The ingredient that produced the Great Leap Forward is an archaeological puzzle without an accepted answer. The missing ingredient doesn’t show up in fossil skeletons. It may have been a change in only one-tenth of i percent of our DNA. What tiny change in our genes could have had such enormous results?
Like some other scientists who have pondered this question, I can think of only one good answer: language. Anatomical or physical changes made complex spoken language possible. To understand how such a change could trigger a rapid burst of human innovation, take a look at how apes use language.
Chimpanzees, gorillas, and even monkeys are capable of symbolic communication, in which a symbol or a sound represents something else—such as a picture of an apple representing a piece of fruit, or a particular cry meaning “snake!” As we will see in chapter 6, apes have learned to use sign language, plastic symbols, and computers to communicate. Some have mastered “vocabularies” of hundreds of symbols. And wild vervets (or green monkeys) have a natural form of symbolic communication based on grunts, with slightly different grunts to mean “leopard,” “eagle,” and “snake.” If these primates are capable of symbolic communication, why have they not gone on to develop much more complex natural languages of their own?
The answer seems to involve the structure of the larynx (or voice box), the tongue, and all the muscles that give us fine control over spoken sounds. Our ability to speak depends on the perfect functioning of many structures and muscles. If, like apes, we could utter only a few consonants and vowels, our vocabularies would be greatly reduced. The missing ingredient that finally made us fully human could well have been some changes to the protohuman vocal tract—changes that gave us finer control and let us make a much greater variety of sounds. Such small changes to muscle and soft tissue need not show up in fossil skulls.
It’s easy to see how a tiny change in anatomy, creating the capacity for speech, would produce a huge change in behavior. With language, it takes only a few seconds to communicate the message, “Turn sharp right at the fourth tree and drive the male antelope toward the red boulder, where I’ll spear it.” Without language, two protohumans could not brainstorm together about how to organize a hunt or invent a better tool. Without language, even one protohuman would have had difficulty thinking about inventing a better tool.
I don’t suggest that the Great Leap Forward began as soon as the mutations for changes in the larynx and tongue appeared. Even with the right anatomy, it must have taken humans thousands of years to develop the structure of language as we know it. But if the missing ingredient did consist of changes that permitted fine control of sounds, then the capacity for innovation would have followed eventually. The spoken word made us free.
Until the Great Leap Forward, human culture had developed at a snail’s pace for millions of years. That slow pace was dictated by the pace of genetic change. Our culture and behavior changed only when a mutation occurred to give rise to the change.
After the Great Leap, cultural development no longer depended on genetic change. People could think, innovate, and communicate in a new way. They could pass ideas and knowledge on to other groups and to the next generation. Even though our anatomy has barely changed over the past sixty thousand years, human culture has evolved far more since the Great Leap than in the millions of years before.
PART TWO
A STRANGE LIFE CYCLE
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.
CHAPTER 3
HUMAN SEXUALITY
THE HUMAN LIFE CYCLE INCLUDES SEXUALITY and family life. It is not always easy, though, to investigate these subjects. One problem is that, where people’s sexual behavior is concerned, there are limits to the scientific approach. We can’t do controlled experiments the way we can with diet or teeth-brushing habits. Another is that the subject can be a sensitive one. Scientists did not begin to study human sexuality seriously until recently, and it still can be difficult to view the subject with a scientific attitude.
Most people regard their relationships with loved ones—family relationships such as parent-child ties, and also romantic and sexual relationships—as deeply meaningful, personal, and pr
ivate. It can seem cold-blooded, even harsh, to put those relationships under a microscope, so to speak, and look at them with a scientist’s eye. Some people might even be offended to see their interactions with the people in their lives compared with the childrearing behavior of apes or the courtship habits of birds.
As you read this chapter and those that follow, keep two important points in mind. First, we are looking at the human life cycle through the particular framework of evolutionary biology. It does not necessarily explain everything about why people do the things they do—it is simply one of many tools that can help us understand ourselves. Second, our focus is on the entire human species, not on specific examples. There are always many exceptions to every rule, and many people behave very differently than science might predict. Our concern is with general trends, not with people as individuals.
In spite of the challenges of studying human sexuality, we are beginning to understand how it is intertwined with other human characteristics, such as tool use, large brains, and child-rearing practices. Our shift from being just another species of big mammal to being uniquely human involved changes not just in our bones and skulls but also in our family lives and sex lives.
Food and Family Life
To understand how human sexuality got to be the way it is, we have to understand the evolution of our diet and our society. From the vegetarian diet of our ape ancestors, we separated within the last several million years to eat meat as well as plant foods, yet our teeth and claws remained those of apes, not of tigers. Instead of on teeth and claws, our success in hunting depended on large brains. By using tools and hunting in organized groups, our ancestors could hunt, and they regularly shared food with one another. We started using tools to gather roots and berries as well, so even the vegetarian part of our diets required large brains.
Human children took years to acquire the information and experience they needed to be efficient hunter-gatherers, just as today they take years to learn how to be farmers or computer programmers. For many years after they are weaned—after they stop nursing on milk from the mother’s body and start to eat food—they are too ignorant and helpless to take care of themselves. Human children depend on parents to bring food to them. This seems natural to us, but it is exceptional in the primate world. Baby apes gather food for themselves as soon as they are weaned.
Human infants are terrible food gatherers for two reasons. One reason is mechanical. Making and using the tools needed to find food requires fine finger coordination that children take years to develop. Just as my sons couldn’t tie their shoelaces when they were four years old, four- year-old hunter-gatherer children can’t sharpen a stone axe or build a dugout fishing canoe.
