Aquatic ape hypothesis

The aquatic ape hypothesis (AAH), sometimes referred to as the aquatic ape theory, proposes that the ancestors of humans went through one or more periods of living in aquatic settings, which was responsible for the development of many of the characteristics of Homo genus that are not seen in other primates. The AAH theorizes that when early or proto-humans lived in proximity to water, they gathered much of their food in or near shallow bodies of water through beach-combing, gathering aquatic plants and animals, coconuts and bird's eggs. Variations propose fresh-water habitats, in the timescale, 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.

The hypothesis and its variations has been received with a mixture of criticism, support and cautious discussion in mainstream paleoanthropology.

History

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 was based on elemental forces of mutation as opposed to evolution.

The history of the modern origin of the AAT is poorly known. For instance, some researchers believe that the first "aquatic hypothesis" was originally suggested in 1972 by Elaine Morgan. Most are sure that this idea was proposed in 1960 by Alister Hardy (who Morgan explicitly cites as the originator of this idea), and few insiders believe that the first author was the German pathologist Max Westenhöfer (1871-1957), who published his "Aquatile Hypothese" in his main work The Road to Man (Der Eigenweg des Menschen). However, Westenhöfer's idea was already proposed as early as 1923.

The origin of the idea of a more aquatic phase in hominid evolution in the English language can be traced to 5th March 1960 when the marine biologist Sir Alister Hardy publicly announced it in a presentation to a sub-aqua club in Brighton and was quickly followed up by two articles published in the scientific magazine "New Scientist". Hardy had had the idea privately since about 1930, independently of Westenhöfer. The early television playwright and later feminist writer Elaine Morgan developed and promoted it, publishing multiple books on the subject.

Arguments for the aquatic ape hypothesis

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, over most of the body, hair is so fine and sparse as to reveal the skin under it into adulthood. Some aquatic mammals such as whales, dolphins, walrus, dugongs, and manatees, and some semi-aquatic mammals hippopotamus also have sparse hair and visible skin into adulthood. The babirusa is a medium-sized littoral mammal which is about as naked as humans are. Therefore, humans have sparse hair because they are semi-aquatic.

Counter claims

Humans also developed sweat glands. These displaced hair follicles in the skin to allow cooling of the body during vigorous exercise, an adaptation which seems useless for water dwellers.

The suggestion that aquatic mammals or semi-aquatic mammals lose their hair is based on a small group of hairless, typically larger aquatic mammals. Many large mammals have shorter, fine hair, regardless of their aquatic involvement. Most aquatic and semi-aquatic species of mammals retain their hair. Even in water, air is a good insulator. Beavers, otters, fur seals, and polar bears all retain 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.

Some parasites live on clothing as well, though these would have developed after humans lost their hair. This is shown by evolutionary studies of the body louse as well as the typical purposes of clothing for warmth being needed after or concurrent with nakedness. Parasite load would still be reduced when the parasite-infested clothing was removed.

Bipedalism

A gorilla wading through water

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. Gorillas, chimpanzees and bears 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 bayou. They often stand on two legs and wade through the water with their 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.

An orangutan on the ground, walking bipedally

Aquatic ape theory proponents argue that as evolution works in small steps and in such a situation it is hard to see how bipedalism could have evolved on the savannah. Water, however, supports the body, and proboscis monkeys as well as lowland gorillas have been observed wading bipedally in mangrove or swamp forests. Baboons have also been observed to cross shallow water bipedally.

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).

Counter claims

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 quadrupedal. 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.

The common ancestor between humans and chimpanzees was quite possibly much closer to bipedal than previously thought, and chimpanzees could have later adopted knuckle-walking.[1] As such, bipedalism would already be available to such an ancestor.

There are a number of bipedal species and nothing suggests that "vertical vertebral columns" are indicative of a water evolution. This claim only serves to exclude all the examples of bipedalism.

Body weight is not an issue for apes walking bipedally. Apes are not often observed torso deep in water. Apes walk in a bipedal fashion quite easily without water to support their torso.

Some chimpanzees do walk in a bipedal fashion without much effort such as Oliver the chimpanzee.

Breathing

Humans are the only land animals with a dive reflex.

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.

Counter claims

All vertebrates have a dive reflex.

All land animals have some control over their breathing, and most have an ability to make voluntary noises such as dogs barking or lions roaring.

Aquatic mammals do not inhale and store the oxygen they need in their lungs; they store it in their blood.

Fat

Aquatic mammals retain fat (blubber) throughout the year. Human infants are especially fat compared with apes. 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.

Counter claims

Despite claims by proponents of AAH, human fat is structured in a very similar way to chimpanzees. The fat in humans is attached to muscle.

