Can invasive placentation be linked to oxygen requirements of the brain?

Skull of a male Western Gorilla
By Didier Descouens CC BY-SA 4.0

Three years ago (here) I reported a study suggesting the metabolic rate of the brain had increased during hominin evolution. This conclusion was reached by extrapolating from measurements of the carotid foramina in fossil skulls of adults (here). I asked whether there was an increasing demand for oxygen during fetal development and whether this could have driven an increase in the depth of trophoblast invasion of the placental bed.

Now Seymour and colleagues return to the fray (here). First, they address criticisms of their previous reliance on an equation for shear stress and introduce a modified approach for estimating blood flow in the internal carotid artery. This replicated their previous findings.

Second and more important, they applied their methods to the great apes Pongo, Pan and Gorilla. A surprising finding was that estimated brain metabolic rate was higher in these apes than in the hominin ancestor Australopithecus. (Brain volume was about the same in extant great apes and australopithecines.)

Decidual myometrial junction in placental bed
of a Western Gorilla with trophoblast stained red
From Carter et al. 2015 (here)

We have shown that the pattern of trophoblast invasion is similar in chimpanzee, gorilla and human (here). This is consistent with an improved oxygen supply to the fetal brain compared to gibbons and Old World monkeys. However, unless this represents convergent evolution, one would infer that the pattern was present in Australopithecus. 

Perhaps the oxygen supply needed to support the brain of the term fetus was present in the human ancestor, but brain development more complete at birth. The human newborn is secondarily altricial, i.e. highly dependent on parental care (previous post). The only clue we have about Australopithecus is that breastfeeding lasted for a shorter time than otherwise expected (here).