Altitude sickness comes from lack of oxygen in the atmosphere, but lack of oxygen can cause other complications.
Despite this, some specific populations across the globe compensate through biochemical specialisations in the way their bodies operate. This is particularly obvious in tribes living the Andes or high up on the Tibetan plateau, some 4000m above sea level, where the air only contains 60% as much oxygen as at sea level. These adaptations are generally acquired through natural selection favouring those in the original population who, through natural variation, are built to cope slightly better than others. Over thousands of years this leads to the evolution of coping mechanisms, although the coping mechanism evolved by a population in the southern Andes may not be the same specific change as that evolved by a tribe in the northern Andes.
An example of this phenomenon can be seen in the appearance of blond hair, which evolved twice as our species, Homo sapiens, spread across the globe from around 100 000 years ago: in Europeans several modified genes contribute to producing blond hair in a person, whereas Solomon Islanders also have modifications in another gene which remains unchanged in Europeans with blond hair.
This effect of selection of favourable traits is particularly strong in relation to overcoming the effects of altitude, as the lower oxygen levels make it harder to conceive children: any would-be parents who were already slightly more able to deal with low oxygen levels would be more likely to have children, and those children would themselves be able to have more children. It’s exactly that kind of cycle which drives most evolution and creates a new or improved function in distant descendents.
The case of the Tibetan people is particularly special however, and this why this story is so interesting. A new study focused particularly on a gene dubbed EPAS1, a specific variant, or allele, of which was found at very high frequencies in Tibetan people compared to the very genetically similar Han Chinese populations (Huerta-Sánchez et al., 2014). While most adaptations to high altitude involve increasing the amount of haemoglobin in the blood in order to carry more oxygen, the Tibetan variant of the gene is seems to be linked to comparatively low haemoglobin levels. High levels of haemoglobin cause the blood to thicken and clot more easily so an allele which puts a cap on haemoglobin levels may be beneficial in pregnancy, where a common side-effect of low atmospheric oxygen is high blood pressure.
As well as being much more common in Tibetan people than the Han population, the Tibetan EPAS1 allele was far more different from the Han allele than expected of a similarly-sized piece of DNA from any other gene. This suggested to the researchers that simple evolution may not be able to explain how the Tibetan allele came to be present on the Tibetan plateau. This view was further supported by comparing a very short repeating region (motif) of the gene across all modern populations. This short stretch proved to be almost unique in structure and, although common in
Han Tibetan populations, only 2 other examples were found globally. Both were of Chinese Han origin.
The team was also able to compare the Tibetan allele with those of Neanderthals and their close cousins the Denisovans. Very little is known about the Denisovans except for two molar teeth, a toe bone and a finger bone which suggest that they were very strongly built, probably similar to Neanderthals. These fossil remains from a cave in southern Siberia date to around 80,000 years ago and have yielded high-quality DNA for analysis because of the cold and dry conditions. Analysis of this DNA suggests that Denisovans may have been around from as long as 800 000 years ago (Meyer et al., 2012).
This comparison turned up a surprise: the unique Tibetan motif was identical to that found in the Denisovan sample. Looking at the allele as a whole, five more specific differences which separated the Tibetan allele from all other modern humans were found to be identical between Denisovan, Tibetan and the two Chinese Han examples already mentioned.
High-altitude hanky panky
All the evidence suggests that this unique allele could not have arisen through evolution in the Tibetan population in such a short time period. This leaves us with one viable option: the allele was common in Denisovans and was somehow passed to Homo sapiens when they settled on the Tibetan plateau. And how does DNA pass from one person to another? Well, DNA only passes from parent to child…
Interbreeding between Denisovans and Homo sapiens must have occurred for Tibetan populations to have acquired this Denisovan allele.
This leaves just one more question: why is it so common in Tibetan people and present but so rare in Han people? The authors of the study suggest that the interbreeding occurred before the Han and Tibetan populations truly split, but once they did it was only advantageous to those living at high altitudes – the allele offered a selective benefit to the Tibetans and was maintained, while it was all but lost from the Han population living lower down.
I think that’s a fairly exciting finding.
As always, if you have any questions, ask away!
Corbyn, Z. 2012. Blonde hair evolved more than once. Nature News. doi:10.1038/nature.2012.10587. Freely available!
Huerta-Sánchez, E., Jin, X., Bianba, Z., Peter, B. M., Vinckenbosch, N., Liang, Y., … Nielsen, R. (2014). Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA. Nature. doi:10.1038/nature13408. Freely available!
Meyer, M., Kircher, M., Gansauge, M., Li, H., Mallick, S., Schraiber, J. G., … Eichler, E. E. (2012). A high-coverage genome sequence from an archaic Denisovan individual. Science, 338, 222–226. doi:10.1126/science.1224344.A. Freely available!
Sean Myles, 2012. Taken from Corbyn, 2012. (above). Sean Myles’ webpage.