Aquatic ape hypothesis

   

The aquatic ape hypothesis (or aquatic ape theory as it is frequently called) holds that the immediate ape ancestors of humans and other hominids lived for a significant time in a semi-aquatic setting on the African seacoast, that they gathered most of their food from the seashore and shallow offshore waters before their hominid descendants returned to a more land-based existence, and that adaptations to this marine Environment of Evolutionary Adaptedness can be identified in the modern human phenotype.

This is a minority position not widely held in biology. The conventional view of human evolution is that the first hominids evolved in terrestrial environments.

The hypothesis was originally suggested in 1960 by the marine biologist Sir Alister Hardy (1896-1985). The feminist writer Elaine Morgan developed and promoted it, publishing her first book on the subject, The Descent of Woman, in 1972. Her later books on the subject are: The Aquatic Ape (1982), The Scars of Evolution (1990), and The Aquatic Ape Hypothesis (1997).

Outline

The aquatic ape hypothesis puts forward these main arguments:

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 and inefficient. Bipedalism is not observed in other savanna animals. Water, however, supports the body, and non-human primates such as bonobos and proboscis monkeys have been observed wading bipedally in their occasionally flooded habitats.
  • Breathing: With the exception of humans and a few other outstanding animals (many of which had aquatic ancestry like the elephant), 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.
  • Childbirth: Dramatic increase in the size of the cranium is a prominent theme in human evolution, making childbirth difficult and dangerous. Water birthing is known to facilitate childbirth and to reduce risks to mother and infant. Human infants are born covered in vernix caseosa, a waterproof coating, and continue to draw oxygen through the umbilical cord while underwater.
  • Tears and excessive sweating, prevalent in humans but not in other primates, are considered further evidence to support the hypothesis, insofar as they are vectors for the removal of excess salts from the body, as might result from the ingestion of saltwater. Other marine animals, such as the marine turtle, secrete tears for this purpose, although these tears are of higher salinity than human tears.
  • 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; hair creates much more surface area for evaporation than skin.
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.

Conclusion

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

External links


de:Wasseraffen-Theorie es:Teoría del Simio Acuático ja:アクア説

Retrieved from "http://www.mywiseowl.com/articles/Aquatic_ape_hypothesis"

This page has been accessed 689 times. This page was last modified 13:19, 20 Nov 2004. All text is available under the terms of the GNU Free Documentation License (see Copyrights for details).