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The aquatic ape hypothesis (sometimes called the aquatic ape theory) proposes that the ancestors of humans went through one or more periods of time living in a semi-aquatic setting and that this history accounts for many of the characteristics of species in the Homo genus that are not seen in other primates, such as chimpanzees, gorillas and orangutans. The theory, often referred to simply as AAT, has been poorly received in mainstream paleoanthropology.
AAT states that human ancestors evolved in warm and wet environments and gathered much of their food from shallow sea-, lake- or riverside environments through beach-combing, wading and diving for foods such as coconuts, bird's eggs, turtles, shell- and crayfish, part of reeds, papyrus and other aquatic plants.  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. The most popular formulation involves a semi-aquatic episode coinciding with the Pliocene-Pleistocene littoral diaspora of the Homo genus along the East-African Rift Valley lakes and the African and Indian Ocean coasts.
A fairly broad definition of the aquatic ape theory is given by Kuliukas :
The Aquatic Ape Hypothesis (AAH):
The hypothesis that water has acted as an agent of selection in the evolution of humans more than it has in the evolution of our ape cousins. And that, as a result, many of the major physical differences between humans and the other apes may be explained, to a large extent, as adaptations to moving (wading, swimming and/or diving) better through various aquatic media and from greater feeding on resources that might be procured from such habitats.
Prior to 546 B.C., the Milesian philosopher Anaximander proposed that mankind had sprung from an aquatic species of animal. He thought that the extended infancy of humans could not have originally permitted survival as a land-based species. This idea, based on elemental forces of mutation as opposed to evolution, does not appear to have survived Anaximander's death.
The modern 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, including The Aquatic Ape (1982), The Scars of Evolution (1990), The Descent of the Child (1994), and The Aquatic Ape Hypothesis (1997).
Arguments for Aquatic Ape Hypothesis
The aquatic ape hypothesis puts forward several main arguments (some of the assertions in these arguments are in dispute).
Humans are the only primate species in which, over 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 (in some larger-sized mammals such as elephants — which are themselves descended from aquatic ancestors — and some rhinoceros species), aquatic (whales, dolphins, walrus, dugongs, and manatees), semi-aquatic or littoral (hippopotamus, babirusas (Babyrousa celebensis)), and subterranean (naked mole rat).
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. Chimpanzees, gorillas and orangutans are able to walk on two legs when they have a particular reason, this can be seen when chimps are feeding on grasses in a biou, they often stand on two legs and wade through the water with thier upper bodies out of the water, they do this with relative ease, but always revert to quadrupedalism as their basic means of locomotion. Some prosimians such as indris skip sideways on two legs when on the ground, because their adaptations to leaping through trees make ground-based quadrupedalism difficult. Kangaroos and hopping rodent species use a bipedal form of locomotion with bent knees and bent hips in rest. Even birds, with exceptions such as (semi-aquatic) penguins which have vertical vertebral columns, walk bipedally but with a horizontal vertebral column. Creatures such as squirrels and meerkats often adopt an upright posture when stationary, but do not walk or run bipedally.
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 proponents argue that if evolution works in small steps (gradualism), it is hard to see how bipedalism could have evolved on the savannah: the mass of the torso makes it inherently unstable and inefficient for locomotion. Water, however, supports the body, and proboscis monkeys as well as lowland gorillas have been observed wading bipedally in mangrove or swamp 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). Kuliukas in 2001 argues that the skeletal morphology of the early hominan Australopithecus afarensis is consistent with adaptation for wading in water. Kawamura in 1962 observed a troop of Japanese macaques developing bipedalism in water through cleaning sweet potatoes therein.
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. Morgan argued that this voluntary breathing capacity was one of the preadaptations to human voluntary speech.
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 even when considered slim. Mammals which hibernate have localised seasonal fat humps; but aquatic mammals retain fat (blubber]]) throughout the year. Human infants are especially fat compared with apes and most other fully terrestrial mammals. The human fatty layer (panniculus adiposus) is also attached to the skin of the central body parts as is the case with most medium- or larger-sized (semi)aquatic mammals, rather than to the muscle as in almost all land mammals. Humans also lack the layer of cutaneous muscle (panniculus carnosus) possessed by land mammals including non-human primates, which allows many land animals to twitch their skin, and which is not present in aquatic mammals.
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 common seals, and continue to draw oxygen through the umbilical cord while underwater. Human infants naturally hold their breath and can swim from birth.
