Admixture is recognized as a widespread feature of human populations, renewing 
interest in the possibility that genetic exchange can facilitate adaptations to 
new environments. Studies of Tibetans revealed candidates for high-altitude 
adaptations in the EGLN1 and EPAS1 genes, associated with lower haemoglobin 
concentration. However, the history of these variants or that of Tibetans 
remains poorly understood. Here we analyse genotype data for the Nepalese 
Sherpa, and find that Tibetans are a mixture of ancestral populations related to 
the Sherpa and Han Chinese. EGLN1 and EPAS1 genes show a striking enrichment of 
high-altitude ancestry in the Tibetan genome, indicating that migrants from low 
altitude acquired adaptive alleles from the highlanders. Accordingly, the Sherpa 
and Tibetans share adaptive haemoglobin traits. This admixture-mediated 
adaptation shares important features with adaptive introgression. Therefore, we 
identify a novel mechanism, beyond selection on new mutations or on standing 
variation, through which populations can adapt to local environments.