Evolution by natural selection is generally understood as a slow, ratcheting process in which major changes in the genes of a species can take millions of years to manifest in any significant way. However, under certain circumstances, evolution can be quite rapid, especially when major changes in a species' environment cause mass die-offs. In the case of humans, the current title holder for fastest evolution goes to the highland Tibetans, who in less than three thousand years underwent a substantial genetic change that allowed them to adapt to the thin air of the environment to which they had migrated.
High Altitudes and the EPAS1 Gene
In a study conducted jointly by researchers at the University of California at Berkeley and the Beijing Genomics Institute, DNA samples were gathered from fifty highland-living Tibetans, forty lowland-living Han Chinese, and 200 Danes to act as a control group. At issue was the presence of a particular allele of the EPAS1 gene, which regulates the human body's response to oxygen deprivation. In members of the general population with the common version of the EPAS1 gene, the body tries to compensate for low oxygen by producing more hemoglobin and red blood cells, causing blood thickening, headaches, fatigue, and general altitude sickness, as well as higher infant mortality.
However, highland Tibetans with two mutated versions of the gene deal with high altitudes easily and efficiently, without producing more hemoglobin. Researchers still don't understand exactly how the mutated allele functions to compensate for oxygen deprivation, but the benefit of the mutation among highlanders is obvious; among modern Tibetans in the study, 87% possessed the allele, in contrast to only 9% of lowland Han Chinese and almost none of the Danes.
Population Bottleneck
Tracing the genes of the subjects, researchers were able to estimate that modern Tibetans are likely descended from a small group of Han Chinese who migrated to the highlands about 2,700 years ago. Since the high-altitude allele is nearly universal among highlanders today, it is also very likely that at some point there was a population bottleneck; in other words, shortly after the migration, many of the group would have died off, leaving only those who were best able to deal with the thin air. These individuals passed their tolerance on to the next generation, and eventually the helpful mutation spread to almost the entire population.
Other Rapid Human Evolution
Until this recent finding, the fastest evolution seen in humans was among Europeans who developed the ability to digest lactose over the course of about 7,500 years, concurrent with the rise of dairy farming. This example of natural selection acting rapidly upon a population, as well as the even more rapid changes undergone by the Tibetans, demonstrate that given the right circumstances, evolution can be anything but a slow process.
Source:
Hsu, Jeremy. "Tibetans Underwent Fastest Evolution Seen in Humans." LiveScience. 1 July 2010. Web. 23 Mar. 2011. www.livescience.com
Jenny Ashford - Jenny Ashford is a writer and graphic artist from central Florida. Her main area of interest in her Suite 101 articles is science, with a ...
Join the Conversation