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.

Did humans reach Asia earlier than we think?

Map of early human migration

NordNordWest (public domain)

An insightful perspective in Nature Ecology and Evolution (abstract here) questions if there was just one successful dispersal of Homo sapiens from Africa to Asia. The current consensus - the Out of Africa Hypothesis (here) - is that modern humans left Africa some 65-55 thousand years ago and rapidly spread around the globe (see map). 

Stone hand axe from Wadi Dabsa, Saudi Arabia

From Foulds et al. (here)

An earlier migration from Africa to the Levant is supported by fossils and increasingly by archeological evidence, including from recent digs in the Arabian peninsular (here). Hitherto this exodus has been regarded as short lived and restricted in geographical extent.

Leveraging the increasing amount of evidence emerging from Asia, Rabett argues that, on the contrary, this first dispersal reached far beyond the Levant with populations surviving for many thousands of years.

Therefore he questions whether all the sites attributed to Homo sapiens in Southeast Asia and Australia stem from a single dispersal. Some sites from 45-65 thousand years ago might represent late-surviving enclaves of an earlier dispersal. At the very least, much of the archeologcal data conflicts with a rapid dispersal along the Southern coastal route.

Genomic data including ancient DNA support the view that all human populations living outside Africa derive from a single exodus albeit there was subsequent admixture with other hominins (including Neanderthals and Denisovans). The earlier migration did not contribute to the extant human gene pool. But to quote Rabett, "should evolutionary success be measured only by genetic and demographic continuity into the present?"

Temerity of the tenrec

Lesser hedgehog tenrec © Peter J Stephenson

After human and mouse the tenrec was one of the first mammals to be sequenced (link here). When this was still in the works, the popular science magazine ScienceNews put a tenrec on the cover. The caption to the article read, "They're sequencing a what?" (here). 

Sadly the tenrec has had it's 15 minutes of fame. When Nature News and Comment ran a report on a recent study from the Chavan lab., it was headlined, "Armadillo, hedgehog and rabbit genes reveal how pregnancy evolved." Once the editors realized Echinops telfairi was a tenrec, this was shortened to "Armadillo and rabbit genes..."

The irony to this was that a previous paper from the lab. (OA here) wrote that their hypothesis - about implantation evolving from an inflammatory reaction - could be tested by looking at the basal eutherian clades Afrotheria and Xenarthra. The lecture on which Nature News and Comment was reporting showed this had now been addressed in a tenrec (Afrotheria) and an armadillo (Xenarthra).

The temerity of a tenrec dressing up like a hedgehog!