Human Evolution Continues: Low Oxygen Environments

Like all creatures, humans are still evolving in response to our environment. Some humans have even evolved to withstand low oxygen environments.

We speak about evolution as if it’s something that previously happened and has been completed. That all that is now is all that will ever be. But this simply isn’t true. From tuskless elephants to antibiotic-resistant bacteria, evolution is constantly around us — the culmination of micro-changes in DNA — and humans are no exception. We are, afterall, organisms of this world and, like everything else, are subject to change.

Oxygen is one of, if not the, most essential substances humans need to survive. But in recent years, scientists have found that there are some humans who have evolved — and are continuing to evolve — to withstand low oxygen environments, such as high altitudes and under water.

The Humans Who Breathe Less Oxygen

In the mountains of Tibet lives the Tibetan highlanders, who live around 4,000 metres above sea level and breathe around 40% less oxygen than they would at sea level.

For the average lowlander (i.e. the average human) living in a 40% lower oxygen concentration environment would cause hypoxia of altitude (hypobaric hypoxia) and would likely result in fatigue, reproduction impairment and potentially lethal diseases. The most commonly known disease being chronic mountain sickness (CMS), which results in high concentrations of haemoglobin (the pigment which carries oxygen) in the blood caused by overproduction of red blood cells. CMS can also result in fluid in the lungs and swelling of the brain, both of which can lead to death. This is why mountain climbers take oxygen tanks with them when climbing to high altitudes.

However, the Tibetan highlanders don’t seem to suffer from the ill-effects of these high altitude low oxygen concentrations. In fact, they’ve been thriving for possibly thousands of years.

It is believed the Tibetan highlander’s have evolved a variant of the EPAS1 gene (also known as HIF2α). The EPAS1 gene is part of a human biological mechanism which controls the body’s response to low blood oxygen concentration. This newly discovered variant, known as an allele, correlates with a marked reduction in response to hypoxic conditions; namely, no-to-little increase in blood haemoglobin concentration. In short, the Tibetan highlanders are not affected by mountain sickness until higher altitudes and lower oxygen concentrations.

It is theorised that living for generations in the low oxygen conditions of the Tibetan highlands has put significant selective pressure on the EPAS1 gene. This has resulted in natural selection giving rise to humans more evolved to living with only 60% of the oxygen concentration than the average human.

How Long Can You Hold Your Breath?

Speaking of low oxygen, how long can you hold your breath underwater? Personally, not very long. After 30 seconds I feel incredibly uncomfortable and have to actively fight the urge to breathe. But the Bajau people, they are truly the champions.

The Bajau people — the ‘sea nomads’ — of Southeast Asia are extraordinary freedivers who have lived in the Indonesian region for over a thousand years. Most notably, they can hold their breath underwater for over 13 minutes.

In a recent study, it has been found that the Bajau people have enlarged spleens, around 50% larger than that of their mainland cousins, the Saluan. The spleen is an organ which, when diving, releases a reserve of oxygenated red blood cells into the bloodstream. Therefore, the spleen aids in breath-holding underwater for extended periods. Enlarged spleens are also found in marine mammals, such as seals.

There is evidence so far that at least two genes may contribute to this extraordinary marine adaptation. PDE10A (Phosphodiesterase 10A) is one of these genes and it is known to be mutated in the Bajau people. PDE10A is associated with smooth muscle tissue contraction, like that of the spleen, and has been found to be related to spleen size. The larger the spleen, the larger the oxygenated blood reserve it can hold, ready for release when diving.

Another gene variant, BDKRB2 (Bradykinin receptor B2), which is associated with vasodilation and vasoconstriction (dilation and constriction of blood vessels) induced by the dive response, is thought to also contribute to the Bajau peoples’ marine adaptations. This variant of BDKRB2 is thought to increase dive time by constricting blood vessels and preferentially oxygenating vital organs, such as the brain.

Adding together both a larger spleen with a larger oxygenated blood reserve, and a more adapted dive response to preferentially oxygenate vital organs, we can start to understand how the Bajau people are continually evolving to adapt to the hypoxic conditions of diving.

Evolution is often slow, to the point of almost imperceptibility, but it is prevalent and occurring all the time in nature. With our tools, we can now peer not just into the huge human evolutionary changes, such as our evolutionary divergence from Neanderthals, but into our more subtle adaptations that help us humans to adapt to our ever changing environments.

Further Reading

Baranova, T., Berlov, D., Glotov, O., Korf, E., Minigalin, A., Mitrofanova, A., Ahmetov, I. and Glotov, A. (2017). Genetic determination of the vascular reactions in humans in response to the diving reflex. American Journal of Physiology-Heart and Circulatory Physiology, 312(3), H622-H631.

Beall, C., Cavalleri, G., Deng, L., Elston, R., Gao, Y., Knight, J., Li, C., Li, J., Liang, Y., McCormack, M., Montgomery, H., Pan, H., Robbins, P., Shianna, K., Tam, S., Tsering, N., Veeramah, K., Wang, W., Wangdui, P., Weale, M., Xu, Y., Xu, Z., Yang, L., Zaman, M., Zeng, C., Zhang, L., Zhang, X., Zhaxi, P. and Zheng, Y. (2010). Natural selection on EPAS1 (HIF2 ) associated with low hemoglobin concentration in Tibetan highlanders. Proceedings of the National Academy of Sciences, 107(25), 11459–11464.

Ilardo, M., Moltke, I., Korneliussen, T., Cheng, J., Stern, A., Racimo, F., de Barros Damgaard, P., Sikora, M., Seguin-Orlando, A., Rasmussen, S., van den Munckhof, I., ter Horst, R., Joosten, L., Netea, M., Salingkat, S., Nielsen, R. and Willerslev, E. (2018). Physiological and Genetic Adaptations to Diving in Sea Nomads. Cell, 173(3), 569–580.e15.

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