From the 1980s, DNA evidence has become almost as important as fossil and archeological evidence in understanding human evolution. In the coming decades it is likely to become hugely more important, especially in tracing the human mind's origins. This is because evolved mental capacities depend on genes, even when they leave no fossil or archeological records. After the Human Genome Project identifies all 80,000 or so human genes in the next couple of years, we can look forward to three further developments that will allow much more powerful tests of my theory and other theories of mental evolution.
Neuroscientists will start to identify which genes underlie which mental capacities, by analyzing the proteins they produce, and the role those proteins play in brain development and brain functioning. (Of course there is no single gene for language or art—these are complex human abilities that probably depend on hundreds or thousands of genes.) Behavior geneticists will also identify different forms of particular genes that underlie individual differences in mental abilities such as artistic ability, sense of humor, and creativity. Psychologist Robert Plomin and his collaborators have already identified the first specific gene associated with extremely high intelligence (a form of the gene labelled "IGF2R" on chromosome 6). Very little such work has been done so far, but the genes that underlie our unique human capabilities will be identified sooner or later, and evolutionary psychology will benefit.
Also, geneticists will find out more about which genes we share with other apes. Research centers in Atlanta and Leipzig are already pushing for the development of a Chimpanzee Genome Project. Since 1975, geneticists have been using a method called DNA hybridization to show that our DNA is roughly 98 percent similar to that of chimpanzees (compared to only 93 percent with most monkeys). However, this method is fairly crude, and we will not know exactly which of our genes are unique until the results of the Chimpanzee Genome Project can be compared to those of the Human Genome Project. Geneticists already know there are some significant differences: humans have 23 pairs of chromosomes whereas other apes have 24 pairs, and the genes on human chromosomes 4, 9 and 12 appear to have been reshuffled significantly compared with their arrangement on the chimpanzee chromosomes. There are plenty of genetic differences to account for our distinctive mental capacities, and the more we know about the unique human genes, the more we can infer about their evolutionary origins and functions.
Finally, it may be possible to recover more DNA from our extinct fossil relatives. DNA decays fairly quickly, and it is very hard to recover DNA from fossils older than about 50,000 years ago (Jurassic Park notwithstanding). However, Neanderthals survived until about 30,000 years ago, and a German team led by Svante Paabo has already succeeded in recovering a DNA fragment from a Neanderthal's arm bone. This fragment, just 379 DNA base pairs long, showed 27 differences compared with modern humans, and 55 differences compared with chimpanzees. This substantial difference between humans and Neanderthals suggests that our lineages split apart at least 600,000 years ago—
much earlier than previously thought. It also shows that humans did not evolve from Neanderthals. Potentially, the same techniques could be applied to Homo erectus specimens from Asia, which also persisted until about 30,000 years ago, but which split off from our ancestors even earlier. It might even be possible, at some future date, to show which other hominids shared the genes underlying our apparently unique mental abilities. For example, if Neanderthals are found to share some of the same genes for language, art, music, and intelligence that modern humans have, then we could infer that those capacities evolved at least 600,000 years ago. Although behavior does not fossilize, some of the DNA underlying behavior does, and it can sometimes last long enough for us to analyze.
The DNA revolution will unveil many more aspects of human evolution and human psychology. I cannot yet show you the many genes that must underlie each of the human mental adaptations analyzed in this book. However, the genetic evidence that will emerge in the coming years will probably render my ideas—even the apparently most speculative ones—fully testable in ways I cannot anticipate. My sexual choice theory sometimes sounds as if it could explain anything, and hence explains nothing. This overlooks the fact that biologists are developing ever more sophisticated ways of testing which adaptations have evolved through sexual selection, and many of these methods—including a range of new genetic analyses—can be applied to human mental traits. Indeed, one goal of this book is to inspire other scientists to join me in testing these ideas.
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