During human birth, the fetus navigates a tight, complicated birth canal, usually by turning and rotating its head at various stages. This complex procedure comes with a high risk of birth complications, from prolonged delivery to stillbirth or maternal death. These complications were long thought to be the result of a conflict between humans adapted to walking upright and our larger brains.
The dilemma between walking straight and a big brain
Bipedalism evolved about seven million years ago and dramatically turned the hominin pelvis into a true birth canal. However, the evolution of large brains did not begin until two million years ago, when the earliest species of the genus fa emerged. The evolutionary solution to the dilemma brought about by these two conflicting evolutionary forces was to give birth to neurologically immature and helpless newborns with relatively small brains – a condition known as secondary altruism.
A research group led by Martin Hausler of the Institute for Evolutionary Medicine at the University of Zurich (UZH) and a team led by Pierre Fremondier of the University of Aix-Marseille has now found that australopithecines, which lived about four to two million years ago, were great Apes had a complex birth pattern compared to that. “Since the brain sizes of australopithecines like Lucy were relatively small but already displayed morphological adaptations to bipedalism, they are ideal for examining the effects of these two conflicting evolutionary forces,” Hausler says.
Typical ratio of fetal and adult head sizes
The researchers used three-dimensional computer simulations to develop their findings. Since fossils of neonatal australopithecines do not exist, they simulated the birth process using different fetal head sizes, taking into account the possible range of estimates. Each species has a specific ratio between the brain size of its newborns and that of adults. Based on the proportion of non-human primates and the average brain size of an adult australopithecinesIn this article, the researchers calculated an average newborn brain size of 180 grams. This would correspond to a size of 110 grams in humans.
For their 3D simulation, the researchers also took into account the increase in pelvic joint mobility during pregnancy and determined a realistic soft tissue thickness. They found that in contrast to the 180 g and 145 g sizes, only a 110 g fetal head size passed through pelvic penetration and midplane without difficulty. “it means that australopithecines Newborns were neurologically immature and dependent on help, similar to human babies today,” Hausler explains.
Long-term learning key to cognitive and cultural abilities
The findings indicate that australopithecines likely practiced a form of cooperative breeding even before the genus fa appeared. Compared to great apes, the brain develops longer outside the uterus, enabling the infants to learn from other members of the group. “This long period of learning is generally considered important to the cognitive and cultural development of humans,” Hausler says. This conclusion is also supported by the earliest documented stone tools, which date to 3.3 million years ago – much earlier than the genus fa appeared.
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