Aquatic ape hypothesis

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The aquatic ape hypothesis is a hypothesis in evolutionary biology proposing that the ancestors of human ancestors went through one or more periods of time living in a semi-aquatic setting and that this accounts for many of the characteristics of species in the Homo genus that are not seen in other primates, such as chimpanzees or gorillas.

Few paleoanthropologists accept the hypothesis. Most adhere to the Savanna Theory, the dominant school of thought among scientists for the past 20 years, which holds that human ancestors evolved in savanna environments.

The minority supports of the aquatic ape hypothesis say that the ancestors of modern humans went through one or more periods of time living in a semi-aquatic setting, gathering much of their food from shallow coastal or other waters, before their descendants returned to a more land-based existence. These proponents hold that that adaptations to this marine environment can be identified in the modern human phenotype when compared to other mammals. There are interpretations which propose fresh-water habitats (Ellis 1993), variations in the timescale (Verhaegen, et al. 2002) and the proposed degree of selection arising from moving through water. One interpretation is that the semi-aquatic episode coincided with the Pliocene-Pleistocene littoral diaspora of Homo along the coasts of Africa.

The hypothesis was originally suggested in 1942, by Max Westenhofer in The Road to Man (Der Eigenweg des Menschen). It became more well-known in 1960 when proposed in academic circles by the marine biologist Sir Alister Hardy. Hardy had had the idea privately since about 1930, independently of Westenhofer. The early television playwright and later feminist writer Elaine Morgan developed and promoted it, publishing in 1972 her first book on the subject, The Descent of Woman, and later other books, inclduing The Aquatic Ape (1982), The Scars of Evolution (1990), The Descent of the Child (1994), and The Aquatic Ape Hypothesis (1997).

1 Outline
2 Comparison with land-based hypotheses
3 Objections to AAH
4 Conclusion
5 Sources
6 See also
7 External links

Outline

The aquatic ape hypothesis puts forward several main arguments (some of the assertions in these arguments are in dispute):

Nakedness: Humans are the only primate species in which hair does not cover almost the entire body. Environments known to give rise to naked mammals are tropical (in some larger-sized mammals), aquatic (whales, dolphins, walrus, dugongs, and manatees), semi-aquatic (hippopotamus, pigs, tapirs, rhinoceroses and elephants), or subterranean (naked mole rat).
Bipedalism: Humans are among very few bipedal mammals, none of which adopt the fully-upright, full-time human posture. Gorillas, chimpanzees and bears are able to walk on two legs when they have a particular reason, but always revert to quadrupedalism as their basic means of locomotion. Some lemurs skip sideways on two legs when on the ground, because their adaptations to leaping through trees make ground-based quadrupedalism difficult. Kangaroos and similar creatures use a bipedal form of locomotion that is quite different from human walking. Even birds, with a few exceptions such as penguins, walk bipedally but with their bodies held horizontally, unlike the human upright posture. Creatures such as squirrels and meerkats often adopt an upright posture when stationary, but do not walk or run bipedally. Although the posture gives humans the ability to use tools while walking or running, bipedalism and upright posture come at a significant cost, from back and knee problems, varicose veins, hemorrhoids, hernias, and problems with childbirth. Proponents of the aquatic ape hypothesis argue that if evolution works in small steps (gradualism), it is hard to see how bipedalism could have evolved on the savanna: the mass of the torso makes it inherently unstable and inefficient for locomotion. Water, however, supports the body, and proboscis monkeys have been observed wading bipedally in mangrove forests. It has been claimed that the one other animal known to have a pelvis adapted to bipedal walking was prehistoric Oreopithecus bambolii (commonly known as the "swamp ape" owing to its flooded habitat.
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 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, though they appear to lack the ability to surface and breathe unassisted. Some young children also have the ability to close their nostrils at will like an aquatic mammal. Elaine Morgan argued that humans have a descended larynx, allowing them to quickly breathe through their mouths, a trait so useful for semi-aquatic activity that the only birds which breathe through their mouths are diving birds.
Fat: Proponents of the aquatic ape hypothesis claim that humans have ten times as many fat cells as would be expected in an animal the same size, and have many adipose cells even when considered slim. Mammals which hibernate have seasonal fat; but aquatic mammals, along with humans, retain fat (blubber) 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 of the central body parts (as is the case with most aquatic mammals), rather than to the muscle (as in almost all land mammals). Humans also lack the layer of cutaneous muscle (platysma), possessed by every land mammal (including all other primates), which allows many land animals to twitch their skin, and which is not present in aquatic 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 are born covered in vernix caseosa, a waterproof coating also seen in newborn harbour seals, 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: Sweating and tears are 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 water and salts from the body, as might result from the ingestion of saltwater (as in eating food from a salt marsh). Other alleged ex-marine animals, such as the elephant, cry saline tears, and the mechanism by which humans produce sweat from eccrine glands could have developed as a means of shedding extra salt. Overheated sealions on land may sweat, though this claim is rather tenuous.
Reproductive traits: The most common human mating practice, ventro-ventral ("missionary position"), is essentially front-to-front, exactly how aquatic mammals must mate. Few other land animals (bonobo, orangutan, potto, sloths, all arboreal) use such a position; instead, mating coitus more ferarum ("doggy style") is the norm, as with, for example, dogs. Marine animals, even non-mammals, also tend to develop a less accessible vagina to keep out water, necessitating a longer penis, a trait long noted as specific to humans and bonobos (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. Furthermore, swamps and marshes are inimical to the creation of fossils.

Comparison with land-based hypotheses

Nakedness; The traditional land-based explanation (the "thermoregulatory hypothesis") is that it was for cooling - humans sweat more per unit surface area than other mammals, 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 supposedly 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 ultraviolet 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. However, this view is not consistent with a model of diving hominids who most certainly would have submerged their heads, nor is it clear that the pattern of hair does minimize drag in anything but the most extreme attempts to keep the head out of the water, something that cannot be acheived at greater swimming speeds.

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. By contrast, 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. However, it is not clear that the biomechanical obstacles involved in wading at such depths are consistent with the morphological changes seen in an obligate terrestrial biped.

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 (cetacea, sirenia, etc.) is suspect, as these animals have evolved characteristics over a far longer period than humans. Further, many 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. The babirusa is a littoral tropical medium-sized mammal which is about as naked as humans are.
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 limbs substantially longer than upper limbs, as is common in wading birds. Human legs do not fit this pattern (although the ratio of leg length to arm length is indeed higher in humans than in other primates).
Breathing: The ability to moderate breathing, to a lesser degree of control, is seen in many other animals, including other apes and dogs. Japanese macaques have been observed diving and swimming beneath the surface as well. The mammalian diving reflex also exists in other mammals.

Conclusion

AAH provokes fierce and often acrimonious contention. Skeptics 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.

Sources

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).

External links

Neutral

AAT discussion group, specifically devoted to discussing AAT, including a lot of files and links.
The Aquatic Ape: Fact or Fiction, proceedings from the Valkenburg Conference 1987. Souvenir Press, 1991, ISBN 0-285-63033-4.
Water and human evolution, proceedings from the Gent Conference 1999.
Wisconsin Primate Research Centre, a list of pro- and con- pages on the web.
Water and human evolution, opinion of professor Phillip Tobias on AAT.