Researchers suggests that, as recently as 25 million years ago, the region wasn’t yet a flat, windswept plateau. Instead, it was a diverse landscape of steep mountains surrounding a deep valley where palm trees grew.
S. Botsyun et al. Revised paleoaltimetry data show low Tibetan Plateau elevation during the Eocene. Science. Vol. 363, March 1, 2019, p. 946. doi: 10.1126/science.aaq1436.