A horse can walk within an hour of being born. A human baby takes about a year. Our pregnancies are long, we usually have one baby at a time, and we put a lot into each one. Animals like that normally have babies that are ready to go at birth. Ours arrive about as helpless as a kitten. Biologists call this secondary altriciality, and they have argued about the cause for decades.

The video takes the argument in three rounds:

• Round one, the textbook answer (1960). Walking upright narrowed the pelvis, and our brains got much bigger. To fit through, babies have to be born early. This idea is called the obstetrical dilemma.
• Round two, the “power bill” (2012). Researchers found that wider hips don’t make walking harder, which weakens round one. They suggested instead that birth happens when the mother’s body reaches its limit on how much energy it can keep supplying.
• Round three, a plot twist (2023). That energy limit traced back to one study of just twelve women. When researchers pooled 12 studies covering 303 women, the limit didn’t hold up. The question is still open.

What nobody disputes

At birth, a chimp’s brain is about 40% of its adult size, but a human newborn’s is only about a quarter. A human baby’s brain keeps growing at the same fast pace as before birth and roughly doubles in the first year. In other words, your baby finishes developing outside the womb, in your arms. That’s why the early months are sometimes called the fourth trimester.

What this means at 3am

A newborn who won’t be put down isn’t being difficult. Wanting constant contact and settling only when held and moved are what an unfinished baby expects. A baby this dependent for this long needs more than one or two adults, and humans have always shared the work. Asking for help isn’t a weakness. It’s how our species is built.

Sources

• Washburn, “Tools and human evolution” (1960), Scientific American
• Portmann (1941), on the “extrauterine year”
• Dunsworth et al., “Metabolic hypothesis for human altriciality” (2012), PNAS
• Warrener et al., “A wider pelvis does not increase locomotor cost in humans” (2015), PLOS ONE
• Haeusler et al., “The obstetrical dilemma hypothesis: there’s life in the old dog yet” (2021), Biological Reviews
• Cordey et al., “Take it to the limit: the limitations of energetic explanations for birth timing in humans” (2023), Evolution, Medicine & Public Health

This is educational content, not medical advice. For anything about your own baby, talk to your pediatrician.