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 fat in humans does not function in this way.

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.

Facial Structure

Human facial structure is quite different from other apes, with thick eyebrow hair and downturned nostrils. The shape of the human nose, with nostrils running perpendicular to the rest of the face, prevents water from entering the nose while upright. Thick eyebrows allow water to flow from the top of the head away from the eyes upon surfacing, allowing for faster adjustment to vision through air. Human facial hair forces water to flow away from and around the nose and mouth to enable faster inhalation upon surfacing.

Counter claims

The fact that the eyebrows and the shape of the skull keep water out of the eyes and nose is no indication of an aquatic origin, as these features are equally useful for keeping perspiration (sweat) out of the eyes and nose, which is necessary for humans as they rely on sometimes copious perspiration to regulate body temperature.

Childbirth

Dramatic increase in skull 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 not found in any other land mammal. A radio programme on the AAH (Scars of Evolution, Episode 2) aired on 2005 postulated that, if vernix is due to the aquatic origin of humans, then other aquatic species may also show this phenomenon. The program searched for this evidence and found a report of the common seal also being born with a layer of vernix.

Counter claims

Water birthing does not reduce risks of child birth. [2] The risks are primarily the result of larger heads and narrower hips.

Breasts

Humans are the only mammal with perpetually swollen mammary glands. Human females have large fatty breasts that float on the surface of water. This would have allowed a mother to feed an infant at varying depth while the baby was kept out of the water and thus allowed to breathe normally.

Counter claims

The most common theory about human breasts are that they exist as sexual selected characteristics. Rather than serving a use to breastfeeding, women have larger breasts than other primates because men like larger breasts. Breasts also provide a sexual signal from the anterior as the buttocks do for the posterior. Others have found that large breasts are by themselves not evolutionarily advantageous and rather exist as a byproduct of selection for fat.

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. 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. Omega-3 fatty acids also promote HDL cholesterol and cardiovascular health in humans, while the saturated fats in pork, beef and other land-based meats do the opposite. Yet for land-based carnivores the opposite is true and they have special digestive enzymes to neutralize the harmful 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.

Counter claims

The human requirement for omega-3 fatty acids is not proof of an aquatic stage in human evolution; it simply tends to indicate that early humans evolved to make use of a diet rich in seafood, which can be obtained by fishing and the gathering of shellfish in intertidal regions. Existing non-aquatic evolutionary theory recognises the importance of seafood in the early human diet (see 'Counter claims' under 'Kidney morphology' below).

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 former 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.

Counter claims

Aquatic mammals dispose of extra salt through urine.

Humans cannot drink much saltwater without dehydration and death.

Aquatic mammals do not sweat.

Reproductive traits

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 (bonobo, orangutan, potto, sloths, all arboreal) 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 (possibly explaining why the human penis is much longer in relation to body size than any other primate penis), a trait long noted as specific to humans and bonobos (who live partially in flooded forest) among primates.

Counter claims

It is true that the human penis is large relative to body size. However, this can more easily be attributed to 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.

The human 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. Although it should be noted that bonobos are also known to frequently use 'missionary position', and to not be stringently limited to 'heat' seasons.

Swimming

Swimming among humans and apes is not innate, and must be learned. Infants are capable of the equivalent of underwater paddling and breath hold. Humans have slight webbing between the fingers.

Also, as exercise physiologists may note, of all exercise types, swimming far and away puts the least strain on human bones and joints. Whereas running, jumping, climbing and virtually all other forms of land-based physical activity put undue and often injurious strain on human joints, swimming is rarely, if ever, deleterious to the human skeleton.

Counter claims

Many mammals are better native swimmers than humans. Humans have trouble keeping their heads above water and, without training, easily drown. Other mammals are situated horizontally rather than vertically and can easily swim to safety.

Despite underwater paddling, infants cannot keep their noses above the water's surface. For this reason, if left unattended in water they can and do drown.

Some apes such as Kloss's gibbon also has webbing between fingers.

Water-based exercise results in greater levels of body fat. [3]

Kidney morphology

Humans parallel marine mammals in having multiple medullary pyramids in their kidneys. To avoid the deleterious effects of saline water dehydration, marine mammals have adaptively thickened the medullas of their kidneys – which enhances their ability to concentrate excretory salts in the urine. However, the lobulation of the kidney’s medullary region in marine mammals appears to be an adaptation to expand the surface area between the medulla and the enveloping outer cortex in order to increase the volume of marine dietary induced hypertonic plasma that can be immediately processed for the excretion of excess salts and nitrogenous waste.

