The evolution of human childbirth

Humans are unique among the great apes in having a tight birth canal and a relatively high risk of feto-pelvic disproportion, when the baby is too big to progress though the canal. Our childbirth difficulties have long been explained as a compromise between efficient bipedal locomotion (favouring a narrow canal) and safe delivery of a large-headed baby (favouring a large canal). However, recent studies have shown that this explanation does not stand to scrutiny and it is time to find a new one! This is what my colleague Todd Rae and I will try to do with a new project funded by the Leverhulme Trust.

Are humans really that unique?

The human birth canal is about the same size, if not a bit smaller, than the baby’s head, leading to a difficult and dangerous birth in our species. Other great apes, on the other hand, have a spacious birth canal in respect to their babies’ size. As we are the only bipedal ape, the obvious conclusion has been that our pelvis changed in response to our adaptation to bipedalism, leading to a tight fit with the baby ’s head, further exacerbated by our large brain. If you expand your view to other primates, however, it appears that we are not that unique: gibbons, macaques, squirrel monkeys and other species have a similar tight fit during childbirth. As part of a new project, funded by the Leverhulme Trust, my collaborator Todd Rae and I are developing new 3D measurements of the birth canal that allow direct comparison across primate species, to investigate in what ways we are unique and in what ways we are not.

The frontispiece to Huxley’s Evidence as to Man’s Place in Nature (1863). The image compares the skeletons of humans to other apes.

Locomotion, posture and obstetrics

Primates are a particularly diverse group of mammals, with very large differences in locomotion, diet and body size. As part of the same project, Todd and I look to investigate how variation in locomotion, habitual posture, adult and baby size and sexual dimorphism have affected the shape and size of the birth canal across primates. By taking a wide comparative approach, we will test alternative hypotheses for why humans (and some other primate species) have evolved a tight birth canal.

3D models of a chimpanzee (left) and a human (right) pelvis reconstructed from CT-scan images. Landmarks for shape analyses have been added to the chimpanzee pelvis.

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