Why do humans give birth to particularly immature babies?

A pat on the back, a big breath of fresh air, and forward! This is what our first moments in the world look like for all of them. Stunted, naked, almost blind, unable to move or even just turn around, the human baby then seems ill-equipped for survival. And as much to say that it takes several long months or even years to work out.

Of the animal kingdom, we can even affirm to be the most badly embarked specimens at birth, which does not, far from it, prevent our species from dominating the world. But then why? What are the evolutionary necessities of such maladjustment to the world?

A question that a team of six CNRS researchers from the University of Aix-Marseille has just answered. Two hypotheses had been on the table for a while: Either this “preterm” birth was a consequence of bipedalism, which would have caused a narrowing of the birth canal. Or, it was due to the post-natal growth of the human skull and brain. To close this debate, the researchers have reconstructed “the australopithecine birthing canal, which, if they are not the first bipedal hominids, were the oldest basins available”, indicates Pierre Frémondière, from the bio-anthropology laboratory. culture and lead author of the study published in the journal Communication Biology.

And as much to say that Lucy, since the researchers used, among other things, the remains of her pelvis to model the birth canal of an Australopithecus, already knew the characteristic “head rotation” movements of babies, explains the researcher . To ensure the validity of their study, the scientific team modeled three different basins from remains. “But they were incomplete, so we had to reconstruct them. For example, in Lucy’s case we only had the left iliac bone, and we mirrored the right. »

Bipedalism, the starting point of higher cognitive skills

Their conclusion suggests that this need (for preterm delivery) is “linked to the acquisition of bipedalism which has restructured the architecture of the birth canal. To test this hypothesis, the study of australopithecines is ideal, because these hominids are already bipedal, but still have a rather small brain in adulthood,” write the authors in their publication. Even more, it is “bipedalism [qui] seems to be the starting point for the acquisition of these new higher cognitive skills”.

Thus, without this modification of the birth canal and the preterm birth it induces, the door to neonatal brain growth would not have opened. At birth, that of an infant is between 28 and 30% of its adult size, compared to 41% in primates. The evolutionary strategy of hominids, “a single individual per litter, a long gestation and a strong maternal investment”, seems to have paid off.

After having devoted twelve years to this research, the ambition for Pierre Frémondière is to integrate the muscles into this modeling and to continue to go back up the chain of time. Because if Lucy lived about 3.2 million years ago, bipedalism appeared 8 or 9 million years ago. “There is indeed an Ethiopian fossil with a basin a little older than Lucy, but it is in great demand…”, concludes the anthropologist who, with his team, has just advanced the understanding of our evolution.

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