A phylogenetic review of freshwater aquatic mammals suggests that most, if not all, nonmarine aquatic mammals inherited the medullary pyramids of their kidneys from ancestors who originally inhabited, or frequented, marine environments. So this suggests that most, if not all, aquatic mammals exhibiting kidneys with lobulated medullas are either marine adapted – or are descended from marine antecedents. Additionally, a phylogenetic review of nonhuman terrestrial mammals possessing kidneys with multipyramidal medullas suggests that bears, elephants and possibly rhinoceroses, also, inherited their lobulated medullas from semiaquatic marine ancestors.

Multipyramidal medullas are rare in the kidneys of terrestrial mammals and are-- non existent-- in catarrhine primates-- except for humans. And those terrestrial mammals that do have them either have coastal marine ancestors (bears, elephants, rhinos) or are capable of drinking salt water (bovines).

Most marine mammals avoid drinking sea water. And experiments have shown that when sea water is inserted into the stomachs of pinnipeds, they usually regurgitate. Most of the intake of hypersaline fluids in marine mammals is from the ingestion of marine invertebrates whose bodies are usually isotonic with their marine environments. The human kidney, however, is fully capable of excreting water with a salinity up to 3.6% salt which is higher than most marine environments. However, since the body also produces hypotonic metabolic water, any hypertonic fluids ingested would be partially diluted by the hypotonic water produced from the oxidation of ingested proteins, fats, and carbohydrates.

Counter claims

Terrestrial mammals also have kidneys with lobulated medullas.

Humans cannot drink much seawater and survive.

Humans, according to fossil evidence, ate sea food and lived near the coast. This is not a significant divergence from mainstream understanding. Kidney adaptations quite possibly are the result of eating seafood. However, this reduces the theory down to the claim that human ancestors ate fish, a conclusion already reached without an overarching water theory, and by scientists working on the problem of human evolution. [4]

Spleen

The human spleen, an organ that stores oxygen-rich blood, is relatively large in humans, serving as a kind of biological "scuba tank". This is suggested after studying physiological changes in freedivers; The contracted and compressed spleen adds extra oxygenated blood to the circulation. The German physiologist Max Westenhöfer already noticed the resemblance of the human spleen with aquatic mammals:

Quoting from Der Eigenweg des Menschen 1942, Berlin,:

"(...) The condition of the 'incomplete uniformation or consolidation' of the human spleen and kidneys has been surpassed only by one species of mammals, namely the Cetaceans, the whales (dolphins and porpoises), compared to the kidneys of cows, of the pinnipeds, of seaotters and bear, hippopotamus and rhinoceros, in other words of animals, which used to or still live in or near water. Of special significance is the observation, that all new-born anthropoid apes have fully consolidated kidneys, as do the majority of mammals, while their spleen may in rare cases show remains of grooving; the consolidation has not yet reached its full extent. Humans (...) stand, in this respect, on one of the lower rungs of the ladder of evolution, while the anthropoids, their so-called closest relatives, as well as the other mammals have by far surpassed them in this respect. Whales, dolphins, seals, sea otters and bears seem much closer relatives to humans when we compare their spleen and kidneys than do anthropoids, who have by the way also developed remarkably far ahead of humans considering the consolidation of the aveolas of the lungs. These remarkable coincidences between such different species must naturally be interpreted as convergences. The fact that these species have a common environment, water, allows the consideration that in an early stage of human mammalian development there existed a being with an aquatic way of life."

The Island Hypothesis

Only the northern Afar region has been repeatedly flooded by marine waters during the late Miocene, Pliocene, and Pleistocene starting about 6.7 million years ago. During the early Pliocene, some small areas of land in the northern Afar region would have been isolated from the rest of Africa for more than 2 million years. The Danakil Depression was also completely underwater during the early Pliocene and has been repeatedly re-flooded by marine waters from the Red Sea since that time. Hominins, possibly in the form of Ardipithecus could have become isolated in the Afar Island region 5.3 million years ago during the re-flooding of the Red Sea, remaining in this insular environment for more than 2.5 million years where they were forced to substantially increase the amount of benthic marine invertebrates in their diets in order to supplement the limited terrestrial food resources found on their small island environment. During this semiaquatic phase, it is hypothesized that these isolated hominins developed the multiple medullary pyramids of their kidneys while evolving into the genus Homo.

The fall of global sea levels after the beginning of a new glacial period, 2.7 million years ago, would have allowed Homo to radiate onto the African continent, possibly with a higher encephalization and a novel lithic technology, the Oldowan, that may have originally been developed to pry open the shells of marine invertebrates. Further evidence for the island isolation of human ancestors during the early Pliocene is evident by the apparent isolation of early Pliocene human ancestors from particular African retroviruses that infected all other species of primates on the African continent, in addition to substantial loss of genetic variation found in humans relative to other extant hominoids.

