In the few moments before he left after the interview, I asked him what he though of the aquatic ape hypothesis. His answer: "So early humans settled near rivers. Well, duh!"
I admit I was so taken aback by such an egregious setting up of a straw man that I was speechless for long enough for the professor to leave before I could point out how wrong he was.
For if proponents of the aquatic ape hypothesis did in fact base it purely on patterns of human settlement, then "duh" would be the appropriate response; it was clear that the good professor had not taken the time to inform himself of the reasoning behind the hypothesis before dismissing it out of hand. Unfortunately, this ant-intellectual reaction seems all too prevalent in academic circles.
So what is the basis for the hypothesis? It's twofold - based on differences and on similarities.
Our closest animal relations are the chimpanzee and the bonobo - we share about 99% of the same DNA. Yet the differences are striking: unlike the chimpanzees and bonobos, humans are almost hairless, walk bipedally, have subcutaneous fat, and communicate with reasoned speech. Something profound must have happened in our past to have caused such changes from creatures so close to us in our DNA makeup, something that conventional anthropologists have no explanation for.*
It's the aquatic or partially aquatic mammals (whales, dolphins, manatees, dugongs) that are similar to us in being hairless and have subcutaneous fat to insulate themselves from cold water, and dolphins communicate with what appears to be reasoned speech (as against the instinctual chatter of chimpanzees).
These two observations are the basis for the aquatic ape hypothesis: that when our early ancestors were somehow cut off from their previous food supply, they took to a shoreside wading and swimming existence, and over a long period adapted to better function as mammals who waded (accounting for our bipedalism**) and swam (accounting for our relative hairlessness and body fat patterns); the hypothesis has the virtue of being a plausible explanation for the differences and similarities described above. Whether it will turn out to be the true explanation will depend on future research, but one observation I might make is that a fossil record will be hard to find if our ancestors' dead bodies were carried out to sea.
The most persistent proponent of the hypothesis has been British writer Elaine Morgan. You can see her 2009 TED talk on the subject here, where she likens people like the anthropologist I met to a priesthood unable to open their minds to possibilities outside their dogma. (I get some encouragement from the fact that the TED audience gave her a standing ovation.)
*The "Savannah Theory" (in fact the savannah hypothesis), which held that humans adapted to their present form when they left the jungle trees to live in grasslands, has finally been discarded, and in any event we know how apes adapt when they move into a savannah existence: they evolve into baboons (with whom we share about 91% of our DNA).
**Bonobos knuckle-walk on land (like chimpanzees), but become bipedal when they wade in water in search of food.