Why Do Humans Have Babies Who Can't Look After Themselves?


THAT’S THE WHY:IF YOU have any experience with a newborn, you’ll know that he or she needs a lot of care and minding in those early weeks and months. At least compared with the new offspring of many non-human species that can move around and fend for themselves sooner.

Why do humans have babies who are so “altricial”, or still need to develop so much?

The often-used explanation is that having a relatively immature brain at birth is a trade-off: the heads of babies need to be relatively small to fit through the pelvis of an upright-walking species, and then once the infant has been born, his or her brain develops rapidly in the first year of life.

But a new paper is challenging that thinking, or at least offering another perspective. “There is little evidence that pelvic constraints have altered the timing of birth,” it states in the Proceedings of the National Academy of Sciences.

The analysis sees the US researchers point to the strong influence of the mother’s metabolism.

“There is a limit to the number of calories our bodies can burn each day,” says researcher Herman Pontzer from Hunter College in New York, in a release. “During pregnancy, women approach that energetic ceiling and give birth right before they reach it. That suggests there is an energetic limit to human gestation length and foetal growth.”

Researcher Holly Dunsworth from Rhode Island University says it would be “physiologically impossible” to have a more developed baby.

“Mom’s energy is the primary evolutionary constraint, not the hips,” she notes in the release. “Our helplessness at birth is just a sign of how much more brain growth we have to achieve once we start living outside our mother.”