The evolution of modern human brain shape

Abstract

Modern humans have large and globular brains that distinguish them from their extinct Homo relatives. The characteristic globularity develops during a prenatal and early postnatal period of rapid brain growth critical for neural wiring and cognitive development. However, it remains unknown when and how brain globularity evolved and how it relates to evolutionary brain size increase. On the basis of computed tomographic scans and geometric morphometric analyses, we analyzed endocranial casts of Homo sapiens fossils (N = 20) from different time periods. Our data show that, 300,000 years ago, brain size in early H. sapiens already fell within the range of present-day humans. Brain shape, however, evolved gradually within the H. sapiens lineage, reaching present-day human variation between about 100,000 and 35,000 years ago. This process started only after other key features of craniofacial morphology appeared modern and paralleled the emergence of behavioral modernity as seen from the archeological record. Our findings are consistent with important genetic changes affecting early brain development within the H. sapiens lineage since the origin of the species and before the transition to the Later Stone Age and the Upper Paleolithic that mark full behavioral modernity.

Fig. 1. Differences in brain shape between a present-day human (left, in blue) and a Neandertal from La Chapelle-aux-Saints (right, in red).

Endocasts are shown together with the triangulated landmark set used in this study and CT scan renderings of the crania.

Fig. 2. bgPCA of endocranial shape.

H. erectus, triangles and orange convex hull; Neandertals, squares and red convex hull; H. heidelbergensis/rhodesiensis, dark red diamonds; present-day humans, light blue convex hull; H. sapiens fossils, circles and dark blue convex hulls for geologic age groups 1 to 3. Evolutionary trends of shape changes in archaic and modern individuals are shown as regressions on geologic age (arrows) (see fig. S2 for labels of fossil individuals).

Fig. 3. Gradual evolutionary changes of endocranial shape within H. sapiens fossils shown as a series of predicted shapes according to the regression on geologic age at 300,000 years ago (top), 100,000 years ago (middle), and 10,000 years ago (bottom and blue area in top and middle).

Color coding in green (bottom) illustrates regions with surface size increase associated with this gradual shape changes. Lateral (left) and occipital (right) views (see also fig. S4 and movies S1 to S3).

Fig. 4. bgPCA of endocranial form.

Symbols and colors as seen in Fig. 2. In addition to the evolutionary trend of shape changes in archaic and modern individuals (like in Fig. 2), allometric relationships are shown as regressions on centroid size (dotted lines) (see fig. S5 for labels of fossil individuals).