Childbirth poses significant challenges across the primate family, notably for humans. Recent research into primate anatomy reveals that many species contend with narrow pelvises that hinder birthing larger-headed infants. This issue dates back to the earliest primates over 50 million years ago.

Traditionally, it was believed that the unique birthing challenges faced by humans emerged from evolution, particularly following the adoption of bipedal locomotion, which constrained pelvic width. As human brains evolved larger, so did infant head sizes, creating a mismatch with pelvis dimensions that couldn’t sufficiently accommodate safe delivery.

Earlier studies suggested that other primates had it easier. Anthropologist Adolf Schulz’s influential research in the 1940s claimed that infants in most primate species fit comfortably within the maternal pelvis.

However, Schulz’s analysis may have contained errors, according to Nicole Torres Tamayo at University College London. “The flaw lay in applying human pelvic measurements across all primate species,” she explains.

Schulz identified certain anatomical landmarks on the human pelvis, hypothesizing they could correspond to those in other primates. However, the human pelvis is uniquely shaped. His landmarks inaccurately defined an inclined plane above the actual birth canal, leading to an overestimation of its size.

In a groundbreaking re-examination, Torres-Tamayo and her research team analyzed the birth canals of 29 primate species while also assessing newborn skull sizes. Their findings indicated that some primate pelvises are ill-suited for childbirth, especially in smaller species like bushbabies and tamarins, where newborn heads can be nearly double the size of the birth canal.

“We were surprised by the high incidence of these size discrepancies across many primate species,” states research team member Leah Betti.

This challenge may even extend back to our primate ancestors, particularly given the small size of early primates, as noted by Betty.

“It’s fascinating to analyze such a diverse dataset,” adds Nicole Webb from the University of Zurich, Switzerland. “These species exhibit varied lifestyles, habitats, and anatomical diversity.”

Different primate species have developed adaptive strategies to cope with these complications. For instance, bush babies and tamarins can dislocate their pelvic bones, temporarily enlarging the birth canal. However, humans are unable to perform this adaptation, as it would make bipedal movement excruciatingly painful.

Interestingly, Torres-Tamayo, Betti, and their colleagues observed that larger great apes experience fewer birthing challenges. This might be due to their size compared to smaller tree-dwelling primates. As such, humans are further distinguished by their unique birth difficulties, making them the only great apes encountering these issues, as Betty noted.

Nonetheless, Webb casts doubt on this conclusion. A study she and her colleagues published in 2024 concluded that even chimpanzees face tight matches between birth canal dimensions and newborn head sizes. “This mismatch is curious and could indicate methodological flaws,” Webb states. “The findings from this new study provide compelling reasons to reassess our assumptions.”