Tibetans are adapted to the low oxygen content at the high (>4000 m) altitudes where they live thanks a lower haemoglobin content in their blood which is again due to a specific gene EPAS1. This gene is more or less restricted to Tibetans, for example it is absent in their neighbours the Han Chinese. But it is also present in the DNA of Denisovans. The most likely explanation is that the gene was introduced to the ancestors of the Tibetans and the Hans thousands of years ago. Since it helped the Tibetans live at high altitudes it survived in their DNA, but died out in the Hans since it was not utilised.
Another example is the gene responsible for the production of body hair, which help humans survive cold climates. It is postulated that this was inherited from the Neanderthals through interbreeding with our ancestors in Europe and help us survive the Ice Age.
This is the first time that scientists have been able to demonstrate that interbreeding with another hominin species has helped humans adapt to adverse conditions. (Nature, vol 512)