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Human work is very painful because we walk upright

Human work is very painful because we walk upright

Human childbirth is much more complex and painful than that of other animal births, including non-human primates. It has long been believed that this is due to the larger size of the brains of infants, and thus the size of the larger heads, and the narrower pelvis of mothers. In other words: the size of the newborn’s skull is relatively large relative to the birth canal. But now, using 3D simulations, scientists from the universities of Zurich and Marseille have shown that labor was also painful and complicated in early hominins who were born with relatively small heads. So the reason must be something else.

During labour, the fetus moves through a narrow, coiled birth canal, bending and twisting the head at various stages. This process is very long and is associated with a high risk of complications, which in some cases may end in the death of the child or the mother. According to the most recent findings, this complication is caused by a conflict between adaptation to vertical walking and a larger brain size. This is known as the birth dilemma.

Walking on two legs evolved about 7 million years ago and brought about a drastic transformation in the hominin basin: it has greatly shortened and widened. In contrast, the brains of our greatest ancestors began to develop only 2 million years ago, when the first species of the human genus appeared. The evolutionary solution to the conflict caused by these two contradictory phenomena was that humans began to give birth to immature and completely helpless newborns whose brains are not fully developed, and thus remain very small.

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A research group led by Professors Martin Hesler and Pierre Fremundier found that Australopithecus, which lived about 2-4 million years ago and was related to hominins, actually had a complex labor history compared to the great apes. “Because Australopithecus, like the famous Lucy from Africa, had relatively small brains, but already showed morphological adaptations to walking on two legs, we found it an ideal model for studying the effects of these two opposing evolutionary forces,” says the professor. Hessler.

The scientists used a 3D computer simulation for their purposes. They took into account the average size of the newborn’s brain, the movement of the pelvic joints as a result of pregnancy, and the thickness of the soft tissues. They found that the heads of fetuses weighing only 110 grams could pass through the entrance to the pelvis and the entire birth canal on their own. “This means that australopithecines newborns were also born with immature brains and were completely dependent on adults, just like children today,” Hessler notes. At the same time, it can be seen that the brains of these individuals were almost half the size of humans.

This suggests that complex and difficult deliveries are due to body position, and more specifically to anatomical adaptation to walking on two legs, rather than to the size of the newborn’s skull.

An additional conclusion from the study in question is that the brains of Australopithecus, like ours now, must have matured outside the womb for a long time. This likely required active gestation of the newborn for an extended period of time after birth, suggesting that these creatures, rather than members of the hominid species, evolved behaviors such as cooperative offspring care. The study authors concluded, “This prolonged period of development and learning outside the womb is key to human cognitive and cultural development.”

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The working source can be found here: http://dx.doi.org/10.1038/s42003-022-03321-z

Katarzyna Czekovic

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