Why Are Human Babies Born So Early?

If you’ve ever seen a nature documentary you’ve probably noticed how new-born babies of most animals can walk and live (mostly) independent of any caregiver; humans on the other hand need a bit more help. Ever since the comparisons between humans and animals of other species have come about, one of the biggest questions has been: Why are Human babies born too early to support themselves?

Even though neonates of other primate species also depend on mothers, human new-borns are uniquely destitute as their brains are relatively undersized. In fact, it is projected that we need a gestational term (pregnancy) of 18 – 21 months, as opposed to the customary nine, to be born at a cognitive growing level akin to that of any other primate new-born.

Figure 1: Table showing the avg gestation periods for primates (in days) taken from http://www.studyofoahspe.com/id43.html

Current Theory: Size Of The Pelvis

The most widely accepted account for our 9-month pregnancy is that natural selection reassures birth at a premature stage of foetal advancement to allow both superior brain masses and erect orientation of human movement. In this regard, evolution to bipedal movement (walking on two feet) has restricted the width of the pelvis and by extension, the birth passage which limits the size of the foetus that can pass through it – Obstetrical Dilemma Hypothesis. As a consequence, human newborns arrive with brains that are barely 30% of the size of the adult brain so they can fit through the slender passage of the birth canal. Following which, they then develop outside the uterus, with brain size growing to 2x the original size within their first year.

However, when Holly M. Dunsworth from the University of Rhode Island and her contemporaries tested this hypothesis, their results and estimates did not concur. The hypothesis expects that: as the female pelvis is broader than the male pelvis, running and other physical strains should be more dynamically and energetically challenging for women than for men. However, innumerable studies on the energetics and mechanism of kinesis in either sex found no such consequences for a broader pelvis.

New Theory: Managing Mother’s Metabolic Rate

The team projects that if a baby ports the pelvic entrance (the upper section of the birth passage) in a primate-like point of brain development – that is, a brain that is 40% of the adult brain. The narrowest part, on average, would need to expand by just three inches. Some women nowadays have big pelvic inlets, and these larger proportions have negligible impact on movement costs. The researchers contend that instead of increasing the foetal brain, which is limited by the proportions of the pelvis, the proportions of the human pelvis have evolved for babies, and another influence has kept the development of the foetus in check.

The mother’s metabolic rate is that other influence. “Gestation places a heavy metabolic burden (measured in calories consumed) on the mother,” Dunsworth and her co-authors clarify. Statistics from an extensive series of mammals propose that there is a bound to how big and energetically exhaustive a foetus can nurture before it can be born. Once outside, the baby’s development slows to a more supportable degree for the mother. Advancing on an idea formerly posited by Peter T. Ellison from Harvard University – the “Metabolic Crossover Hypothesis”, the team suggests that “energetic constraints of both mother and foetus are the primary determinants of gestation length and foetal growth in humans and across mammals.”

By the ninth month, the metabolic strains of a human foetus surpass the mother’s capacity to meet both the baby’s energy necessities and her own, thus, delivery happens.

In their statement, issued in the Proceedings of the National Academy of Sciences USA, Dunsworth and her co-authors settle that “if the human reproductive system poses a dilemma between competing needs, then foetal energy needs and maternal energy supply are the competitors, rather than [brain expansion] and bipedalism.”


You can find the entire research paper here

If you are looking for more about human cognitive development, this paper may also interest you: Modularity, comparative cognition and human uniqueness by Sara J. Shettleworth


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