ash) found within Member I at levels. This makes them substantially older than the Herto remains discovered in 1997, previously thought to be the earliest-known fossil remains of modern humans. What is curious is that the 156,000 year old Herto remains, despite being around 40,000 years more recent than the Omo remains, retain more primitive features and were originally assigned their own subspecies, Homo sapiens idaltu.

Reference:

McDougall I, Brown FH & Fleagle JG (2005): Stratigraphic placement and
age of modern humans from Kibish, Ethiopia, Nature 734 Vol 433 17 Feb 2005.

© Christopher Seddon 2008