A paper in Nature (Altitude adaptation in Tibetans caused by introgression of Denisovan-like DNA Emilia Huerta-Sánchez, Xin Jin, Asan et al.) says that DNA in Tibetans to adapt to the altitude can be attributed to an extinguished hominid precursor, the Denisovan: (cited from Nature)

Admixture with other hominin species helped humans to adapt to high-altitude environments; the EPAS1 gene in Tibetan individuals has an unusual haplotype structure that probably resulted from introgression of DNA from Denisovan or Denisovan-related individuals into humans, and this haplotype is only found in Denisovans and Tibetans, and at low frequency among Han Chinese.