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.
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.
It’s important to understand what these conclusions—conclusions that are broadly accepted by human geneticists—do and do not mean. They do mean that our species possesses what population geneticists call “population structure”: that is, certain DNA sequences are found at slightly higher frequencies in Africa, others in Asia, and so on. In a sense, it would be miraculous if these frequency differences did not exist. In view of the fact that contemporary groups of people have been separated for a few thousand generations by oceans, deserts, or mountains, it’s inevitable that their gene pools will show some differences, at least statistically. These findings also mean that different groups of people might differ in subtle aspects of their biochemistry. Again, this should come as no surprise. It’s well known, for example, that many Asians blush after drinking alcohol and it’s hardly shocking that this response has a genetic, not cultural, basis.
On the other hand, these findings do not mean that human races necessarily or even probably differ in profound ways. We’re all strikingly similar at the level of DNA sequence—all human races are far more closely related to one another than any is to any other species—and geneticists have to look hard to find sets of DNA sequences that reliably distinguish Asians from Africans, for example. (In fact, the Feldman study showed that 93–95 percent of all human genetic variation is found among individuals within populations, not between them.) Again, this isn’t surprising. The major groups of people have been separated for only tens of thousands of years, a blink of the evolutionary eye and a small proportion of the time over which Homo has evolved. Second, there is no reason to conclude that one race has arrived at a “better” set of genes than another. Each race spent thousands of generations adapting to its environment and it’s hard to see why one race would be particularly adept at this process. Third, these findings do not mean that any existing difference in traits that reliably distinguishes the races has a genetic basis. To take a trivial example, Asians typically speak different languages than do Africans but this fact has nothing to do with differences in genes.
Last, these findings do not mean that human races possess some permanent, essential status. Indeed one of the few things that occur faster in evolution than the appearance of genetic differences between populations is their disappearance once populations begin to interbreed. It’s not entirely clear that Wade appreciates this point and he bizarrely suggests that human beings might one day even speciate, i.e., splinter into truly distinct species that can no longer reproduce with each other. But this outcome is extremely unlikely in an era of airplanes, global trade, and routine marriage between persons of different ethnic origins. Instead, our genetic fate almost surely lies in the opposite direction: the eventual erosion of differences between all groups of human beings. Despite this misunderstanding, Wade’s discussion of the genetics of race is, for the most part, sensible. He makes most of the right distinctions and provides many of the necessary caveats.
Unfortunately, the same cannot be said of his discussion of another controversial topic, the evolution of the human psyche. The transition from African ape to behaviorally modern human involved a good number of psychological and social adjustments and some were surely genetically based (the most obvious is the capacity for language). Along with other writers of recent popular books on human evolution, Wade is fond of evolutionary psychology, the attempt to discern the Darwinian reasons for the evolution of human behaviors and cognitive processes. But in an odd move, he attempts to outdo evolutionary psychologists at their own game. Evolutionary psychology concentrates on the origins of so-called human universals, traits like cooperation or laughter that are shared across all peoples and that therefore plausibly constitute a true human nature. But because Wade concentrates on the last 50,000 years, he frequently finds himself concocting adaptive tales about behaviors or psychological features that arose in the very recent past and that, in some cases, characterize only one or a few groups of human beings. Despite his earlier reliance on hard genetic data, these stories are almost entirely speculative. And they pile up fast.
Here are some examples, taken more or less at random. Wade speculates that dialects may be an adaptation. Because our ancestors lived in a dangerous world in which neighboring tribes warred incessantly—Wade has no patience with those who pretend warfare is a modern aberration—it would have benefited people, he argues, to be able to distinguish those of their own village from those several villages away. Hence dialects. In another story, Wade suggests that the mellowing of Vikings from vicious thugs to benign Scandinavians may have involved changes in genes, not culture. Similarly, he suggests that “the conformity for which eastern societies are known” may be an adaptation to the emergence of a civilization based on rice farming, a type of farming that required centralized control of irrigation systems, not individual initiative. Last but certainly not least, Wade recounts a story in which the intellectual achievements of Jews (in particular, Ashkenazi Jews) might reflect past natural selection for mathematical and verbal prowess, selection that centered on Jews, not Christians, since the latter were barred by the Church’s proscription against usury from the intellectually demanding job of moneylending.5
These adaptive tales (and there are more) suffer from so many problems that it’s hard to know where to begin. For one thing, the evidence that most of these traits has a genetic basis, as required by Darwinism, approaches zero. In some cases a role for genes seems highly unlikely. If Asians, for example, are predisposed genetically to conformity, and not to the “rugged individualism of the west,” why do Asian entrepreneurs so often do well when transplanted to America?
Some of Wade’s stories, moreover, employ questionable logic. Is it really obvious, for instance, that it takes more intellectual firepower to know when to make a loan (and at what rate of interest) than to know when to take out a loan (and at what rate of interest)? Moreover, Wade’s focus on recent human history saddles him with a systematic problem: adaptation by natural selection takes time and many of his stories provide precious little of it, at least by evolutionary standards. The Church’s proscription against usury, for example, was in effect for centuries, not millennia. This might be long enough to yield a perceptible response to natural selection on intelligence but the case is far from obvious.