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. Additionally, these omega-3 fatty acids promote (good) HDL cholesterol and cardiovascular health in humans, while saturated fats in pork, beef and other land-based meats do the opposite. Yet for land-based carnivores the opposite is true as they have special digestive enzymes to neutralize the deleterious effects of dietary cholesterol. It is noteworthy that many nutritionists find seafood to be the healthiest protein source for humans, whereas the meat of land-based mammals such as from beef or pork are the most harmful.
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). 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 sea lions on land may sweat, though this claim is rather tenuous.
The most common human mating practice, ventro-ventral ("missionary position" or "dolphin-style"), is essentially front-to-front, exactly how aquatic mammals must mate. Few other land animals (chimpanzees, gorillas and orangutans) use such a position more or less frequently; instead, mating coitus more ferarum 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.
Inherent difficulties in the evaluation of the aquatic ape theory
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 terrestrial models
The traditional land-based explanation is that fur loss 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. More recently, Pagel and Bodmer hypothesize the loss of human hair serves to reduce parasite load. In either case, humans can support hairlessness due to their own distinct advantages with shelter, clothing and fire; using these advantages enables humans to regulate their own temperature better than having a thick layer of fur: the thickness of fur is fixed, but one can remove clothing, move away from a fire, or leave a shelter. This is the same solution humans have for shaved sheep; they are given sheep blankets. Pagel and Bodmer claim "[o]ur hypothesis explains features of human hairlessness--such as the marked sex difference in body hair, and its retention in the pubic regions--that are not explained by other theories." 
Body hair helps protect against direct sun (shaved sheep overheat more easily) and extreme heat as well as cold. Human sweating is highly wasteful of water and salts, which is a distinct disadvantage on the savanna; and that exposed skin might not be 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 aquatic ape theory, 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 (some) infants.
On the second point, it is possible to suggest an aquatic 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 difficult 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.
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. Usually these are given as a continuation of ape bipedalism which is less adaptive
The difficulty with this, according to aquatic ape proponents, is that they only apply for a small amount of time, unlike wading which could have happened frequently; 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.
For apes to become bipedal they would need more of a curved spine, a basin-like pelvis, and the development of an arched foot with short toes. Apes are capable of walking upright but only for a short distance. Humans can walk long distances but cannot reach speeds on two legs that an ape can on four limbs.
Although, according to savanna hypothesis long term bipedalism would be the preferred mode of locomotion for most activities as carrying things in a savanna would be very frequent. Also, an assumption of reverting to quadrupedalism assumes quadrupedalism was the previously preferred method of transportation (see Bipedalism Objections hereinafter). While modern humans are not engaged in bipedal activities we do not revert to quadrupedalism, while in water humans are frequently not bipedal.
Fat is often believed to be important in developing and maintaining the brain, which is a very expensive organ in terms of energy requirements. This suggestion, as with body hair, requires an additional explanation for women and babies having a much higher proportion of body fat than men -- such as that babies need fat to aid high levels of post-natal brain growth not seen in other primates, and women need fat to aid in pregnancy and lactation.
The aquatic ape hypothesis accounts for the sex differences in fat (as with body hair) by suggesting that nursing mothers would have spent more time in water than adult males. Terrestrial models suggest that the difference is due to the need for it during pregnancy and lactation.
Objections to Aquatic Ape Hypothesis
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 the aquatic ape hypothesis 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. Sweating is an adaptation to thermo-homeostasis, particularly important for brain function. For an animal whose success is mostly due to a large brain, this is a key adaptation. Sweating as a means of thermo-regulation is best achieved with sparse fine hair, permitting access of air to the skin. It is further possible that the relative hairlessness of humans has nothing to do with either thermo-regulation or resistance to motion in water; it may rather be the result of sexual selection for hairlessness in females, (but this does not explain why baldness occurs more frequently in men than in women).
The suggestion that aquatic mammals or semiaquatic mammals lose their hair is based on a small group of aquatic mammals that did, and typically larger aquatic mammals. Many large mammals have shorter finer hair, regardless if they are aquatic or not. Most aquatic and semiaquatic speciese of mammals retain their hair. Even in water, hair is a good insulator. Beavers, otters, fur seals, polar bears have all retained their hair.
Many juvenile primates, such as chimps, have larger heads, less hair, and a greater ability to learn. This combined with similar skeletons has led many evolutionary biologists, including Stephen Jay Gould, to conclude that the nakedness of humans is due to a neoteny of our chimp/human ancestor.
