Aquatic ape hypothesis

• Bipedalism: Humans are the only surviving bipedal mammal. Although it gives us the ability to use tools whilst walking or running, it directly causes the following problems: back problems, varicose veins, hemorrhoids, hernias and problems with childbirth.

Since evolution works in small steps, it is hard to see how bipedalism could have evolved on the savanna: the mass of the torso makes bipedalism inherently unstable. Water, however, supports the body.

• Breathing: With the exception of humans, land mammals have no conscious control over their breathing. The voluntary control humans have over their respiratory system is similar to that of 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 much fat in our bodies as is normal in an animal of our size. We are by far the fattest primates. Mammals which hibernate have seasonal fat: aquatic mammals, like humans, retain fat throughout the year.

• Tears and excessive sweating are considered further evidence to support the hypothesis. Supporters of the hypothesis also mention the webbing between the human thumb and forefinger, which has no apparent value on land.

One difficulty in evaluating this hypothesis is that the places it suggests fossils might be found are mostly below sea level at the present epoch.

Comparison with land-based hypotheses

• Nakedness: The usual land-based explanation (the "thermoregulatory hypothesis") is that it was for cooling - humans sweat more per unit surface area than any other mammal, and proponents of this idea claim that it makes us particularly effective at remaining active during the heat of the day. A layer of hair would reduce the effectiveness of this (human sweat may be seen as an analogue of the water-seeking behaviors of the animals mentioned above).
  • Problems with this explanation are that body hair is needed to protect against direct sun and extreme heat as well as cold; that human sweating is highly wasteful of water and salts, which is a distinct disadvantage on the savanna; and that exposed skin is not, after all, essential for sweating to be effective.

In addition, any such hypothesis has to explain the pattern of hair that we do have, and why women and children have less body hair than men. On the first point, why should we have retained head hair if the purpose of a naked skin is to keep cool? On the side of AAH, it may be noted that the top and the back of the head are the areas least in contact with water in the human pattern of swimming, and also the only areas covered with thick hair in both mature individuals and infants.

On the second point, it is possible to suggest an AAH scenario in which mature males spent more time near the shore, while mothers with babies stayed in deeper water out of reach of land predators; it is hard for the temperature regulation hypothesis to accommodate a case where females and infants were more active than males, and therefore more in need of sweat-cooling, in the heat of the day.

• Bipedalism: There are several land-based suggestions as to why the first hominids became bipedal: carrying behaviour, tool-making, and sentry behaviour, for example.
  • The difficulty with all of these is that (unlike the putative Aquatic Ape, which would have been a full-time wader) none of them apply for more than a small amount of the time; when not engaged in these behaviours, the proto-hominids would simply have reverted to quadrupedalism.

• Fat is very important in developing and maintaining the brain, which is a very expensive organ in terms of energy requirements.
  • However, this suggestion doesn't account for the fact that women and babies have a much higher proportion of body fat than men; while within the AAH scenario this, as with the contrast in body hair, further suggests that nursing mothers would have spent more time in water than adult males.

Objections to AAH

• Nakedness: Human hair is drastically different from all of the aquatic species named above. The comparison to fully aquatic mammals (cetaceans, sirenians, etc.) is suspect, as these animals have evolved characteristics over a far longer period than humans.

• Bipedalism: No aquatic mammal is bipedal. Those animals that are temporarily bipedal (such as kangaroos and some primates) use their upright state for locomotion, feeding and sentry behaviour, which are all useful for terrestrial life. Furthermore, for standing in shallow water, it is useful to have lower legs substantially longer than upper legs, as is common in wading birds. Human legs do not fit this pattern.

• Breathing: The ability to moderate breathing, to a lesser degree of control, is seen in many other animals, including other apes and dogs. The mammalian diving reflex exists in other mammals as well.

AAH provokes fierce and often acrimonious contention. Sceptics criticise the lack of direct fossil evidence; the sometimes amateurish way in which it is presented; and the occasional over-emphasis of tenuous arguments. Proponents complain about a dismissive and superior attitude; attacks on methods and personalities rather than substance; and the failure to provide land-based alternative hypotheses that survive the very criticisms levelled at AAH.

Resources

• Hardy, A. C., "Was man more aquatic in the past?", New Scientist, 7,642-645 (1960).
• Morgan, Elaine, The Aquatic Ape, 1982, Stein & Day Pub, ISBN 0-285-62509-8
• -- The Scars of Evolution, 1990, Souvenir Press, ISBN 0-285-62996-4
• -- The Aquatic Ape Hypothesis, 1997, Souvenir Press, ISBN 0-285-63377-5

See also

• Theories of the origin of humans
• Aquatic adaptation