Neanderthal brain size at birth provides insights into the evolution of human life history

Abstract

From birth to adulthood, the human brain expands by a factor of 3.3, compared with 2.5 in chimpanzees [DeSilva J and Lesnik J (2006) Chimpanzee neonatal brain size: Implications for brain growth in Homo erectus. J Hum Evol 51: 207-212]. How the required extra amount of human brain growth is achieved and what its implications are for human life history and cognitive development are still a matter of debate. Likewise, because comparative fossil evidence is scarce, when and how the modern human pattern of brain growth arose during evolution is largely unknown. Virtual reconstructions of a Neanderthal neonate from Mezmaiskaya Cave (Russia) and of two Neanderthal infant skeletons from Dederiyeh Cave (Syria) now provide new comparative insights: Neanderthal brain size at birth was similar to that in recent Homo sapiens and most likely subject to similar obstetric constraints. Neanderthal brain growth rates during early infancy were higher, however. This pattern of growth resulted in larger adult brain sizes but not in earlier completion of brain growth. Because large brains growing at high rates require large, late-maturing, mothers [Leigh SR and Blomquist GE (2007) in Campbell CJ et al. Primates in perspective; pp 396-407], it is likely that Neanderthal life history was similarly slow, or even slower-paced, than in recent H. sapiens.

The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation

“…This implies a primitive (non-rotational) birth mechanism in both H. heidelbergensis and Neanderthals, although this is debated partly because of the paucity and incompleteness of pelvic fossils [1,2,16,37,96,124,129]. However, birth canal size in Middle Pleistocene Homo is clearly larger than in earlier hominins, generally within the modern human range [16,74,79,119,121,124,127].…”

“…Adult body size in H. heidelbergensis and the Neanderthals, which was within or above the modern human range, changed little during the Middle Pleistocene, but adult brain size did increase [24,60,74,113,114,116]. Fossil evidence suggests that Neanderthal infants were born with a body and head size similar to modern newborns, implying a similar level of obstetric difficulty in Neanderthals and modern humans [1,16,74,77,124] and probably 'obligate midwifery' (cf. [35,130]).…”

“…While the birth canal is larger in females than males, this is primarily owing to rotation of the pubis, rather than lengthening as in modern humans. In archaic Middle Pleistocene Homo, the pubic rami are actually longer in males than in females [74,79,116,120,121,124].…”

“…The birth canal now varied in proportions along its length: the inlet retained its M-L orientation (although less pronounced), especially in females, whereas the midplane and outlet became larger in the A-P dimension. The fossil evidence suggests, therefore, that the modern human pattern of rotational birth evolved only with the origin of H. sapiens [2,74] (although there is some disagreement [124,129]). …”

“…The pelvis in Neanderthals is typically very similar to H. heidelbergensis in its morphology and large size [1,74,100,[120][121][122][123][124][125][126] (but see [127]). Except for a few uniquely derived characteristics, particularly in the cross-sectional shape of the superior pubic ramus (see [100,120] for reviews), recent work has shown that most of the features of the Neanderthal pelvis that were once thought to be distinctive are in fact retained primitive traits [37,100,[119][120][121]123,128].…”