The aquatic ape hypothesis is a theory that, at some point in the past several million years ago, the ancestors of Homo sapiens lived on the seashore and spent enough time in the water that their bodies developed a number of adaptations to an aquatic lifestyle, which remain with us today and separate us from many other primates. No "aquatic ape" fossil specimens have ever been discovered, and at least for the moment, the theory is accepted only by a minority of anthropologists. However, it is important to note that a life in the water would intrinsically leave fewer fossil remains than a life on land, and that future discoveries could, as they have in the past, revolutionize our understanding of the evolution of the human race.
- Suggested Evidence for the Aquatic Ape Hypothesis -
There are several curious features of humans which might seem to indicate, at the very least, that we have spent a lot of time close to the marine environment in the past. Bipedalism, or the capacity to walk comfortably on two legs, is a skill that might have been easier to develop while wading in the ocean, where the water would provide greater support to the first clumsy walkers than they ever could benefit from on land. (It would also allow them to range farther out into the surf in search of food.) Humans also possess an involuntary physical response known as the mammalian diving reflex, which is mostly (though much more strongly) present in dolphins and seals.
In all cases of the mammalian diving reflex, when water is splashed on the animal's (or human's) face, the heartrate slows, the circulatory system begins to reduce its flow through the vulnerable and inefficient extremities, and, as we immerse in the water, a blood shift occurs which can reduce our vulnerability to increased water pressure well below the surface. This reflex only occurs when the water splashed is cold, and implies that we continue to maintain an automatic reflex from a point in our past where we actually needed these things in order to survive.
There are a few other features of human beings and human evolution which advocates suggest that the aquatic ape hypothesis could explain, as well. Babies are born with a layer of fat under the skin, known as subcutaneous fat, which is somewhat analogous to the thicker layer of "blubber" in cetaceans - and could suggest we once relied on similar insulation for long swims in the ocean. We also have very little body hair compared to our biological relatives. How humans lost their hair is a subject of inquiry in its own right, although from what we know about mammalian evolution, it is logical to believe that our ancestors were once light-skinned but covered by hair or fur (like most mammals), lost their hair for uncertain reasons, and then developed darker skin to protect themselves from exposure to sunlight. One possible reason we lost our hair is because it slowed us down in the water - other hairless mammals in Africa, like the rhinoceros, are descended from mammals who we think would have lost their hair for the same reason.
- Origins and Current Status of the Theory -
The aquatic ape hypothesis emerged from brief speculation in the 1930s by authors such as Alister Hardy and Max Westenhofer. However, its strongest advocate was Elaine Morgan, a screenwriter who has written popular anthropology books since the 1980s. Morgan argues that the aquatic ape hypothesis is the best alternative to what she regards as the savannah ape hypothesis: the theory that our ancestors began their rapid evolutionary progress when they descended out of the trees onto the African plain.
Among professional anthropologists who study human evolution, however, the aquatic ape hypothesis generally has been greeted with much greater skepticism. It is possible that we obtained some important benefits from living alongside the sea; for instance, fish might have provided a rich source of nutrients encouraging the early growth of the brain. In general, however, the list of inherited features which could have developed as a result of an aquatic lifestyle has never been linked to any fossilized remains of an actual aquatic ape species. (Admittedly, this would be difficult, since fossilized remains on the shore will necessarily be less common and far more difficult to find.) Moreover, each of these individual alleged adaptations also has a number of proposed explanations which do not rely on the aquatic lifestyle.
Finally, skeptics also point out that we seem to be lacking one key set of adaptations that one would expect from an aquatic ape: a set of defences against predators in the water. Although individual humans are quite vulnerable to large predators on land, they are even more defenceless in the water. In the dangerous environment of Africa, being vulnerable could not have been beneficial. For these reasons, the majority of people studying the early evolution of human ancestors currently regard the aquatic ape hypothesis as intriguing but unsupported. At the very least, however, it raises some important points about the numerous gaps which still remain in our understanding of how our species evolved.