On the Time Scale of Human Evolution: Evidence for Recent Adaptive Evolution

Article excerpt

To what extent did the genetic constitution of human population change in the recent past, and how fast are evolutionary changes proceeding in present-day populations? This review summarizes the available evidence about genetic adaptations that evolved after the dispersal of modern humans from Africa about 50,000 years ago and especially since the Neolithic Revolution and the emergence of the first civilizations. Evidence for recent evolutionary change is obtained from the molecular study of individual genes that have been or are currently under selection, and genomic studies that identify genomic regions under recent or ongoing adaptive evolution. The molecular evidence is complemented by archaeological evidence about population dispersals and prehistoric living conditions, and by evidence from historical demography about differential fertility and mortality in historic populations. The conclusion is that evidence for recent adaptive evolution is overwhelming, in many cases at the level of individual genes. There is also evidence that in many cases the selection of adaptive traits occurred in recent historic times. These adaptations evolved in response to climatic conditions, the nutritional changes brought about by the introduction of settled farming, exposure to new diseases, and the social conditions of civilized life. There is evidence that compared to earlier times, human adaptive evolution has accelerated massively since the Neolithic Revolution, possibly by a factor of about 100. Because historic and evolutionary time scales overlap, gene-culture coevolution is emerging as a major unifying theme in anthropological research.

Key Words: Evolution; Adaptation; Race differences; Linkage disequilibrium; Haplotype analysis; Natural selection; Mutation.

According to sociobiological lore, "human nature" evolved in the "environment of evolutionary adaptation" over some millions of years, and remained virtually unchanged since the emergence of anatomically modern Homo sapiens in Africa about 100,000 years ago. The dramatic behavioral changes that occurred in our species after that time are assumed to be produced entirely by cultural evolution, based on the same unchanging genetic foundations across space and time (Tooby & Cosmides, 1992). The assumption is that natural selection cannot produce substantial genetic changes in such a short time. This belief has been reinforced by the theory of "punctuated equilibrium" which claims, based on paleontological evidence, that speciation occurs in rapid bursts that are followed by long periods of stasis (Eldredge & Gould, 1972; Gould & Eldredge, 1993; Pagel et al., 2006). Effectively, Tooby & Cosmides (1992) described Homo sapiens as a species that has remained in stasis for the past 100,000 years.

This belief, which has never been supported by hard evidence, is being challenged by findings in molecular population genetics. This review assesses the evidence for recent adaptive evolution. After providing a general background about selection effects in animals and about the relevant molecular methods, it presents examples of genetic adaptations to new environmental challenges that our ancestors encountered after the recent out-of-Africa dispersal. These challenges included the adaptation to climate zones that had not previously been occupied by anatomically modern humans; adaptations to the nutritional changes brought about by the Neolithic Revolution; adaptations to newly-emerged infectious diseases; and behavioral adaptations to the conditions of civilized life.

The emphasis is on individual genes that have been identified as the targets of recent selection. In other cases, adaptive evolution can only be inferred from the observation of phenotypic change. Further evidence comes from genomic studies that identify selected regions of the human genome without prior knowledge about the biochemical or phenotypic nature of the selected traits. The extent of recent adaptive evolution is of central importance both for our understanding of genetic diversity in modern human populations and for our understanding of gene-culture coevolution. …

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