Obstetrical Constraints and the Origin of Extended Postnatal Brain Maturation in Hominin Evolution

[Primum Sapienti]

https://www.mdpi.com/2079-7737/13/6/398
Published: 31 May 2024

Simple Summary
The unique complexity of human childbirth
is traditionally attributed to the
opposing selection pressures of bipedal
locomotion and large brains. Here, we
explore this trade-off in Australopithecus
with canonical discriminant analyses using
different fetal head sizes. We reveal that
the shape of the pelvis in Australopithecus
led to a tight fit between the mother’s
pelvis and the newborn head despite their
relatively small brain sizes. To alleviate
this obstetrical dilemma,
australopithecines must have already given
birth to secondarily altricial infants that
were helpless at birth like those of extant
humans. Cognitive development and some
aspects of the modern life history pattern
therefore likely originated prior to the
appearance of the genus Homo.

Abstract
The origin of difficult birth is still a
matter of debate in obstetrics. Recent
studies hypothesized that early hominins
already experienced obstructed labor even
with reduced neonatal head sizes. The aim of
this work is to test this hypothesis using
an extant obstetrical sample with known
delivery outcomes. Three delivery outcomes
(i.e., instrument-assisted, Caesarean
section, and vaginal birth) were evaluated
using a discriminant analysis based on 131
mother–baby dyads and 36 feto-pelvic
variables. This obstetrical sample was
compared with 20 australopithecine “dyads”
generated from the combination of six pelvic
reconstructions (three for Australopithecus
afarensis, two for A. africanus, and one
for A. sediba) and three fetal head size
estimations. The obstetrical analysis
revealed that dystocic births can be
predicted by pelvic features such as an
anteroposteriorly flattened pelvic inlet.
Australopithecines shared these pelvic
morphologies with humans and had eutocic
birth only for infants of 110 g brain size
or smaller, equaling a human-like
neonatal/adult brain size ratio of 25–28%.
Although birth mechanism cannot be deduced,
the newborn/adult brain size ratio was
likely more human-like than previously
thought, suggesting that
australopithecines were secondarily
altricial to circumvent instances of
obstructed labor and subsequently require
a prolonged postnatal brain growth period,
implying some aspects of life history
pattern similar to modern humans.

[JTEM]

Post by Primum Sapienti
The unique complexity of human childbirth
is traditionally attributed to the
opposing selection pressures of bipedal
locomotion and large brains.

What the hell does this mean?

Bipedal locomotion works well with large brains. It's
not a competing "Selection."

Post by Primum Sapienti
We reveal that
the shape of the pelvis in Australopithecus
led to a tight fit between the mother’s
pelvis and the newborn head despite their
relatively small brain sizes. To alleviate
this obstetrical dilemma,
australopithecines must have already given
birth to secondarily altricial infants that
were helpless at birth like those of extant
humans.

And mice.

So BECAUSE BIG BRAINS & BIPEDAL LOCOMOTION,
mice give birth to tiny, helpless babies?

It's not at all clear WHAT this "Cite" of yours
is even trying to say, let alone WHY and HOW it's
supposed to be important.

So just explain it. Because you understand it so
well.

Right?