It took approximately five million years for
Pakicetus, the artiodactyl ancestor of the cetaceans, to evolve into
Protocetus, the first mammal that could be called a primitive whale. And the cetaceans have had forty million years since then to evolve into the much better adapted species we have today. (The ancestors of the cetaceans were basically primitive hippopotamuses who swam all the way down the river to the sea, and rather liked it there.)
It takes a lot of major changes to turn a land mammal into a fully marine mammal. (Not partially marine like seals, who sleep and give birth on land.)
Going back five million years from today takes us to our own ancestral species that weren't even human yet. These were creatures that were still largely arboreal, barely adapted to living on the ground. There's no way that, in addition to completing their evolution into ground-dwelling primates, they could have made the additional, much more complex evolution into marine mammals. Just not enough time!
The most recent primates that we can call proto-human lived two or three million years ago. It's even more impossible that one of those creatures evolved into a marine mammal in such a short timespan.
The "Aquatic Ape Hypothesis" suggests that at some time in the last four or five million years our ancestors took a detour and became
partially aquatic. Supposedly, they took up residence on the shore of a lake. This is said to have caused them to adapt into better swimmers: humans are more buoyant than other apes, we have those little vestigial webs between our fingers, and we still have a remnant of the Mammalian Diving Reflex that occasionally allows a baby who's been underwater for ten minutes to be pulled out no worse for the wear. Furthermore, warm-blooded air-breathing animals (mammals and birds) have a much higher-energy metabolism than cold-blooded and/or gill breathing animals (fish, amphibians, reptiles, crustaceans, cephalopods, etc.) and this typically allows them to utterly rule aquatic environments as the apex predator.
Almost no one takes this hypothesis seriously (except here on SciForums
) but even if it were true, these aquatic apes have a long way to go before they could become completely marine mammals who never have to return to shore, even to sleep and give birth.
Thus the mermaid hypothesis qualifies as an
extraordinary assertion. It violates everything we know about mammals returning to the sea. Therefore it invokes the Rule of Laplace:
Extraordinary assertions must be supported by extraordinary evidence before anyone is obliged to treat them with respect.