Animal Planet will be airing a special on mermaids which promises to be about as accurate as the 1990s classic "Alien Autopsy" video. But one thing the faux-cumentary mentions is the "aquatic ape" hypothesis, which is a pseudoscientific theory that just won't quite die.
According to this theory of evolution, humankind went through an aquatic phase. This phase is said to explain several mysteries, including our diving reflex, our hairlessness, the pattern of hair on our bodies, and our lack of a "missing link." Fossil evidence would be missing, according to this theory, because we were living in the water at the time.
This isn't entirely without precedent. Whales famously came ashore, lived as land animals for a while, then returned to the water. However, the evidence against the Aquatic Ape Hypothesis is both voluminous and convincing.
Problem 1: We found the missing link
This "missing link" was a big deal back in the day. For a long time, archaeologists were unable to find fossil evidence of the bridge between homo sapiens and earlier forms of primate. These forms have since been unearthed, and the continuum of fossil evidence is complete. There are no missing links.
Problem 2: Our hair pattern isn't that strange
Granted, it is odd that we are hairless but for the tops of our heads. But as for the contention that our hair pattern would make us more streamlined, it just isn't so. First of all, if you compare our hair pattern to other animals, it's pretty much the same. Our hair grows from our shoulders down and out, for the most part. Just like most animals.
Problem 3: Our hair pattern isn't aquadynamic
The only place where our hair pattern differs is on our forearms and calves, where it grows sideways. Not so unusual for primates - the same pattern is found on orangutans and other apes. And it's only "streamlined" if you can figure out how aquatic apes propelled themselves face-first through the water, with their arms out at their sides like airplane wings.
Problem 4: Subcutaneous fat is actually pretty common
Many proponents claim that we share the property of subcutaneous fat with other aquatic mammals (like whales and seals) but not land animals. I have an overweight cat sitting on the desk beside me who is ample proof to the contrary.
Problem 5: It wasn't long enough to evolve this way, anyway
It took whales millions of years to evolve away from their land-based forms. However long we theoretically spent in the water, it wasn't nearly long enough to account for all these changes.