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The aquatic ape hypothesis (or aquatic ape theory as it is frequently called) is most commonly interpreted to hold that ancestors of humans and other hominids went through one or more periods of time living in a semi-aquatic setting on an African seacoast, that they gathered most of their food from the seashore and shallow offshore waters before their 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. However, there are other interpretations which propose fresh-water habitats (e.g. Ellis 1993), variations in the timescale (e.g. Verhaegen et al 2002) and the proposed degree of selection arising from moving through water.
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, although the proximity to fresh-water sources of the great majority of hominid paleohabitats is not in doubt.
The hypothesis was originally suggested in 1942, by Max Westenhőffer in The Road to Man (Der eigenweg des menschen), but became more (in)famous in 1960 when proposed in academic circles by the marine biologist Sir Alister Hardy (1896-1985). The early television playwrite and, later, 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), The Descent of the Child (1994) 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. The one other animal to develop a pelvis designed for walking like a human's was the Oreopithecus, known commonly as the Swamp Ape.
- Extant apes most predictably move bipedally in waist deep water.
- 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. Human babies also reflexively hold their breaths both when submerged, as well as swimming without training. Humans also have the "diving reflex" otherwise unique to aquatic animals, where their heart rate decreases and blood distribution is automatically altered to support the brain when swimming under water. Some young children also have the ability to completely close their nostrils at will like an aquatic mammal, which then explains the sinus cavity as a means of equalizing pressure. And humans have a descended larynx, otherwise found only in sea otters and dugongs, allowing them to quickly breath through their mouths, a trait so useful for semi-aquatic activity that the only birds which breath through their mouths are diving birds.
- 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. Human infants are especially fat compared to apes and most other fully terrestrial mammals. The human fatty layer is also attached to the skin, like most aquatic mammals, not the muscle, like almost all land mammals. Humans also lack the layer of cutaneous muscle, present in every land mammal (including all other primates), that allows many land animals to twitch their skin, but is absent in aquatic mammals.
- 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.
- 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. Indeed, most animals which move to plains life tend to develop smaller brains, while aquatic animals tend to evolve larger ones, quite possibly because of access to Omega 3.
- 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 sea 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. Humans also have hands and feet which are useful, among other things, for paddling in water.
- Reproductive Traits: The most common human mating practice, missionary position, is essentially front-to-front, exactly how aquatic mammals must mate. No other land animals use such a position, instead mating coitus more ferarum, as with dogs. Marine animals...even non-mammals...also tend to develop a less accessible vagina to keep out water, and thus larger male reproductive organs, a trait long considered unusual to humans and bonobo chimps (who live partially in flooded forest), among primates.
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. Although fresh-water forms of this hypothesis are consistent with the fossil evidence.
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. A prime example of this is the horse, which does sweat when hot, and yet is covered in hair. Indeed, most savanna animals have hair in part because it provides protection to skin from the heat and ultraviolete radiation of direct sunlight, not as important to semi-aquatic animals which are cooled and sheltered by water.
- 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 over a dozen 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 could have waded much more frequently) 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. In waist deep water, apes have little choice but to move bipedally and do so, very predictably. This is unusual for mammals which typically continue to wade quadrupedally, or switch to swimming.
- 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. Further, most proponents of AAH claim that the putative aquatic ancestor was never that aquatic, thus presenting an internal inconsistency in their arguments when a feature of dedicated marine mammals appears without similar selective stimulus.
- 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; an exaggeration of the degree of aquaticism being proposed; and the failure to provide land-based alternative hypotheses that survive the very criticisms levelled at AAH.
Resources - Ellis, D. V., "Wetlands or Aquatic Ape? Availability of food resources." Nutrition and Health, 9, 205-217 (1993).
- 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
- Verhaegen, M., Puech, P-F, Munro, S., "Aquarboreal Ancestors?" Trends in Ecology and Evolution, 17, 212-217 (2002).
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