Brain development after birth differs between Neanderthals and modern humans

Here, we test whether we could find evidence for a globularization phase in the ontogenetic trajectory of Neanderthals, and thus whether the adult endocranial differences are already established at the time of birth, or develop later. We statistically compared shapes of virtual endocasts extracted from computed-tomographic scans of crania of 58 modern humans [7] and virtual reconstructions of 11 fossil humans, including the Neanderthal neonate Le Moustier 2. Three lines of evidence suggest that Neanderthals did not have a globularization-phase after birth. First, both Neanderthal and modern human neonates have relatively elongated braincases at the time of birth (Figure 1A), but only modern human endocasts change to a more globular shape between dental age groups 1 and 2 (see Supplemental Information, published with this paper online). By contrast, the endocranial shapes of the two youngest Neanderthal specimens in our sample, the neonate Le Moustier 2 (dental age group 1) and Pech de l’Azé (dental age group 3), are so similar that their reconstruction distributions (which reflect the estimation uncertainty) overlap (Figure 1B). Second, if Neanderthals and modern humans had the same globularization-phase after birth, a Neanderthal neonate would need to have an extremely elongated neurocranium and a very poorly developed cerebellum (Supplemental Information). However, it is not possible to reconstruct the Neanderthal neonate Le Moustier 2 in a way that would match this prediction. Finally, given that large portions of the braincase of Le Moustier 2 had to be estimated (Figure 1A), we also conducted additional tests that did not rely on subadult fossils (Supplemental Information). We simulated the development of modern human neonates along the average human trajectory from age groups 2–6, thus skipping the globularization phase. The adult crania resulting from these simulations bear a striking resemblance to the Neanderthal average shape, even though they were based on modern human neonates. We find that the modern human pattern of brain development is derived compared to Neanderthals. The pattern of endocranial shape changes between age groups 2 and 6 is similar among modern humans, Neanderthals and chimpanzees [7]. Neanderthals Brain development after birth differs between Neanderthals and modern humans