The aquatic ape hypothesis (AAH), sometimes referred to as the aquatic ape theory is an alternative explanation for human evolution which theorizes that the common ancestors of modern humans spent a period of time living in an aquatic environment. The theory suggests that a wide variety physiological differences between humans and primates, including bipedalism, relative hairlessness, fat distribution and secondary sex characteristics are better explained through aquatic adaptations than through the conventional scenario of human evolution within a savanna. The hypothesis was first published with an evolutionary explanation by Max Westenhöfer in 1942. The hypothesis was proposed again in 1960 by a marine biologist, Sir Alister Hardy, and subsequently promoted by the writer Elaine Morgan for more than forty years. Morgan has stated she believes the theory has not been seriously examined within the scientific community in part because it was proposed by a non-academic outsider.

The aquatic ape theory has not been accepted within the scientific community as a valid explanation for human divergence from related primates. There is no fossil evidence to support the theory, and it has been criticized as pseudoscientific, untestable, lacking parsimony, and supported only by circumstantial evidence. And it has been noted that the purported traits of aquatic adaptation are not actually uniquely human^{[citation needed]}, uniquely aquatic^{[citation needed]}, or misrepresented and fail to support the theory^{[citation needed]}. In addition the theory has been criticized for posing a false dichotomy between aquatic adaptation and the supposed savanna hypothesis, though mainstream paleoanthropology no longer believes human ancestors evolved within such an environment^{[citation needed]}.

Contents 1 Hypotheses 2 History 3 Reception 4 See also 5 References 6 Further reading 7 External links

Hypotheses

As compared to the great apes, their nearest living relatives, humans exhibit many significant differences in anatomy and physiology, including bipedalism,^[1] almost hairless skin like some marine mammals,^[2] hair growth patterns following water flow-lines,^[3] increased subcutaneous fat for insulation,^[4][5] descended larynx,^[3][6] vestigial webbing between the fingers,^[7] vernix caseosa,^[3] a hooded nose, muscular nostril aperture control and the philtrum preventing water from entering the nostrils,^[3] voluntary breath control like marine mammals and birds,^[3] and greasy skin with an abundance of sebaceous glands, which can be interpreted as a waterproofing device.^[8] It has also been suggested that the abundance of docosahexaenoic acid and iodine in seafood would have been helpful in the development of a large brain.^[9]

Morgan has pointed out that other hairless land-based species such as the elephants and the rhinoceroses also have aquatic ancestors, the main exception being the naked mole rat which lives underground. However there are several variants on the broad theme that early or proto-humans lived in close proximity to water, gathering much of their food in or near shallow bodies of water and developing and adapting new modes of locomotion in order to move and gather food (possibly including wading,^[1] swimming,^[10] and diving^[4]). Proponents have disagreed on the relative importance of fresh water^[11] versus coastal salt- or brackish-water^[12] habitats. Although the earliest proponents argued for an early (Miocene, about 6 million years ago) timescale,^[4] most now favour the view that the critical period of close association with waterside habitats was much later, Pleistocene or possibly late Pliocene (i.e., less than 2 million years ago).^[13][14] One hypothesis suggests that a semi-aquatic phase happened when ancestral Homo population spread along the South Asian coasts where during the Ice Ages the lowered sea levels exposed large areas of the continental shelves. In these conditions shell and crayfish would be easily procurable by a dextrous, tool-using, thick-enameled, omnivorous primate. This timing could explain why this seaside phase (100-120 metres below sea level now) did not leave many traces in the fossil and archaeological record. From the coasts their descendants might have trekked into the continents along lakes and rivers.^[15]

History

Sometime prior to 546 BC, the Greek 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 rather than natural selection.

