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.

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.