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The Aquatic Ape Hypothesis, sometimes referred to as the Aquatic Ape Theory or AAT, is a questionable theory from paleoanthropology that enjoyed popularity in the 80s and early 90s. The basic idea is that human evolution was heavily influenced by the presence of bodies of water, and many of our signature characteristics and differences from other primates, such as hairlessness and bipedalism, can be explained by reference to this aquatic habitat. The Aquatic Ape Hypothesis' most vocal proponent is Elaine Morgan, a television playwright and feminist writer. Although the theory was ultimately dismissed by the paleoanthropological community, being aware of it and the reasons for why it was refuted can help us learn more about the nature of the evolutionary process.
The first argument for the Aquatic Ape idea comes from hairlessness. Getting rid of our thick primate hair makes it easier to swim and faster to dry off when exiting a body of water. The next argument comes from bipedalism. It is argued that the buoyant properties of water would have made the incremental evolution from quadrupedalism to bipedalism easier. Another argument comes from control over our breathing. We can deliberately control our breathing like many aquatic and semi-aquatic creatures, but unlike other land creatures.
There are many other anecdotal arguments for the Aquatic Ape hypothesis. A few are our fat surplus, perpendicular nostrils, the ability of infants to hold their breath and swim from birth, the greater nutrition of fish relative to land animals, and face-to-face sex, like in dolphins, are all cited as possible evidence for the influence of aquatic environments over our evolution.
There are many arguments against the Aquatic Ape Hypothesis. The most obvious is that arguments in its favor tend to be vague, offer few testable predictions, and change their assumptions based on what trait it is they are trying to argue is related to an aquatic past. The premises of the theory have not changed substantially since the 50s, when the theory was originally introduced.
Another argument is that most of the bodily features attributed to water evolution by the Aquatic Ape enthusiasts are either not truly exclusive to aquatic animals or their evolution can be explained by other means. For example, many species of non-aquatic apes are capable of walking bipedally, at least temporarily, which places doubt on the idea that water was necessary to facilitate permanent bipedalism. Our hairlessness is probably a result of walking longer distances and their corresponding need to dissipate heat more effectively. Our fat surplus is common among all animals with no natural predators and substantial amounts of food. The Aquatic Ape hypothesis is not necessary to explain any of this.
Sometimes theories teach us even more about science when they're wrong than when they're right. The Aquatic Ape hypothesis is frequently studied by paleoanthropologists as a way of how evolution theories should be falsifiable and as amenable to scientific testing as possible.