Sea otters probably diverged from other otters during the Pliocene, approximately 5 mya.[1] They probably arose from the closely related Enhydritherium.
Enhydra reevesi, the oldest known species, has its origins in the Atlantic, suggesting this may have been where sea otters originated.[1] Fossil evidence indicates the Enhydra lineage became isolated in the North Pacific approximately 2 million years ago, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter.