Many human cultures have been polygamous. This photograph, taken around 1900, shows Joseph Smith, founder of the Church of Jesus Christ of Latter-Day Saints, or Mormon religion, with his large family, including his children’s spouses. Smith had multiple wives, a practice eventually banned by the Mormon Church.
OUR EVOLUTION OF LARGE BRAINS AND UPRIGHT posture was needed before we could develop language and art, but it wasn’t enough by itself. Human bones don’t guarantee humanity. Our rise to humanity also required drastic changes in our life cycle.
Every species has what biologists call a life cycle. It is made up of traits such as the number of offspring produced in each litter or birth, the amount of care that the mother or father gives to the offspring, the way adults form social relationships, how males and females select mates, and how long individuals typically live.
We take the human forms of these traits for granted, as if they were normal. But our life cycle is bizarre by animal standards. To mention just a few examples, most animals produce litters much larger than one baby at a time, most animal fathers provide no parental care to their offspring, and few other animals live even a fraction of seventy years, which is not an unusual life span for a human.
Apes share some of these unusual features. Unlike cats, dogs, songbirds, and goldfish, apes usually have one baby at a time, and they live for several decades. In other ways, however, we’re greatly different even from apes. Young chimpanzees are cared for by their mothers, but among humans, most fathers as well as mothers are closely involved in caring for their young. Our infants require a long period of being fed, trained, and protected—a much greater investment of time and energy than ape mothers face. Human fathers who want their children to live and grow up have generally helped their mates raise them.
Our life cycle differs from that ofwild apes in other ways. Human females often live for many years after menopause, the point in their lives beyond which they can no longer have children. This is almost unheard of among other mammals. Humans are unusual in their sexual activities, too. Apes engage in sex publicly, in front of other members oftheir group, and only when the female is ready to bear young. Among humans, sexual activity is usually private, and childbearing is not the only reason for it.
Human society and child-rearing rest on both the skeletal changes mentioned in part 1 and also these remarkable new features of our life cycle. Unlike our skeletal changes, however, our life cycle changes left no fossils. We do know that our life cycle traits have some genetic basis. Among those 1.6 percent of our genes that are different from chimpanzees’ genes and that have any function, a significant part is likely to be involved in shaping our life cycle.
Three aspects of our distinctly human life cycle are explored in the next three chapters. The first is human social organization and sexuality. Next is racial variation, the visible differences among humans native to different parts of the globe. I’ll argue that these differences arose as a result ofthe way we humans choose our mates. Finally, I’ll ask why we grow old and die. Aging is a part of our life cycle that we take for granted: of course everyone grows old and eventually dies. But why do we have to age, when our bodies are able to repair themselves to a great extent?
Here, more than anywhere else in this book, it is important to think in terms of “trade-offs.” In the animal world there’s nothing that’s free or purely good. Everything involves not just benefits but also costs, by using space, time, or energy that could have been devoted to something else. In the framework of evolutionary biology, success is measured in terms of leaving more offspring. As you’ll see in chapter 5, this view of success helps explain why it just wouldn’t pay for us to make the increased investment in self-repair that we would need to live longer lives. The idea of the trade-off also explains the puzzle of menopause: a shutdown of childbearing so that women can leave more surviving children.