AS THE PRIMATE BRAIN grew larger in our early ancestors, infants had to be born more and more immature. Why? Because early hominids walked upright before the brain began its monumental growth. For a biped, there is a limit to how wide the hips can be. If they get too wide, walking on two feet becomes difficult. So as the brains of our ancestors got bigger, there was a strong selection for premature babies. The more fully-developed babies had heads so large that childbirth killed both the mother and child.

But this solution to the hip-width limit (birthing babies prematurely) brought another problem. An immature baby requires more support for more time. A single female on her own, at some point in our evolution, could no longer find or kill enough food to feed her and her offspring by herself. She needed help.

The obvious solution to this problem would be to enlist the male. But our primate relatives are all promiscuous, with fathers giving very little support to offspring, if any. How did humans make the transition to a more monogamous standard? The solution was pure genius, but to really understand how brilliant it was, I want to tell you a true story.

Robert Sapolsky studied baboons for decades. He traveled to Africa and watched the same troop of baboons year after year. He got to know them, and discovered that they are individuals, each different and easily recognizable both by looks and by personality (behavioral traits).

Baboons are promiscuous. When a female comes into heat, she usually mates with quite a few males, which means none of those males knows whether her offspring are his or not. And the males don't help the female much with the raising of her offspring. In fact, Sapolsky found the amount of support was fairly proportional to his chances of being the father. If the female mated with a lot of males, each male offered very little support. The fewer males she mated with, the more support each of those males gave her. And Sapolsky saw one female who devoted herself to a single male, and only mated with him. And what do you know? He was a devoted father.

This female baboon's strategy, it seems, would be one way for our female ancestors could have enlisted their males' help in raising their offspring: Be true blue. A female could be devoted to only one male. But there's a catch, isn't there? How does he know for sure she isn't mating with someone else on the sly? In a baboon troop, they all travel together, hang around together, and eat together for the most part. One male who is spending time with one female can spend all his time with her, so he would know if she mated with another.

But what if the early hominids weren't together all the time? What if males went off in search of food? How could a female prove to her male that she will not mate with someone else (so he will know her offspring are his, so he will lend considerable support)? This is evolution's brilliant solution: The female became hard to please. The female was hard to get. Choosy. Had high standards. Wouldn't "put out." As a matter of fact, the more hoops she makes him jump through (without sending him away) before she has sex with him, the more sure he is that she is not easy. And the more sure he is that she is not easy, the more sure he is that her offspring are his. The end result is: He is very willing to give help to her and her offspring.

In other words, women being hard to get is the evolutionary driver for monogamy. In a way, that one feminine trait is what allowed humans to become intelligent.