A 2003 study holds that nakedness may have developed as a way to reduce parasite load once hominids could regulate heat more effectively with fire and clothing. This, combined with sexual selection, may explain the hairlessness as well as the pattern of hairlessness for each gender.  However parasites live on clothing as well, and if reducing parasite load was significant, then head hair would likely have been lost as well in males and females equally.
Most apes are at least temporarily bipedal, using their upright state for locomotion, feeding and sentry behavior all of which are useful for terrestrial life. Brachiators such as orangutans and gibbons, typically move by swinging in trees, when they come down to the ground they typically walk in a bipedal fashion, they are not quadupedal. The reason for brachiation is speed over bipedalism, not inability. There is no requirement to evolve this in water as bipedalism already exists in the primate family.
There is a growing school of thought that the common ancestor between humans and chimpanzees was actually a brachiator and chimpanzees simply reverted to knuckle-walking. As such, bipedalism would, when the African forests changed to savanna, be the best mode available to such apes.
No aquatic mammal is bipedal. Few tetrapods evolve walking on two feet in an environment where swimming is preferred. Beyond this, some chimpanzees do walk in a bipedal fashion without much effort such as Oliver the chimpanzee.
Some ability to moderate breathing is seen in many other mammals, including other primates (for instance, macaques have been observed diving for foods underwater) and dogs. The descended larynx in humans, which functions both for breath-holding and vocalisation, is claimed by Aquatic ape theory to have developed to assist in breath-holding first and subsequently led to speech. However, the human larynx only descends in early childhood, at the same time as children learn to speak. Although, not as developed as human speech, most mammals (including all primates) and many animals, in general, do make voluntary sounds.
Almost all mammals have a diving reflex to some degree, and many non-aquatic mammals can hold their breath longer than the typical human. Dogs for example can hold their breath for three minutes.
The quality of having many small and numerous fat cells under the skin is not unique to humans among land animals. Rather it is shared with many species including hedgehogs, monkeys and badgers. In addition, the distribution of these fat cells in humans does not correspond with the distribution of fat cells in whales, seals or other aquatic mammals. Fat in aquatic mammals had evolved for use in streamlining and insulation (though not as insulating as blubber), and fat in humans does not function in this way nor to the same extent. Fat distribution in humans corresponds with developing via sexual selection and as a luxury evolved from having a lack of predators. Deers isolated from predatory wolves have been shown to become fatter in the same way, as have monkeys kept on special diets.
The argument that humans are generally fatter than other primates is questionable in the first place, as human studies tend to use city-based subjects, who have more fatty diets than rural people and especially rural people in ancient times. Fat amount seems to be more aptly explained by self-domestication rather than an aquatic ancestry.
The difficulty in childbirth is due to two primary factors, the increased size of an infant's head (due to increased brain size) and narrowing of the hips (a byproduct of bipedalism). There are advantages to waterbirths, but the typical method for human childbirth before hospitals was standing or leaning while on dry land.
Although there may be less Omega-3 fatty acid on land than in fish, it exists in sufficient quantity for modern humans. In fact, humans can have an adequate diet without eating anything aquatic.
Removal of salt
Marine mammals usually remove extra salt via urine. Humans cannot drink much salt water and survive. This is inconsistent with an oceanic origin for humans, requiring that we have a nearby freshwater source. It is not inconsistent with a freshwater origin.
It is true that the human penis is the largest of all primates. However, this can more easily be attributed adaptations in favor of bipedalism, such as the loss of the os penis rather than an aquatic stage of man. Human females do not have an inaccessible vagina.
Humans' tendency to mate ventro-ventral does not necessarily imply aquatic adaptation. It could just as well be explained as another byproduct of bipedalism, having an entirely different cause than the parallel development of ventro-ventral mating in marine mammals. In addition, humans are able to mate in a variety of positions, including coitus more ferarum, but dolphins are only able to mate ventro-ventral due to the location of their reproductive organs. Quadripedal animals mate coitus more ferarum because their limb function does not permit otherwise, again suggesting that this property of humans is merely a byproduct of bipedalism.
Most mammals are better native swimmers than apes and humans. Swimming among humans and apes (some, but not all, can learn to swim) does not come easily and is a learned skill. Most mammals when dropped in the water for the first time can swim well enough to survive. However an ape and human without training would typically drown. Although infants can propel themselves under water, they cannot keep their nose above water and would drown after a short period of time. As of 2005, in the United States the lifetime chance of dying by drowning was 1-in-8,942 .
