Stop the Planet of the Apes: I Want to Get Off!

By way of the BBC comes word that Homo erectus neonates may have had proportionally larger heads than Homo sapiens, extrapolated from a female H. erectus pelvis that is proportionally wider than H. sapiens ones. The conclusion drawn is that

the young hominid was dependent on its mother for less time than a modern human baby, a useful survival adaptation

Well, maybe. First, it's not clear to me (and maybe the article discussed this but the BBC story doesn't mention it) that neonates "30% bigger than previously thought" would convey much of an adaptive advantage (ooh, Henry Gee isn't going to like that), in terms of reduced dependence upon adults, in a species whose development rate is measured in years anyway (on the other hand, what I recall of the mathematics of evolutionary biology suggest that very small variations can have enormous effects). Second, though – duh. H. sapiens neonates, unlike other extant species', maintain the fetal growth rate for about 3 months after birth (equal to about 30% of gestation). This is because H. sapiens have, proportionally, freaking huge heads, and fetal head size at 12 months gestation would require freaking huge pelves. Imagine if the last month of pregnancy lasted four months, getting worse each week. Imagine trying to squeeze out a 3-month-old. (Fuuuuck.) Also consider the locomotive engineering disadvantage female H. sapiens are already at with a 9-months-gestation–adapted pelvis (which is why we have sex categories in post-adolescent sports, not just size/age categories, as well as sexy hip-swaying). For obligate bipeds like us hominids there's a functional limit to how much locomotive efficiency can be sacrificed to gestational adaptation and still permit escape from marauding saber-toothed cats. Beyond that limit something else has to give, and what gave was intrauterine development. In terms of developmental rate our babies are born 3 months early.

That, to me, is what makes this report particularly interesting. H. sapiens babies are born "early," middle-aged hominids didn't have big enough heads to require "early" birth. So...when did "early" birth appear? If this report is correct, it happened sometime after H. erectus, but it's not likely to have happend much later, because a wide-hipped H. erectus was probably already near the intersection of birth and locomotive energetic efficiency.