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.

Advertisements

Author: prehistorian

Prehistorian & author

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s