Human babies are born quite immature to fit through the narrowness of the birth canal caused by upright walking. This problem has to do with our large brains, it was thought until now. But now a study has shown that, despite their still small brains, walking upright led to birth complications even in pre-humans and thus to the birth of babies that were immature compared to great apes. This, in turn, may have played a role in cognitive development, the researchers explain. Because more intensive group care was necessary and newborns learned earlier.
While the birth of dogs, cats and Co. as well as great apes is usually relatively unproblematic, it is often a drama in humans: the birthing process is not only comparatively strenuous and painful, but also involves risks for mother and child. Because in order to pass through the narrow birth canal, a baby has to perform complex twisting movements and bends. But why hasn’t nature produced smoother solutions for us as well? According to the usual explanation, women have to pay the price, so to speak, for walking upright and for the large human brain: There was a problematic compromise between the anatomical adaptations to the new way of moving and to the larger heads.
Why is it stuck?
Anthropologists currently assume that our ancestors developed the ability to walk upright around seven million years ago. This resulted in a characteristic remodeling of the pelvis with a shortened distance between the hip joint and the sacrum. However, the enormous increase in brain size did not begin until two million years ago, when the earliest members of the genus Homo evolved. According to the previous assumption, the birth problems only then appeared: the anatomical system for upright walking was difficult to expand in order to give birth to babies with larger heads without any problems. Evolution then solved this dilemma by giving birth to neurologically underdeveloped, helpless newborns with a relatively small brain size, according to the previous assumption. That is why human babies are decidedly nesty compared to those of great apes.
As part of their study, the researchers led by Martin Häusler from the University of Zurich have now investigated the extent to which these connections are already apparent among the early members of our family tree. Specifically, they focused on the australopithecines, which lived two to four million years ago. Among them was the famous Australopithecus afarensis lady “Lucy”, whose remains were discovered in Ethiopia. “Pre-humans like Lucy are ideal for investigating the effects of the different evolutionary forces: They still had a relatively small, ape-like brain, but their pelvis already showed clear adaptations to upright walking,” explains Häusler.
The researchers’ findings are based on three-dimensional computer simulations: since no fossils of newborn australopithecines have survived, they simulated birth with different fetus head sizes to cover the full possible range of variation. They used a rule that can be observed in non-human primates as a data basis: the brain size of newborns is in a typical ratio to the brain size of adults. Based on the ratio of non-human primates and the brain size of an average adult Australopithecus, the researchers then determined a mean theoretical newborn brain size of 180 grams.
Immature babies even with Lucy
Using the information about the characteristics of the pelvic area in the australopithecines, the researchers were then able to perform 3D birth simulations. It turned out that problem-free passage is only possible with a fetus head with a brain mass of 110 grams, but not with 180 grams or an intermediate size of 145 grams. It is therefore obvious that these pre-humans, in contrast to great apes, were not already relatively far develop babies. “This means that the Australopithecus babies were just as neurologically underdeveloped at birth and dependent on help as human babies are today,” says Häusler.
He and his colleagues see this as an interesting clue to the history of the development of cognition in the human family tree: According to the results, the Australopithecines had to make comparatively intensive efforts to raise their offspring, which was possibly associated with developments in cooperation and culture. In addition, compared to great apes, the infant brain was exposed to the stimuli of the world earlier and the newborns learned more intensively from other group members. “This extended learning phase is generally regarded as a decisive factor for human cognitive and cultural development,” says Häusler. Archaeological finds also support this theory: The oldest stone tools, dated 3.3 million years ago, come from a time when there were only Australopithecines and no representatives of the genus Homo.
The research team concludes: “The prerequisites for advanced cognitive development appear to have been a consequence of the skeletal adaptations to biped locomotion that preceded the appearance of the genus Homo and the increasing brain growth”.
Source: University of Zurich, specialist article: Communications biology, doi: 10.1038/s42003-022-03321-z