Monday, 29 January 2018

Evolution of brain shape - and why babies are not better developed at birth

Brain shape of present-day human (left) and a Neanderthal (right)
From Neubauer, Hublin and Gunz (OA here) CC BY-NC
The difference between Neanderthals and modern humans lies not so much in brain size as in brain shape. In modern humans, the brain changes from an elongated to a more globular shape. This occurs mainly after birth (OA here). 

Now a remarkable paper in Science Advances (OA here) shows evolution of brain shape was a gradual process in Homo sapiens. The earliest modern humans from North and East Africa (300,000 to 200,000 years ago) had much more elongated brains.

Brain shape comparable witrh modern humans is first seen in fossil skulls that are 35,000 to 10,000 years old. These are associated with the more advanced culture of the Upper Paleolithic, i.e. with behavioural modernity.

Conveniently, there is a group of fossils from 130,000 to 100,000 years ago with intermediate brain shapes. They include some from the Levant mentioned in my last post (here). It has been suggested that they represent an early dispersal from Africa that reached far into Asia and persisted for a long period of time. Perhaps they were supplanted by a second wave of humans with more globular brains and a more advanced culture.

Quite a lot has been written about the constraints on human childbirth consequent on evolution of a larger brain (e.g. OA here and here). Changing the shape of the brain and skull could have complicated matters further. Maybe that is why most of the rounding up of the brain, which includes rapid growth of the cerebellum, occurs in the first months of life. The other region that rounds up is the parietal area. It is thought that this reflects changes in "an important hub of brain organization" called the precuneus.

Most mammals with a long gestation and a single young have newborns that are well developed (previous post). Babies need extensíve parental care leading Portmann to describe humans as "secondarily altricial." Maybe the postnatal growth of critical brain areas, reflected in a more globular brain, is part of the explanation for this conundrum. 

P.S. Right now there is a great deal of buzz about a modern human fossil from Israel dated to 177,000 years ago (OA here). As there is no cranium it cannot yield data on brain size.

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