The 10 , 000 Year Explosion : How Civilizations Acceler - ated Human

In the early 1970s David Frayer, who was to be my first Ph.D, was looking for references to evolution within early modern humans and was surprised to find that at the end of David Pilbeam’s new book reviewing human evolution, The Ascent of Man (1972), there were none. The book ended with the first appearances of (what Pilbeam considered) the first modern humans. This was conventional wisdom; when modern humans appeared, evolution was essentially over. However, Frayer (1976) did not believe it. He researched this issue and subsequently wrote his dissertation examining patterns and rates of dental change in upper Paleolithic and Mesolithic populations of Europe. Later, he showed that the rates of dental change within the upper Paleolithic were greater than the rates of change between the Middle and upper Paleolithic (i.e., between European Neandertals and post-Neandertal populations, reviewed in Frayer, 1992). Frayer (virtually alone) had found that the rate of human evolutionary change had accelerated in the Late Pleistocene, but it turns out that even he really underestimated how much so. In their highly accessible book, Cochran and Harpending show how much so, explaining the genetic basis for accelerating human evolution, and illustrating it with some—let’s say—unexpected examples. Like many real advances in science, the prerequisites for their insight have long been before us, but only recently did the pieces fit together (Hawks et al., 2007). Their thesis is that human civilization greatly accelerated increases in the size of the human population. More people mean more mutations and thereby more opportunity for evolutionary change under selection because some mutations are favorable. The spread of rapidly expanding populations eventually outpaced the spread of favorable mutations under selection in those populations, so for the first time in human history favorable mutations could not disperse through the human species. Why was this unexpected? The relation of number of mutations to population size has been known for a very long time, although there is been a certain amount of confusion about the suspension of selection when population size is expanding. The significant recent increase in human population size was established by archaeologists some time ago (c.f. Hassan, 1981), even the basic shape of the pattern of population increase—an exponential curve virtually flat for most of human prehistory and virtually vertical today—was well understood. But until now the link between a recent very large population increase and accelerated evolutionary change because of many favorable mutations was below the radar screen and took the discovery of the mutations themselves to establish. It was not obvious because of the two reasons why evolutionary stasis is still conventional wisdom. First, stasis is expected from the widely believed theory that modern humans appeared recently as a new species that replaced archaic species everywhere because they were better adapted. Second, stasis follows from the widely accepted belief that the technological innovations of the Upper Paleolithic and beyond, and the consequences of complex social organization that followed the Neolithic, relaxed selection (Brace, 1995), even though the association of domestication and disease evolution was well established (Cohen and Armelagos, 1984). Cochran and Harpending bring this together in a highly readable form, starting with their own take on the conventional wisdom that asserts the evolutionary process stopped when modern humans appear. They rely on quotes from the elders, in this case Ernst Mayr, and Stephen Gould who explicitly wrote: ‘‘there’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain’’ (p. 1). They begin their discussion of ‘‘how wrong this is’’ with the Neandertals and their understanding and presentation of the Neandertals’ significance and fate is quite good. These authors ‘‘get it’’ when so many paleoanthropologist specialists don’t seem to—the significance of Neandertal genetic contributions to the modern gene pool is found in the importance of the genes that persisted, not in their quantity. This reflects the clear understanding that natural selection is the central driving force of human evolution, one of the key insights and most constant themes of this book. But for natural selection to have a chance there needs to be favorable mutations, or favorable combinations of existing alleles. Cochran and Harpending concentrate on the former, and focus on the Neolithic as the beginning of the significant population size expansions that provide the mutations to accelerate genetic change. Infectious diseases are a well-known consequence of the Neolithic and a new source of selection, but the origins of many more genes have now been traced to this period, with effects such as regional differences in skin color and skeletal gracility. More controversial speculations are about mental functions and behavioral innovations. By 5000 years ago the authors estimate that adaptive alleles were coming into existence at a rate about 100 times faster than during the Pleistocene. This is the ‘‘explosion’’ of the book’s title. Yet, the most interesting aspect of this book is what follows—the interpretation of the subsequent history of (for the most part) Africa and Europe in a biological framework that emphasizes evolutionary change. The recent history of so many genes is now so accurately known that in many ways this is no longer an exercise in speculation, but one of interpretation. ‘‘Recent studies have found hundreds of ongoing [selective] sweeps—sweeps begin thousands of years ago that are still in progress today. Some alleles have gone to fixation, more have intermediate frequencies, and most are regional. Many are very recent . . .’’ (p. 75). If natural selection is a key ingredient of the historic overview (from the genetic perspective), surely gene flow is another. From the progeny of Genghis Khan on, ‘‘history is full of examples expanding at the expense of their neighbors’’ (p. 155). Important biological aspects of dispersions and replacements are found in the role of selection. Perhaps the biggest example is the role of European infectious diseases, a consequence of domestication and urbanization, in the political domination of native populations AMERICAN JOURNAL OF HUMAN BIOLOGY 22:137–142 (2010)