Humans likely first took to the seas about 50,000 years ago. But there's mounting evidence that our Neanderthal cousins were routinely sailing throughout the Mediterranean twice as long ago. Alternatively, they were just really good at long distance swimming.
We can't know what sorts of boats Neanderthals might have used. Presumably, they were made out of wood, which is exactly the problem — there's just no way wood is going to last 100,000 years, without rotting in even a tiny fraction of that time. But we can identify ancient Neanderthal presence through the distinctive Mousterian stone tools that they left behind, which have been found on the coastal Greek islands of Kefallinia and Zakynthos. Neanderthals must have crossed the water for their tools to end up there.
The only potential objection to this was the possibility that shifting sea levels might have once connected these islands to the mainland via a land bridge. But it looks like we can rule that possibility out, according to George Ferentinos of Greece's University of Patras. As New Scientist reports, Ferentinos has found that the sea levels in the Mediterranean were significantly lower 100,000 years ago, they were still about 180 meters higher than the bed of the Ionian Sea off the Greek coast, meaning no land bridge would have been possible.
It wouldn't have taken the Neanderthals too much effort to reach these islands — they're only about three to eight miles away, depending on the particular configuration of the coast. Ferentinos and his fellow researchers estimate that Neanderthals began seafaring sometime between 110,000 and 35,000 years ago. We should have a better idea of the exact date range once the stone tools on the islands have been more accurately dated.
The big question is precisely how they got there. That sort of distance wouldn't necessarily require boats to make the journey — the Neanderthals might just have swum across. But these aren't the only islands that show signs of Neanderthal habitation. The much more distant island of Crete also shows signs of Mousterian tools, and if those results hold up it's almost certain that the Neanderthals did indeed start sailing the Mediterranean — and quite possibly on a regular basis — some 50,000 years before humans first conquered the seas.