Traits atypical of aquatic mammals
Many features humans possess are not associated with aquatic or semi-aquatic creatures:
- large ears
- long limbs
- broad rounded shoulders
- large breasts
- current lack of aquatic behavior
- inability to see under water effectively (although Moken have better underwater vision than other people (Gislén et al. 2003)—certainly an aquatic adaptation, though very recent)
- long hair on top of head (not typical of any aquatic animal and covered by professional swimmers)
Since the 1960s, the theory hasn't improved; it has simply become more vague in time period(s), degree and water source (Ellis 1993, Verhaegen et al. 2002, Morgan 1982, Hardy 1960). Rather than suggesting an aquatic stage of evolution, some are reduced to suggesting some general association with water (which is true of all animals). This vagueness makes such a theory impossible to evaluate scientifically. Furthermore, there is much to suggest that hominids were still covered with hair long after they began to walk bipedally full-time . There is no suggestion that humans had more fat than other species before modern civilization enabled a sedentary lifestyle. Many humans without exposure to plentiful food do not have more fat than other animals.
Recent fossil finds
Since the 1960s, more fossils have been found to fill in the gaps in human evolution. It can be argued that these fossils support the terrestrial theories of human evolution. Orrorin tugenensis for example appears to show bipedalism and tree-climbing skills, which could cut the time for an aquatic stage completely out of the picture.
Attacks on proponents
Although it has very little to do with the theory itself, many skeptics of the aquatic ape hypothesis attack the proponents of the hypothesis, rather than address the hypothesis itself. Ad hominem attacks have been repeatedly launched against supporters of the theory. Much has been made of the fact that Alister Hardy pondered the role of telepathy on evolution. Also subject to ridicule have been lesser claims made by some proponents in non-scholarly works, such as a suggestion the mermaids are some kind of race memory of an aquatic stage. Others have gone so far as to parody the theory, such as with the Pliocene Pussy Cat Theory, which suggests that early humans used cats to hunt for them.
The AAT Debate
The aquatic ape hypothesis provokes fierce and often acrimonious contention. Skeptics criticise the lack of direct fossil evidence; the sometimes amateurish way in which the theory 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 assumed; and the failure to provide land-based alternative hypotheses that survive the very criticisms levelled at AAT.
- 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 (1982). The Aquatic Ape, Stein & Day Pub. ISBN 0-285-62509-8.
- Morgan, Elaine (1990). The Scars of Evolution, Souvenir Press. ISBN 0-285-62996-4.
- Morgan, Elaine (1997). The Aquatic Ape Hypothesis, 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).
- Kuliukas, A., "Wading for Food: The Driving Force of the Evolution of Bipedalism." Nutrition and Health, 16(4), 267-290, (2002). html
- Gislén, A. et al., 2003. Superior underwater vision in a human population of sea gypsies. Current Biology 13: 833-836. http://www.current-biology.com/content/article/abstract?uid=PIIS0960982203002902 See also Pilcher, Helen R., 2003. How to see shells on the sea floor, http://www.nature.com/nsu/030512/030512-14.html
- Mark Pagel, Walter Bodmer, A naked ape would have fewer parasites, Proceedings of the Royal Society B: Biological Sciences , Volume 270, Issue 0, 07 Aug 2003, Pages S117 - S119, DOI 10.1098/rsbl.2003.0041, URL http://dx.doi.org/10.1098/rsbl.2003.0041
- Aquatic adaptation
- Human evolution
- Savanna Theory
- Aquatic ape theory discussion group - specifically devoted to discussing Aquatic ape theory, 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 the aquatic ape theory.
Pro-aquatic ape hypothesis
- A Critique of Jim Moore's 'Aquatic ape theory Sink or Swim?' Web Site - (see below).
- The Aquatic Ape Theory - an argument for the hypothesis by Elaine Morgan.
- Aquatic Ape Info - by Dewi Morgan (grandson of Elaine Morgan).
- Aquatic Ape Theory: The Most Plausible Explanation of Human Origins - from Riverapes.com.
- Publications and presentations on the aquatic ape hypothesis by Kuliukas
- Scars of Evolution (audio) - a BBC Radio 4 radio documentary.
- The role of floods in ape evolution - a BBC article.
- Seashore Man and African Eve A search for the origins and evolution of humankind in Africa - Denis Montgomery
Anti-aquatic ape hypothesis
- Aquatic Ape Theory: Sink or Swim?
- Did humans descend from 'aquatic apes'? - a Staff Report on the aquatic ape hypothesis by The Straight Dope.
- Pliocene pussy cat theory
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