Human brains rounded into shape over 200,000 years or more

Big brains outpaced well-rounded brains in human evolution.

Around the time of the origins of our species 300,000 years ago, the brains of Homo sapiens had about the same relatively large size as they do today, new research suggests. But rounder noggins rising well above the forehead — considered a hallmark of human anatomy — didn’t appear until between about 100,000 and 35,000 years ago, say physical anthropologist Simon Neubauer and his colleagues.

Using CT scans of ancient and modern human skulls, the researchers created digital brain reconstructions, based on the shape of the inner surface of each skull’s braincase. Human brains gradually evolved from a relatively flatter and elongated shape — more like that of Neandertals’ — to a globe shape thanks to a series of genetic tweaks to brain development early in life, the researchers propose January 24 in Science Advances.

A gradual transition to round brains may have stimulated considerable neural reorganization by around 50,000 years ago. That cognitive reworking could have enabled a blossoming of artwork and other forms of symbolic behavior among Stone Age humans, the team suspects. Other researchers have argued, however, that abstract and symbolic thinking flourished even before H. sapiens emerged (SN: 12/27/14, p. 6).

Ancient DNA studies indicate that genes involved in brain development changed in H. sapiens  following a split from Neandertals more than 600,000 years ago ( SN Online: 3/14/16 ). “Those genetic changes might be responsible for differences in neural wiring and brain growth that led to brain [rounding] in modern humans, but not in Neandertals,” says Neubauer of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Still, a lack of fossilized brains means scientists have to rely on braincase data. But these data don’t directly measure brain shape, making it difficult to untangle precisely how quickly or slowly human brains became as round as they are today, says paleoanthropologist Christoph Zollikofer of the University of Zurich. In general, though, the faces of H. sapiens got smaller over time, a skull change that Zollikofer contends critically influenced the evolution of rounded braincases described in the new report.

Neubauer’s team studied 20 ancient H. sapiens skulls. The three oldest specimens included two Moroccan finds dating to around 315,000 years ago that may be the earliest known H. sapiens (SN: 7/8/17, p. 6). A second group of four skulls date to between 120,000 and 115,000 years ago. Estimated ages for the remaining 13 skulls range from around 36,000 to 8,000 years old.

Comparison skulls came from 89 present-day humans, eight Neandertals dating to between 75,000 and 40,000 years ago and 10 members of other ancient Homo species dating to between 1.78 million and 200,000 years ago. Progressive rounding of braincases appeared only in the sample of ancient H. sapiens.

Neubauer considers it unlikely that the gradual evolution of smaller faces with the same general skull shape altered braincase shapes. The oldest known H. sapiens skulls, which his team considers to be the two Moroccan finds, have faces shaped like those of modern humans, Neubauer says.