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Talking Genes

Within the world of popular science the subject of human evolution never goes out of fashion. While formerly one could complain that the number of new books on this topic bore little relation to the volume of new discoveries—a stray skull unearthed in Tanzania might unleash four books—this is no longer true. The growing literature on human origins reflects a real and substantial increase in our understanding of the biological history of human beings. Much of this improved understanding derives from new genetic data that allow us to date important evolutionary events and, in some cases, to trace the actual geographic routes traveled by early peoples over the earth.

The latest book to summarize the state of our understanding of human evolution comes from the science journalist Nicholas Wade, a reporter for The New York Times who covers biological and medical developments. Wade has also worked for the prestigious journals Nature and Science and has written a number of previous books, including The Nobel Duel (1981), about the competition over the 1978 prize for work on brain hormones, and Betrayers of the Truth: Fraud and Deceit in the Halls of Science (with William Broad, 1982). Wade’s latest work, Before the Dawn, is a broad survey of human evolution for lay readers which considers the emergence of man in his entirety: physical, psychological, and social. Unlike most popular treatments of human biology, Wade’s book concentrates on the recent evolutionary past: our last 50,000 years. Also unlike many popular treatments, Before the Dawn emphasizes genetic over paleontological evidence. In Wade’s story, it’s the genes, not the bones, that do most of the talking.

Before the Dawn suffers from some of the problems that mar many popular books on biology—misplaced cheerleading for big science (Wade credits the human genome project with findings that had nothing to do with that endeavor), lapses into breathless prose (“Some mysteries lie beyond the power even of DNA to resolve”), and unnecessary extended quotations from Charles Darwin (it helps if one’s views appear endorsed by the Master). But the book is, on the whole, a fascinating account of recent scientific findings. Wade is an especially skillful narrator and his recounting of the twists and turns of early human history is superb. He sketches the many physical and social changes that occurred as an African ape morphed into Homo sapiens, describes our species’s departure from Africa, and chronicles our subsequent migrations to different parts of the planet.

Before the Dawn is by design, though, more than a narrative. It is a book with a purpose, and a book that goes out of its way to court controversy. Wade is certain that natural selection shaped many aspects of human nature and that some of the genetic changes underlying the psychological adaptations he describes occurred surprisingly recently. Wade is plainspoken and he has a number of provocative things to say about the genetic basis of both human nature and race. Indeed he so enjoys playing the provocateur that I sometimes found myself resisting being provoked. In the end, his controversial claims are oddly mixed: some are both surprising and likely true, while others are simply silly. More than any other recent book on human evolution, Wade’s requires careful picking and choosing on the part of the reader. Reject the book wholesale and you reject important truths; embrace it wholeheartedly and you embrace a good deal of nonsense.

1.

Although they’re our closest relatives—chimps and humans split from their common ancestor in Africa a mere five to six million years ago—chimpanzees are very different from human beings. Chimpanzee males are larger than females and the two sexes have little to do with each other apart from procreating.1 Instead, males are concerned day to day with their status in a continually contested hierarchy of dominance, while females struggle for rank within their own pecking order. Despite previous claims to the contrary, chimpanzee life is almost unimaginably brutish. Small bands of males ambush stray chimps from neighboring groups, often savagely killing them. Females, for their part, are less violent, though hardly paragons of good manners; they have been known, for example, to consume the offspring of other females. Chimpanzees do not speak sentences, they do not produce art, and they do not build cities.

There’s good reason, Wade argues, to believe that the common ancestor of chimps and people was more chimp-like than human. If so, the big question in human evolution is: How did we get here from there? The answer, Wade says, involves a set of morphological and cognitive transformations, some of which can be ordered in time.2

The first significant step toward humanity occurred surprisingly soon after our split from chimps. About 4.4 million years ago, our ancestors began to walk on two feet. The importance of this event had less to do with feet and more with hands, which were then freed to do more than knuckle-walking. The second key step built on the first. About 2.5 million years ago, our ancestors began using tools. Not coincidentally, at about the same time the human lineage initiated what would become a defining trend of the genus Homo: the evolution of progressively bigger brains. The third key transformation was subtle but profound. About 1.7 million years ago, the difference in size between males and females shrank. This event, Wade argues, might well mark the collapse of distinct male and female dominance hierarchies and their replacement by a stable bond between males and females of the sort that now characterizes human beings.

By 100,000 years ago or so, creatures that looked essentially like you and me—so-called anatomically modern humans—roamed Africa. According to Wade, these proto-humans were not, however, yet behaviorally modern. They did not produce art, for example, or live in large communities. Then, roughly 50,000 years ago, things changed. Sophisticated tools made of antlers, ivory, and bone appear in the archaeological record.3 So do impressive new weapons, such as improved spears capable of bringing down large animals. At about the same time, both flutes and a kind of primitive jewelry appear, as does evidence of ritualistic burial of the dead. Something significant seems to have happened at about this time and in Wade’s account, which largely echoes that of the paleoanthropologist Richard Klein, that something was likely the emergence of language. The appearance of true language—featuring a complex syntax and capable of conveying detailed information—allowed, he says, the emergence of a rich human culture. Wade suggests that language evolved in a group that he calls the ancestral human population. This population lived in Africa, was the ancestor of all present human beings, and, on at least one occasion, barely escaped extinction.