The German biologist Max Westenhöfer was perhaps the first to publish the idea in an evolutionary context, writing in 1942 that "The postulation of an aquatic mode of life during an early stage of human evolution is a tenable hypothesis, for which further inquiry may produce additional supporting evidence."^[16]

The similarity of the subcutaneous fat in aquatic birds and larger aquatic mammals to the fat in humans had already been noticed by marine biologist, Sir Alister Hardy in 1930, while reading Frederic Wood Jones' Man's Place among the Mammals, which included the question of why humans, unlike all other land mammals, had fat attached to their skin. Hardy realised that this trait sounded like the blubber of marine mammals, and began to suspect that humans had ancestors more aquatic than previously imagined. Because it was outside his field and aware of the controversy it would cause, Hardy delayed reporting his theory. After he had become a respected academic, Hardy finally voiced his thoughts in a speech to the British Sub-Aqua Club in Brighton on 5 March 1960.

News of Hardy's speech generated immediate controversy in the field of paleoanthropology, and Hardy followed up by publishing two articles in the scientific magazine New Scientist. In the article of 17 March 1960 Hardy defined his idea:

My thesis is that a branch of this primitive ape-stock was forced by competition from life in the trees to feed on the sea-shores and to hunt for food, shell fish, sea-urchins etc., in the shallow waters off the coast. I suppose that they were forced into the water just as we have seen happen in so many other groups of terrestrial animals. I am imagining this happening in the warmer parts of the world, in the tropical seas where Man could stand being in the water for relatively long periods, that is, several hours at a stretch.^[4]

The idea received some interest after the article was published,^[17] but was generally ignored by the scientific community.

In 1967, the hypothesis was positively reviewed in The Naked Ape, a book by Desmond Morris in which can be found the first use of the term "aquatic ape".^[18] Feminist writer Elaine Morgan read about the idea in Morris' book and was struck by its potential explanatory power. She developed and promoted it over the next thirty years, publishing six books on the subject.^[19] Several other proponents have published work in favour of the aquatic ape hypothesis over subsequent decades.^{[9][11][12][13]}

Reception

The hypothesis and its variations aim to explain a number of unique features of humans compared to their ancestors but has largely been ignored by mainstream paleoanthropology or met with significant skepticism,^[20] and Daniel McNeill has described it as "...one of the most ridiculed and interesting ideas in prehuman anthropology" that has yet to undergo serious scrutiny.^[21] It has been suggested, for example, that because a broad terrestrial diet would ensure sufficient access to docosahexaenoic acid, there was no requirement for high consumption of seafood and accordingly no reason to posit an aquatic phase in human evolution for dietary reasons.^[22] The "savanna model" which the AAH purports to debunk is also no longer believed to be the primary environment that humans evolved in, with more recent theories propose a tree or forest-based habitat as providing the driving forces for adaptation.^[23]

Opponents dispute some of the claims made by Alister Hardy and Morgan: humans are inefficient swimmers; though babies can propel themselves in the water, they can not swim efficiently or lift their faces out of the water to breathe; many truly aquatic animals retain much more hair than humans; human bodies are not hydrodynamically shaped to travel easily through water; the subcutaneous fat distribution in humans is more similar to a domesticated animal than an aquatic one; bipedalism is more common in brachiating apes than in ground-dwelling ones and no aquatic species travel bipedally on land; and the human fossil record demonstrates a gradual adaptation from tree-dwelling to bipedalism rather than an abrupt transition to an aquatic environment. It is also claimed that the mammalian diving reflex is also more likely to be a reaction to immersion in cold water used to avoid hypothermia rather than drowning.^[24]

In 1987 a symposium was held in Valkenburg, the Netherlands, titled "Aquatic Ape: Fact or fiction?", which published its proceedings in 1991.^[25] The chief editor Vernon Reynolds summarized the results of the symposium as "overall, it will be clear that I do not think it would be correct to designate our early hominid ancestors as 'aquatic'. But at the same time there does seem to be evidence that not only did they take to the water from time to time but that the water (and by this I mean inland lakes and rivers) was a habitat that provided enough extra food to count as an agency for selection. As a result, we humans today have the ability to learn to swim without too much difficulty, to dive, and to enjoy occasional recourse to the water."^[26] The results of the conference were reported in the anthropological press as having rejected the hypothesis.^[27]

