Once upon a time, an ape took a swim. The end.
Actually, it was probably twice. Approximately-- two million years-- feeding on freshwater aquatic plants and shellfish on the isolated Miocene island of Tuscany-Sardinia. The second phase probably occurred in isolation on an island off the coast of Africa for nearly three million years during the Pliocene, feeding mostly on marine shellfish and what little grew on the island itself. For approximately, the last 2.6 million years, Homo has essentially been a tool using terrestrial hunter-gather with some populations still exploiting coastal marine resources. So for the approximately 8 million years that hominins have probably been bipedal, five million have been spent during two semiaquatic phases-- at least along the line that led to Sahelanthropus, Ardipithecus, and Homo. Marcel F. Williams