Human newborns arrive remarkably underdeveloped. The reason lies in a deep evolutionary trade-off between big brains, bipedalism and the limits of motherhood.

MEXICO CITY — When she walked into an IVF clinic in June, Alin Quintana knew it would be the last time she would try to conceive a child. She had prepared herself spiritually and mentally for the ...

A newborn baby cannot walk, feed itself or even lift its head. Compared with most animals, human babies are remarkably helpless. Yet this dependence is not a weakness; it is an evolutionary advantage.