Births in humans are reputed to be difficult, compared to those of other primates. In female chimpanzees, childbirth is
almost a non-eventunderlines Pierre Frémondière, main author of the study published Wednesday in the journal Communications Biology.
In question: the shape of the female pelvis, which makes the exit of the human fetus perilous. The only solution to overcome the obstacle without difficulty: to have a small skull, and therefore to be born a little prematurely.
The size of his skull, very small compared to his future adult size, makes him a particularly
immature, unable to survive without assistance. A singularity in mammals which seems linked to the acquisition of bipedalism: by starting to walk on two legs, the first humans would have seen the architecture of their pelvis change and, in women, the birth canal shrink.
To test this hypothesis, the scientists went back to Australopithecines, an extinct species of hominins that lived in Africa between 3.2 million and 1.8 million years ago.
This very ancient group of human lineage, made famous by the discovery of the Lucy fossil in 1974, were mostly bipedal, but still possessed small brains as adults.
It is almost comparable to that of a chimpanzee, so we imagine primitive beings, living very simple deliveriesexplains to Agence France-Presse Pierre Frémondière, anthropologist and midwife teacher at the University of Aix-Marseille.
In order to reconstruct the scenario, he and his team resorted to numerical simulations, using software usually used for automotive tests, but adapted to the
The aim was to calculate the ratio between neonatal cranial size and adult cranial size in australopithecines; a parameter hitherto unknown, paleontologists only have fossils of adult skulls, from three species of Australopithecus.
They therefore suggested different neonatal sizes, corresponding to brain masses of 110 grams, 145 grams (close to chimpanzees) and 180 grams (close to modern humans). At each simulation, the software had to calculate the size to find the right way in the virtual parturients.
Result: only the brains between 110 and 145 grams crossed the canal without incident, what is called a eutocic delivery, that is to say normal. The bigger ones got stuck.
%, which is very close to the configuration ofHomo sapiens“,”text”:”With this smallest option, we calculated a ratio of 28 to 30%, which is very close to the configuration of Homo sapiens”}}”>With this smallest option, we calculated a ratio of 28 to 30%, which is very close to the configuration ofHomo sapiens, details the researcher in biocultural anthropology. This proportion almost identical to ours suggests that the baby Australopithecus also had a small skull compared to that of adulthood, the only way to get out of its mother’s shrunken pelvis.
The authors of the study conclude that immaturity at birth was already present in australopithecines, and that it comes from bipedalism.
From this vulnerability, they deduce a need for infant care to protect him from the cold (since he cannot regulate his temperature on his own) and from predators (since he is still far from being able to walk).
The earlier the birth, the more deprived the baby and the greater the postpartum parental investment, on the part of the mother, but also of the group.analyzes Pierre Frémondière.
It is this practice
assisted childbirth that allowed humans to be socialized at an early stage and mobilized higher cognitive functions, leading to the increase in brain size over the course of human evolution.