Tibetans Evolved Altitude Tolerance in 3,000 Years
Tibetans and other peoples who live at high altitudes possess a remarkable tolerance to the thin atmosphere. Now, scientists at UC Berkeley have identified some 30 genes related to oxygen regulation that differ in Tibetans from Han Chinese. Since those tribes are thought to have diverged 3,000 years ago, natural selection for these changes must have occurred in that time.
PhysOrg reported the findings that were published in two papers in Science.1,2 According to Rasmus Nielsen, lead author of one of the studies, “You look for rapid evolution in genes because there must be something important about that gene forcing it to change so fast.” That’s why they were keen to find genetic differences between the 50 Tibetans and 40 Han Chinese whose genomes they compared. Surprisingly, Nielsen said that “The new finding is really the first time evolutionary information alone has helped us pinpoint an important function of a gene in humans.” The way evolutionists talk about their theory as the key to biology, one would think thousands of studies would have preceded this one.
New Scientist called this a “remarkable” case of adaptation at “a record-breaking rate.” They quoted Nielsen calling it “the fastest genetic change ever observed in humans.” Still, it is not clear what evolution was doing. The genetic differences do not result in a different blood concentration of hemoglobin, the protein that carries oxygen to the tissues, from the values for people living at sea level. Jay Storz [U of Nebraska], commenting on the papers in the same issue of Science,3 spoke of the mutations as representing possible “candidate” targets of selection, but that the functional significance of the changes has not yet been established:
In the case of hemoglobin concentration in Tibetans, for example, selection has not favored a trait value outside the ancestral range of variation. Instead, selection appears to have favored a blunted erythropoietic response such that hemoglobin concentration at high altitude is maintained at the sea-level status quo. Although the mechanism has yet to be elucidated, it appears that regulatory changes in EPAS1 and other HIF-related genes have recalibrated the set point for hypoxia-induced erythropoiesis in Tibetans. Andean highlanders have not evolved a similar mechanism for attenuating the erythropoietic response to hypoxia, possibly because of their shorter history of residence at high altitude.
It remains to be seen whether hemoglobin concentration represents the direct phenotypic target of selection in Tibetans, or whether changes in hemoglobin concentration represent an ancillary effect of selection on some other physiological trait that is altered by regulatory changes in the HIF cascade. These studies of Tibetan highlanders provide compelling proof of principle that the integration of population genomics and association studies can successfully identify targets of recent positive selection.
The authors of the two primary papers similarly spoke of “putatively advantageous genes” but did not establish actual functional advantages they confer on Tibetans living in oxygen-poor, high-altitude environments. Storz thus raised two questions: (1) are the genetic changes related to Tibetans’ altitude tolerance, or just ancillary effects of selection for other traits? Neutral genetic drift should be ruled out. (2) If these changes are functional in Tibetans, how do the Andeans tolerate high altitudes without similar genetic changes?
It should be noted that it’s difficult to identify genes under selection pressure. Scientists sometimes assume that changes are targets of selection without checking to see if those changes produce functional advantages for the organism in its environment. Last year, three researchers indicated that many studies for “positive selection” are based on flawed methods and statistics (03/30/2009).
A previous study did not link Tibetan altitude tolerance to genes for hemoglobin, but rather to nitric oxide levels (see 10/31/2007, bullet 4). Furthermore, it appears that only regulatory changes to existing genes, not new genetic information, may be involved. A more thorough study would compare comparable genes for other mammals, birds, and reptiles at high altitude with their sea-level counterparts, and see whether gradations exist for animals at mid-altitudes. Before one can claim, therefore, that these studies represent “the first time evolutionary information alone has helped us pinpoint an important function of a gene in humans,” the researchers need to get beyond “proof of principle” and show whether natural selection accomplished anything at all.
1. Yi, Lyang et al, “Sequencing of 50 Human Exomes Reveals Adaptation to High Altitude,” Science, 2 July 2010: Vol. 329. no. 5987, pp. 75-78, DOI: 10.1126/science.1190371.
2. Simonson, Yang et al, “Genetic Evidence for High-Altitude Adaptation in Tibet,” Science, 2 July 2010: Vol. 329. no. 5987, pp. 72-75, DOI: 10.1126/science.1189406.
3. Jay F. Storz, “Evolution: Genes for High Altitudes,” Science, 2 July 2010: Vol. 329. no. 5987, pp. 40-41, DOI: 10.1126/science.1192481.
Natural selection may be responsible for the changes, or it may not be. Researchers assume that mutations are targets of selection. Scientific rigor demands that those genetic changes actually be linked to adaptive changes. Darwinian theory would require that the adaptations allow for survival of the fittest – i.e., that individuals lacking the mutations die out. The study found that only 87% of the Tibetans had one of the leading candidate mutations. How do the remaining 13% breathe and live up there? And why did 9% of the Chinese have the mutation, if they were not under selection pressure by living at high altitude? Furthermore, if this “fastest genetic change ever observed in humans” represents evolutionary progress, why don’t the people in the Andes have it? How do they live and breathe at their high altitudes? Responding that the lack of genetic changes is “possibly because of their shorter history of residence at high altitude” is a cop-out. Evolution can be as fast as evolutionary biologists need it to be to fit their story.
Despite the hype in the headlines, there has been no demonstration of a Darwinian evolutionary adaptive change here. But even if there were, creationists would have no problem accepting it. Creationists accept natural selection operating within created kinds producing “horizontal” changes (i.e., adaptive changes that do not add any new genetic information). The Tibetans are still interfertile with the Chinese, after all, and the time frame for the changes fits easily within a Biblical timeframe.
What evolutionists should be worried about is why some humans did not evolve altitude tolerance hundreds of thousands of years ago. By now, shouldn’t there be a separate species, Homo everestus, the obligate mountain men? According to Darwinian expectations of geographical isolation and allopatric speciation, such high-altitude humans should be unable to breed with sea people or even approach sea level without bursting their lungs. This story looks more and more like a confirmation of the Biblical creation model for human history and origin by design (see the 06/17/2005 entry).