Ancient Denisovan genes enable modern Tibetan to handle high altitude
The authors of a paper in Nature this week conclude a modification of a haplotype found in the Denisovan genome apparently has given modern Tibetans (and their ancestors) an adaptation to high altitude, thus avoiding the hypoxia thinner air causes in individuals lacking this genetic combination.
“Haplotype” is defined as a set of DNA variations, or polymorphisms, that tend to be inherited together. A haplotype can refer to a combination of alleles or to a set of single nucleotide polymorphisms (SNPs) found on the same chromosome. “Hypoxia” (also known as Hypoxiation or Anoxemia) is a condition in which the body or a region of the body is deprived of adequate oxygen supply. Hypoxia may be classified as either generalized, affecting the whole body, or local, affecting a region of the body.
The paper by Emilia Huerta Sanchez and others is summarized in the abstract as follows:
“As modern humans migrated out of Africa, they encountered many new environmental conditions, including greater temperature extremes, different pathogens and higher altitudes. These diverse environments are likely to have acted as agents of natural selection and to have led to local adaptations. One of the most celebrated examples in humans is the adaptation of Tibetans to the hypoxic environment of the high-altitude Tibetan plateau. A hypoxia pathway gene, EPAS1, was previously identified as having the most extreme signature of positive selection in Tibetans and was shown to be associated with differences in haemoglobin concentration at high altitude. Re-sequencing the region around EPAS1 in 40 Tibetan and 40 Han individuals, we find that this gene has a highly unusual haplotype structure that can only be convincingly explained by introgression of DNA from Denisovan or Denisovan-related individuals into humans. Scanning a larger set of worldwide populations, we find that the selected haplotype is only found in Denisovans and in Tibetans, and at very low frequency among Han Chinese. Furthermore, the length of the haplotype, and the fact that it is not found in any other populations, makes it unlikely that the haplotype sharing between Tibetans and Denisovans was caused by incomplete ancestral lineage sorting rather than introgression. Our findings illustrate that admixture with other hominin species has provided genetic variation that helped humans to adapt to new environments.”
(Those with a subscription to Nature can read the full article.
Some years ago a cave called Denisova in the Altai mountains of Siberia yielded a partial finger bone dated to around 70,000 years ago. Altitude and low temperatures preserved enough DNA for researchers from the Max Planck Institute to analyze. These scientists were able to reconstruct the individual’s genome and announced that while genetically similar to Neanderthal remains found in the region, these were sufficiently different so as to be distinct from both Neanderthals and modern humans. The individual was designated a “Denisovan” but cannot be assigned a formal species designation because the protocols by which scientists distinguish species were adopted before modern genetic analysis. Differences in the size, shape and function of bones are used to establish genus and species attribution (for example, Australopithecus robustus is distinguished from Homo habilis on the basis of cranial and facial characteristics, among other things). While scientists can make a genetic distinction they cannot assign a species name to the bones found in the Denisova cave.
Enough genetic material was extracted from the middle joint of the pinky finger from the left hand to determine the individual was a young female with brown eyes and hair but until more bones are found that can be related reliably to the Denisovan girl, we cannot speak of Homo denisovensis (or similar) but simply say the people to whom she was related were “Denisovans”.
A friend reading this as a draft suggested the high altitude residents of the Andes might have a similar adaptation. While it could not be derived from the Denisovans, it bears looking into.