Aquatic ape hypothesis

The Aquatic Ape hypothesis argues that in the past, humanity's primate ancestors dwelt in shallow waters for the sake of safety and food supply. The German biologist Max Westenhõfer was perhaps the first to publish the idea in an evolutionary context in 1942. The hypothesis seeks to explain human traits such as hairlessness, salty tears, fat tissue, bipedalism, and sweat glands. It also purports to explain why human body is streamlined and why most apes quickly drown when put in water while humans do not. Using the principle of convergent evolution, it says that life in an aquatic environment explains these features, and that a transition from ape to hominid in a non-aquatic environment cannot.

The hypothesis is somewhat out of favor because several key pieces of evidence fail to support it. For example, hairlessness, while found in several aquatic mammals, is not indicative of aquatic mammals (see aardvark and rhinoceros).