Criticisms of the aquatic ape hypothesis

Traits atypical of aquatic or semi-aquatic mammals

Many features humans possess are not common amongst aquatic or semi-aquatic mammals:

• large ears
• long limbs
• large breasts
• long hair
• bipedalism
• non-native swim ability
• sweat glands
• external testicles
• inability to see under water effectively (although Moken have better underwater vision than other people (Gislén et al. 2003))

Vagueness

Since the 1960s, the theory hasn't changed much nor increased its testable predictions; in most respects it has become less specific as objections have been proposed. The theory has become less specific as to time period ranging from many thousands of years ago to millions. The theory is purported to explain bipedalism more than 4 million years ago, hairlessness 1.2 million years ago , or the increased fat within recorded history. The water source is either freshwater or seawater or some combination of the two, often depending on the objection or trait the theory is explaining. To deflect from these problems some proponents become more vague on the number of aquatic stages suggested. Rather than suggesting an aquatic stage of evolution, some proponents have been reduced to suggesting some general association with water (which is true of many, indeed most, mammals) or more association with water than that of chimpanzees or other apes.

This vagueness and the many different versions of the theory make the theory impossible to evaluate scientifically. This, combined with the lack of evidence, makes suggestions concerning the validity of AAH seem like special pleading.

Lack of evidence

No direct fossil evidence for AAH has ever been found. The vagueness of the theory makes it difficult to determine anything which could be evidence, what time period this evidence should be found in, what environment the it should exist in, or where it should be found.

Arguments from ignorance

Rather than citing evolutionary or fossil evidence, proponents of aquatic ape hypothesis often rely on arguments about how certain human morphological characteristic are only or heavily restricted to aquatic mammals. This is, opponents claim, an argument from ignorance; simply because another explanation is not forthcoming does not mean that there is no other explanation. Comparing human morphology to that of other mammals requires one to ignore all the known ways humans are different from other mammals: full time bipedalism, use and control of fire, use of more advanced technology, large brains. Anthropology is finding explanations for these characteristics and typically they heavily relate to other unique characteristic of humans; it is based on theories which not only explain the characteristics in question but also have evidence to support why the theory advanced is the actual cause of those characteristics.

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 less scholarly works, such as a suggestion the mermaids are some kind of race memory of an aquatic stage.

AAT in popular culture

• In the album This Toilet Earth by thrash metal band GWAR the song The Insidious Soliloquy Of Skulhedface features the lyrics: "I am he who has been him back when man swam Pangean seas as aquatic apes."

• In Michael Crichton's novel Next the passage: "According to her reading, the loss of hair had occurred after human beings separated from chimps. The usual explanation was that human beings had become for a time swamp creatures, or water creatures. Because most mammals were hairy their coats of fur were necessary to help maintain their internal temperature. But water mammals, such as dolphins and whales had lost their hair in order to be streamlined. And people, too, had lost their hair." The aquatic ape theory is addressed as being the primary model for human evolution.

See also

• Aquatic adaptation
• Human evolution

References

• Bender, Renato; Verhaegen, Marc & Oser, Nicole (1997): Der Erwerb menschlicher Bipedie aus der Sicht der Aquatic Ape Theory. Anthropologischer Anzeiger 55 (1), 1-14.
• Bender, Renato and Oser, Nicole (1997): Gottesanbeterinnen, Maulwürfe und Menschen. Unipress 95, 20-26.
• Cameron, D.W. (2004). Bones, Stones and Molecules:" Out of Africa" and Human Origins. Academic Press. ISBN 0121569330. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Ellis, D.V. (1986). "Proboscis monkey and aquatic ape". Sarawak Mus J. 36: 251–262.
• Ellis, D.V. (1995). "Human Ancestors in Wetland Ecosystems". ReVision. 18 (2): 8–12.
• Craig Hagstrom. The Passionate Ape. Riverforest Press. ISBN 0970262655.
• Kuliukas, A. (2002). "Wading for food the driving force of the evolution of bipedalism?". Nutr Health. 16 (4): 267–89. PMID 12617279. {{cite journal}}: |access-date= requires |url= (help)
• Montgomery, Denis. Seashore Man and African Eve. Lulu Enterprises, UK Ltd. ISBN 1847538118.
• Tobias, P. (1998). "Water and human evolution". Out There, December.

External links

• Did humans descend from 'aquatic apes'? - a Staff Report on the aquatic ape hypothesis by The Straight Dope.
• Water and human evolution – proceedings from the Gent Conference 1999.