Finally, Wade has a tendency to blur the distinction between two different processes: the evolution of a single, general cognitive feature and the evolution of many particular ones. Consider again dialects. It’s relatively easy to believe that human beings evolved a general inclination to imitate those around them. It’s much simpler, after all, to mimic successful behavior—how to make an ax, sing a song, or cut a hide—than to reinvent each from scratch. One of the many consequences of such an adaptation is that people might end up speaking like those around them. This seems, at least, a reasonable speculation, although it too is wholly speculative: we have no physical evidence of genetic changes that caused the human brain to favor imitation.
It’s far harder, on the other hand, to believe that dialects were themselves directly chosen by the process of natural selection in order to distinguish friend from foe. (And another thing: Why are there not only spoken dialects but musical dialects? Why do reggae and bluegrass sound different?) Wade’s tendency to lose sight of the difference between general cognitive features and more particular ones leads to an explosion in the number of adaptive tales needed to make sense of people, an explosion that will likely unsettle even some evolutionary psychologists.
In the end, Wade’s undisciplined storytelling spoils an otherwise fascinating survey of the genetic history of human beings. Worse—and Wade seems not to see this—his weakness for adaptive stories runs the risk of undermining his larger project. If one hopes to convince the general reader that biology can at last provide objective insight into the origins of humanity and the reality of race, it would surely help to avoid the sorts of fanciful speculations that have plagued so many earlier scientific attempts to understand our species.
Before the Dawn is almost completely silent on one of the most obvious questions about population genetic studies of human beings, especially human races: Should they be performed? The question is a moral, not scientific, one and much rests on its answer.
Population genetics now provides a set of reasonably powerful statistical tools that allow us to determine whether a gene evolved under Darwinian natural selection.6 In principle, then, one might ask questions like: Do genes that play a role in the brain evolve much faster in certain human races than in others? If so, were the DNA changes involved driven by natural selection? The answers to such questions could clearly be awkward, if not incendiary.7 Wade’s only comment on this issue is to insist that by turning our backs on free inquiry into the human genome, we would “retreat into darkness.” While I tend to agree, the issue is not merely whether refusal of such studies would mean a retreat into scientific darkness, but whether performing them would also mean a march into moral darkness. After all, we rightly forbid scientists from performing all manner of repugnant or disturbing experiments (think of those committed under the Nazis).
The interesting point—and it’s not widely appreciated—is that this question is rapidly becoming moot. Vast quantities of information about the human genome now pour into publicly available databases on a daily basis. These data are collected with the noblest of intentions (often medical) and are also made public for perfectly good reasons: citizens should have ready access to the fruits of publicly funded science. Indeed it’s almost impossible to imagine how one could stop the sorts of studies I described above. In previous times, granting agencies, such as the NIH or NSF, could block funding for undesirable experiments or scientific journals could refuse to publish them. But with genomic data, minimal money is required (an Internet connection is enough) and any bright graduate student working in his parents’ garage could ask and answer any awkward question he likes. And the Internet thoroughly dashes any chance of preventing the publication of unpleasant results.
The reality, then, is that the pace of technological change is rendering irrelevant what is essentially a question of policy. On the other hand, I suspect we have little to fear. Over the next decade or so, discomfiting analyses will surely be performed and bold claims will certainly be made but, in the end, biologists will likely conclude that we are all much more alike than we are different. It’s hard to believe, after all, that in genetics, millions of years of shared evolution don’t count for far more than a few tens of thousands of years of separate history.
September 21, 2006
We are considering the common chimpanzee, Pan troglodytes; the bonobo, Pan paniscus, has a very different, and less violent, social structure. ↩
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. ↩
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. ↩
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). ↩
Wade’s account is based on recent work by G. Cochran, J. Hardy, and H. Harpending, all of the University of Utah, who claim that the moderately high frequencies of several disease mutations, including that causing Tay-Sachs, among Ashkenazi Jews might be explained by natural selection. Because the genes involved also play some role in the brain, Cochran and colleagues speculate that the relevant mutations might increase intelligence, perhaps reflecting a history of selection among medieval European Jews, who often worked in finance or related professions. Such a hypothesis is certainly possible; the critical issue is the strength of the empirical evidence. ↩
Two approaches are common. The first searches for regions where DNA shows little or no variation among individuals. These regions plausibly represent genes that experienced a recent “selective sweep”: a rapid increase in the frequency of an especially fit DNA sequence that drove less fit sequences out of the population. The second approach is subtler and contrasts the kinds of genetic variants found at a gene within a species (e.g., human beings) with those found between species (e.g., human beings and chimpanzees). ↩
This scenario is not entirely hypothetical. See, for example, Antonio Regalado, “Head Examined: Scientist’s Study of Brain Genes Sparks a Backlash,” The Wall Street Journal, June 16, 2006, p. A1. ↩