Our red blood cells contain hemoglobin, a molecule that carries oxygen from our lungs to our tissues. When someone who normally lives at low elevation travels to a higher elevation to live, their body makes more red blood cells and more hemoglobin to compensate for the reduced oxygen level in the air. This enables their body to obtain the oxygen that they need, but it can also make their blood thicker. The thickened blood can lead to hypertension or a heart attack.
|A Tibetan yak; photo by Dennis Jarvis,|
CC BY-SA 3.0 License
The Denisovans were a member of the genus Homo that became extinct 50,000 to 40,000 years ago. Although only a few Denisovan bones have been discovered, both nuclear and mitochondrial DNA have survived, enabling a fascinating analysis of Denisovan genes.
A human gene known as EPAS1 is responsible for the increase in concentration of red blood cells and hemoglobin at high elevations. The gene exists in several variants, or alleles. Most people have a variant that produces a strong effect at high elevations and can indirectly lead to health problems. The Chinese population contains a variant that is very different from that possessed by other humans (as far as is known) and is very similar to a Denisovan gene. This variant increases the red blood cell and hemoglobin level only slightly, enabling survival at high elevations without cardiovascular problems. The useful variant is present in 87% of Tibetans today, who live at high elevation, and in only 9% of Han Chinese people, who live at lower elevations. The two groups share a common ancestor.
The researchers theorize that long ago Denisovans and modern humans who had traveled to China from Eurasia interbred, supplying the modern humans with the EPAS1 variant. This variant was advantageous for the Chinese people who chose to live at a high elevation and spread through the population. It's an interesting idea. It will also be very interesting to learn more about the Denisovans and to discover if they had other effects on our genome.