Despite proponents claiming the hypothesis has not had a fair hearing, a variety of scholars have examined the evidence for the AAH. Specific criticisms include inconsistencies within the hypothesis, the failure of the fossil record to support the claims^[27][28] and the hypothesis is not parsimonious despite claims that it is.^[27] To Morgan's statement that wading into shallow water would help proto-humans avoid predation from lions and hyenas, it was pointed out that this would present the risk of being attacked by aquatic animals such as crocodiles and hippopotami that present a current risk to Africans living near bodies of water,^[29] and that protohumans lacked the fangs, claws or size to defend themselves from these threats.^[30]

Humans would lose body heat quickly out of water; most similarly-sized aquatic mammals have dense, insulating fur. Hairless skin is only an advantage for fully-aquatic mammals that dive, swim quickly or migrate long distances such as whales and dolphins.^[30] Many aquatic mammals are covered with dense fur but swim very well, and have fatty layers beneath the skin; in many cases the imperfections help with streamlining due to laminar flow.^[29] Humans also lack immune system defences to protect against the waterborne parasites.^[30] The position, evolutionary timing of changes, and size of the nerve openings in the vertebra suggest that breath control in humans improved because of the increased complexity and use of speech rather than an aquatic phase of evolution.^[31] The loss of body hair is also explainable through a lower parasite load and maintained through sexual selection.^[32] Other criticisms include other water-dwelling mammals that have dense hair, such as otters, beavers and seals, that subcutaneous fat is the result of a sedentary lifestyle rather than aquatic adaptation, that swimming must be learned (just like walking) rather than being an innate reflex, a lack of a waterproof skin (as demonstrated by the "pruning" of the fingertips when submerged for long periods), eyes that are ill-adapted to see underwater, and the adaptation of the mammalian diving reflex being more easily explained as a reaction to hypothermia than underwater swimming.^[33]

Though describing the theory as plausible, Henry Gee went on to criticize it for being untestable, as most of the evolutionary adaptations described by Morgan would not have fossilized. Gee also stated that while purely aquatic mammals such as whales show strong skeletal evidence of adaptation to water, humans and human fossils lack such adaptations; that there are many hypothetical and equally plausible scenarios explaining the unique characteristics of human adaptation without involving an aquatic phase of evolution; and that proponents are basing arguments about past adaptations on present physiology, when humans are not significantly aquatic.^[34] There is ultimately only circumstantial evidence to suggest, and no solid evidence to support the AAH.^[23][35]

In a review of The Scars of Evolution, professor of anthropology Adrienne Zihlman states that Morgan sets up a false dichotomy between the AAH and the 'savanna hypothesis', which Zihlman calls "Morgan's code name for other prevailing reconstructions of human evolution". Zihlman states Morgan's first book did not address the central questions of anthropology - how the human and chimpanzee gene lines diverged - which was why it was ignored by the scholarly community. Zilman also says that Morgan ignores the fossil record, skirts the absence of evidence that australopithecine underwent any adaptations to water, making the hypothesis impossible to validate from fossils.^[28] The AAH is thought by some anthropologists to be accepted readily by popular audiences, students and non-specialist scholars because of its simplicity.^[27]

Morgan has claimed a variety of reasons why the AAH has not been accepted - old academics protecting their careers, sexism on the part of male researchers and her status as a non-academic intruding on academic debates. Despite modifications to the theory and occasional forays into scientific conferences, the AAH has neither been accepted as a mainstream theory nor managed to venture a genuine challenge the orthodox theories of human evolution.^[36] Colin Groves, professor of biological anthropology at the Australian National University also speculated that Morgan's status as an outsider may have affected reception of the AAH, and that it could not be discounted as a possible explanation for bipedalism.^[37]

See also

• Aquatic adaptation
• Human evolution

References

Further reading

• Williams, M. Morphological evidence of marine adaptations in human kidneys. Medical Hypotheses. 66: 2. 2006. p. 247-257

External links

• Aquatic ape hypothesis at the Open Directory Project
• Scars of Evolution (online series of two BBC radio programmes about AAH/AAT)
• Proponents presentation Elaine Morgan at TED July, 2009