The second reason is mental. More than other animals, we depend on brainpower when we look for food, because we have a much more varied diet and more complicated food-gathering techniques. New Guineans whom I work with typically have separate names for about a thousand different plant and animal species in their vicinity. For each species, they know something about where to find it, whether it is edible or useful, and how to capture or harvest it. All this information takes years to acquire.
Weaned human infants not only need adults to feed them; they need adults to teach them for a decade or two. As with so many human hallmarks, these needs occur in other species. Young lions, for example, must be taught how to hunt by their parents. Chimpanzees, like humans, have a varied diet and use a number of techniques to obtain food. Chimpanzee parents do help their young find food, and common chimps also make some use of tools, although bonobos do not. But for humans, the necessary survival skills, and the burden on the parents, are much greater than for lions or chimps.
Family life in the species Homo sapiens is organized around a simple fact: human children cannot fend for themselves at birth or for a long time afterward. They need parental care, not just for food, shelter, and protection, but to learn the skills they will need to survive in society.
That parental burden means that care by the father as well as the mother is important if the child is to survive. Orangutan fathers provide their offspring with nothing. Gorilla, chimpanzee, and gibbon fathers do more, providing some protection for their young. Human hunter-gatherer fathers do even more, providing some food and much teaching. Our complex food-gathering habits require a social system in which a male has a longterm relationship with a female, so that he can help rear their child. Otherwise the child will be less likely to survive, and the father will be less likely to pass on his genes.
A Social System That Meets Our Needs
The orangutan system, in which the father simply departs as soon as he and his female partner have mated, wouldn’t work for us. The chimpanzee system also wouldn’t work for us. Among chimpanzees, a female who is ready to be fertilized and become pregnant is likely to mate with several adult males within a short time. As a result, a chimpanzee male has no idea which infants in the troop he has fathered. This is no big loss to a chimp father, because males don’t do a great deal for the troop infants. A human father, though, spends a lot of time and energy on the care of his child. From the point of view of evolution, a human male had better have some confidence that the child is his, or his child care contributions may help pass on some other man’s genes.
Confidence about fatherhood would be no problem if humans, like gibbons, were scattered across the landscape in isolated couples, so that a female almost never saw a male other than her mate. Almost all human populations, however, have consisted of groups of adults. Hunting and gathering often involve cooperative group efforts among men, women, or both. Groups also offer protection against predators and enemies, especially other humans.
To meet our need for both confidence in fatherhood and group living, humans evolved a social system that seems normal to us, although it is strange by ape standards. Adult orangutans are solitary. Adult gibbons live as solitary male-female pairs. Gorillas live in harems consisting of several adult females and usually one dominant adult male. Common chimpanzees live in communities of scattered females plus a group of males, in which individuals mate with more than one partner. Pygmy chimps, or bonobos, form colonies of both sexes that are even more promiscuous, meaning that individuals have multiple sex partners.
Human societies resemble none of these primate societies. Like our food habits, our social system resembles more that of lions or wolves. We live in bands containing many adult males and females. Among lions, however, any male can and does mate with any female, meaning that the fatherhood of lion cubs is unknowable. Among humans, males and females are paired off with each other. The closest thing to our social system in the animal world is large colonies of seabirds, such as gulls and penguins, which also organize into male-female pairs.
Officially, at least, in most modern political states human pairing is more or less monogamous, meaning that each individual has a single partner. Among hunter-gatherer bands, which are better models for how humans lived over the last million years, most men can support only a single family, but a few powerful men have several wives. The huge harems some human rulers have maintained weren’t possible until the rise of agriculture and centralized government let a few princes tax everyone else to feed the royal harem’s babies.
Why Men Are Bigger Than Women
Adult men are, on average, slightly bigger than women of the same age. Although there are many individual exceptions, across a whole population men weigh about 20 percent more than women and are about 8 percent taller. Why is this the case? The answer lies in our social and sexual organization.
The typical hunter-gatherer social system, in which most men have one partner but a few men have several wives, can be called “mildly polygynous.” (Polygynous means “with multiple wives.”) Because humans were hunter- gatherers for many millennia before the rise of agriculture, that particular social organization explains why men are bigger than women.
Among polygynous mammals, the average difference in size between males and females is
related to the number of females that mate with a single male, and only with that male. The more females in a male’s harem, the greater the size difference between the sexes. The biggest harems are seen in species with males much larger than females. Three examples from the animal world show how this works.
Surrounded by his harem of smaller females, a large male elephant seal basks on the not-so-balmy beach of King George Island, Antarctica.
Gibbons are monogamous. Each individual has only one partner. The male gibbon has no harem, and there is no average size difference between the sexes. Males and females are the same size. Male gorillas, on the other hand, typically have harems of three to six wives. This is reflected in a size difference: a male weighs about twice as much as a female. The average harem of a southern elephant seal is forty-eight females. With such a big harem size, you would expect a big difference in size between the sexes, and you would be right. A three-ton (six-thousand-pound) male seal dwarfs his seven-hundred-pound wives.
The explanation is that in a monogamous species, every male has the opportunity to win a female. In a very polygynous species, such as the elephant seal, many males go without mates, because a few dominant males have succeeded in rounding up all the females into their harems. The bigger the harem, the fiercer the competition among males—and the more important it is for a male to be big, because the bigger male generally wins a fight.
We humans, with our slightly bigger males and slight polygyny, fit this pattern. At some point in human evolution, though, male intelligence and personality came to count for more than size. Big men don’t tend to have more wives than smaller ones.
Our Unusual Sex Lives
Human sexual activity is freakish by the standards of other mammals. For one thing, most mammals are sexually inactive most of the time. They copulate, or engage in sex, only when the female is estrous. That means that she has entered estrus, the part of her biological cycle when she is ovulating—her ovaries are preparing to release an egg. During this time she can be fertilized and become pregnant.