Although we’re all derived from the ancestral human population, we do not all live in Africa. The reason is that—again somewhere around 50,000 years ago—a group of human beings left East Africa, beginning the long process of peopling the earth. From here on, Wade’s story relies increasingly on genetic evidence. Two types of data have proved particularly valuable in reconstructing the recent evolutionary past. One involves variations in DNA sequences that reside on the Y chromosome, which is transmitted solely from fathers to sons. The other involves variations in DNA sequences that reside in mitochondria, cellular bodies that are transmitted solely from mothers to children. Because both the Y chromosome and the mitochondria are, unlike our other chromosomes, passed through one sex only, they show simple patterns of inheritance that make it easy to determine which populations are most closely related to which others.4 If, for example, two populations differ at a single site in a DNA sequence on their Y chromosomes, the two populations probably split from each other quite recently. If another pair of populations differs from each other at five such sites, they likely split from each other longer ago. Using this logic, geneticists have pieced together a reasonably coherent picture of which human populations descended from which others.

These kinds of genetic studies—part of the scientific discipline called population genetics—suggest two key findings about early humans. The first is that the exodus from Africa involved a modest number of people. Modern African populations harbor much more genetic variation than do modern non-African ones, implying that the migrants who departed from Africa were few enough in number that they took relatively little African genetic variation with them. Indeed, the exodus may have involved as few as 150 people. Second, genetic data say something about the likely routes taken by human beings as they abandoned Africa for new worlds. Wade claims, for example, that people may have left Africa via the southern tip of the Arabian peninsula, a route that would have required crossing the waters of the Gate of Grief at the narrowest point of the Red Sea. Remarkably, then, human beings may have departed from Africa by boat. (People could clearly cross vast stretches of open water by about 45,000 years ago when human beings arrived at the geological equivalent of modern Australia.)

It also seems likely, Wade argues, that travelers hugged the coast as they then made their way eastward to India. Somewhat later, humans began to radiate in different geographic directions, with some lineages moving east into present-day Australia, Japan, and China, and others veering northwest into present-day Iran and Turkey, and eventually populating large parts of Europe. Much later, migrants crossed from Siberia into the New World, ultimately peopling North and South America. And permanent human settlements appeared surprisingly recently in our history—perhaps 15,000 years ago.

Any discussion of the migration of early peoples into distant lands leads inevitably to a discussion of race and Wade does not duck the issue. He appreciates, of course, that scientific discussion of race is, for good reason, sensitive and often suspect. We all know that much evil has been committed in the name of various crackpot theories of race. But it does not follow that racial differences do not exist or that science can say nothing sensible about them. Wade’s discussion of the reality of race focuses on recent surveys of DNA sequences among people belonging to various human populations. Perhaps the best of these was performed by Marc Feldman of Stanford University and his collaborators. Wade summarizes their work:

Feldman and his colleagues looked at 377 sites throughout the genome [i.e., the complete set of chromosomes of the people they studied]…. This was done for each of 1,000 people from 52 populations around the world. A computer was then instructed to group the individuals, based on their DNA differences at the 377 sites, into clusters. They fell naturally into 5 clusters, corresponding to their five continents of origin—Africa, western Eurasia (Europe, the Middle East, the Indian subcontinent), East Asia, Oceania and the Americas.

Wade draws two conclusions from this and similar studies: human races are real and they correspond reasonably well to our folk distinctions between peoples from different continents.

  1. 1

    We are considering the common chimpanzee, Pan troglodytes; the bonobo, Pan paniscus, has a very different, and less violent, social structure.

  2. 2

    The narrative that follows is based on Wade’s account. As he acknowledges, many of the details are hotly debated by experts and some pieces of the story will likely prove wrong. But the story represents his attempt to arrive at a synthesis derived from a contentious literature.

  3. 3

    Although the clearest evidence of this cultural transformation is found in Europe, and dates to about 45,000 years ago, Wade argues that the shift itself likely began in Africa. He further concludes that these cultural artifacts were likely the handiwork of modern humans, not of a Neanderthal lineage that had left Africa earlier. Wade’s narrative focuses on modern humans, not on Neanderthals, since only human beings gave rise to present-day people; Neanderthals were a genetic dead end.

  4. 4

    More specifically, neither the Y chromosome nor mitochondria undergo recombination, in which genetic material from one’s mother and father are swapped during the production of sperm or eggs. As a result, Y chromosomal and mitochondrial DNA are inherited as intact blocks from one generation to the next. Although mitochondria are passed by mothers to both sons and daughters, only the daughters will, in turn, pass their mitochondria on to their children. For a fascinating account of the biology and evolution of mitochondria, see Nick Lane’s recent book, Power, Sex, Suicide: Mitochondria and the Meaning of Life (Oxford University Press, 2005).

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