Steller's sea cow, an extinct sirenian and one of the largest Quaternary mammals, was described by Georg Steller in 1741 and eradicated by humans within 27 years. Here, we complement Steller's descriptions with paleogenomic data from 12 individuals. We identified convergent evolution between Steller's sea cow and cetaceans but not extant sirenians, suggesting a role of several genes in adaptation to cold environments. Among these are inactivations of lipoxygenase genes, which in humans and mouse models cause ichthyosis – a skin disease characterized by a thick, hyperkeratotic epidermis that recapitulates Steller's sea cows' reportedly bark-like skin. Finally, we found that Steller's sea cows' abundance was continuously declining for tens of thousands of years prior to their description, implying that environmental changes also contributed to their extinction.

We extracted ancient DNA from fragmentary remains of 12 Steller's sea cow individuals recovered from beaches of Bering Island, which we radiocarbon dated to 2205–1155 BP. We sequenced the genomes of two of the best preserved of these (SNMB N51667 from the Braunschweig Natural History Museum and SC16.JK045 from the collection of the Kamchatka Branch of the Pacific Geographical Institute, RAS Museum) to 15.86 and 15.63× coverage, respectively, and the remaining ten to an average coverage of 2.78× (range 1.97–3.95×).