You know what? I know I'm going to be eating crow about this for the foreseeable future but I've changed my position about the Aquatic Ape Hypothesis. I just watched the Elaine Morgan
In a nutshell, Elaine Morgan has been arguing for decades (from outside the academic mainstream, which may be important in this case) that the major phenotypic differences between humans and chimpanzees are the characteristics we humans share with aquatic mammals. These include hairlessness and the subcutaneous fat layer which no other primate possesses. She also notes that apes, which are all capable of walking upright when they feel like it, always walk upright when entering the water.
She makes other points which I can't independently verify, that all hairless terrestrial mammals (e.g. elephants and rhinos) have aquatic forebearers (save the naked mole rat, a freak by anyone's measure) and that breath and diaphram control is common to aquatic animals but otherwise unknown in apes. This control gives us the power of speach.
I've read her arguments before and there wasn't anything new in the TED talk that I hadn't heard already. So I googled around a bit to see if she was under-representing the strength of arguments made by her detractors. Most of the arguments I found rely either on the lack of fossil evidence to support the theory, or on logical arguments which don't seem to me to be significantly stronger than those in favor of the aquatic ape.
Evolutionary change can occur when a fortuitous coincidence of environmental pressures with the occurence of a mutation in some population which directly affects the likelyhood of survival relative to those pressures. It also occurs when random mutations which don't affect the likelyhood of survival are also prevalent in the selected population - so it is not reliable to look at a single characteristic and deduce a past environmental pressure. In the long term, traits which advance survival or reproductive success are most likely to survive. However, the rate of mutations is such that this model is never run out to its conclusion. No organism exists, or is likely to ever exist, which is perfectly adapted to its environment at the time we observe it. The disadvantage to survival caused by the occasional appendicitis is not sufficient to cause the next generation to be born without an appendix. The reproductive struggles introduced by the upright human pelvis are likely a more significant challenge to survival than the appendix. But these are accomodated by cultural means, or technology, and have clearly not affected the survivability of the species. In short: the logical arguments made on both sides of the aquatic ape hypothesis so far fail to produce a unique conclusion, only explore what could have been possible.
So. Is it possible that a semi-aquatic hominid existed in our lineage, which could have perservered long enough in the lakes of the East African Rift to adapt some characteristics common to aquatic mammals, although not our nearest ancestors? Logical arguments could be resolved by adequate data from the fossil record.
The hominid fossil record is painfully scarce (relative to other more numerous, longer lasting species). In order to be fossilized, an animal has to die in the right place at the right time. On the savannah, animals are dismembered, desicated, and the bones dry and crack in the sun. There is almost no potential for preservation. Fossil beds from which we understand the record of terrestrial animals come from environments where rapid burial in sediment can take place - lakes and rivers - where we know that terrestrial and aquatic animals gather together. Hominid fossils are also found primarily in lake sediments but this does not really address where they lived, only where they died. Even so, it is not possible from the fossil remains that have been found to determine soft tissue characteristics such as identifying the emergence of a subcutaneous fat layer in our hominid ancestors.
So... Dad... although I am unwavering in my opinion that when discussing scientific theories, "interesting" and "likely to be true" are mutually exclusive - I am no longer hostile to the Aquatic Ape Theory. This has mostly to do with a recently developed appreciation for the scarcity of the hominid fossil record, and therefore the higher degree of uncertainty, than anything else. Now I'm going to the pool.