The modern hypothesis of the aquatic ape theory (AAT) was originally suggested in 1942, by Max Westenhofer in The Road to Man (Der Eigenweg des Menschen). The writer Elaine Morgan developed and promoted it, publishing in 1972 her first book on the subject, The Descent of Woman, and later other books, including The Aquatic Ape (1982), The Scars of Evolution (1990), The Descent of the Child (1994), and The Aquatic Ape Hypothesis (1997).
The aquatic ape hypothesis puts forward several main arguments:
Nakedness: Humans are the only primate species in which, in most of the body, hair is so fine and sparse as to reveal the skin under it. Environments known to give rise to naked mammals are tropical (like elephants, which are themselves descended from aquatic ancestors, and some rhinoceros species), aquatic (whales, dolphins, walrus, dugongs, and manatees), semi-aquatic or littoral (hippopotamus), and subterranean (naked mole rat).
Bipedalism: There exist very few bipedal mammals, and humans are the only ones which adopt a full-time, fully-upright posture with a vertical vertebral column. Gorillas, chimpanzees and bears are able to walk on two legs when they have a particular reason. Although the posture improves the ability to carry objects and use tools while walking or running, bipedalism and upright posture are believed to come at a significant cost, from back and knee problems, varicose veins, hemorrhoids, hernias, and problems with childbirth. Aquatic ape theory argue proposes water supports the body.
Breathing: Most land mammals have no conscious control over their breathing. The voluntary control humans have over their respiratory system can be compared to that of (semi)aquatic mammals which inhale as much air as they need for a dive, then return to the surface for air.
Fat: Humans have ten times as many fat cells under the skin as would be expected in a non-aquatic animal the same size, and have many adipose cells. Aquatic mammals retain fat throughout the year, some terrestrial do, but in some seasons to hibernate. Human infants are especially fat compared with apes and most other fully terrestrial mammals.
Childbirth: Dramatic increase in cranium size is a prominent theme in human evolution, making childbirth difficult and dangerous. Water birthing is believed to facilitate childbirth and to reduce risks to mother and infant. Human infants naturally hold their breath and can swim from birth.
Nutrition: Human brain tissue requires comparatively large amounts of omega-3 fatty acids, which are uncommon in the land food chain but prevalent in the marine food chain.
Tears and excessive sweating: Sweating and tears are prevalent in humans but not in other primates. They are considered further evidence to support the hypothesis, insofar as they are vectors for the removal of excess water and salts from the body as might result from the ingestion of saltwater (as in eating food from a salt marsh).
