Aquatic ape hypothesis

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The aquatic ape hypothesis (AAH), sometimes referred to as the aquatic ape theory, is a theory about the Human evolution. It proposes that distinctive human characteristics such as subcutaneous fat and bipedalism are the result of a period of evolution in which human ancestors lived an aquatic or partially aquatic lifestyle.

According to this theory, evolutionary pressures could have arisen from aquatic methods of feeding (underwater foraging) and the types of aquatic foods available.

The theory has antecedents in Ancient Greece but was first published with an evolutionary explanation in 1942. The aquatic ape hypothesis was discussed by ethologist Desmond Morris in his 1960 book The Naked Ape, and was popularized and promoted over the next 30 years by television screenwriter Elaine Morgan.

The theory has great popular appeal, some supporting publications in scientific journals and some support within paleoanthropology, at least for a weak version that doesn't accept there was an actual "aquatic ape". Criticisms include that the human dietary needs are not necessarily a result of eating seafood and that human skin and shape are not well adapted to swimming.

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

[edit] 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, 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.^[7]. 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.^[8]

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,^[9] and diving^[4]). Proponents have disagreed on the relative importance of fresh water^[10] versus coastal salt- or brackish-water^[11] 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).^[12][13] 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.^[14]

[edit] 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."^[15]

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.^[16] 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".^[17] 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.^[18] Several other proponents have published work in favour of the aquatic ape hypothesis over subsequent decades.^{[8][10][11][12]}

[edit] Reception

The hypothesis and its variations explains a number of unique features of humans compared to their ancestors but has largely been ignored by mainstream paleoanthropology or met with significant skepticism.^[19] 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.^[20]

In 1991 a symposium was held in Valkenburg, Holland, titled "Aquatic Ape: Fact or fiction?", which published its proceedings.^[21] The chief editor, Vernon Reynolds, rejected the strong version of the hypothesis, but accepted a weaker form, summarizing that "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."^[22] The results of the conference were reported in the anthropological press as having rejected the hypothesis.^[23]

However there has since been some acceptance. In 2004 Colin Groves, Professor of Biological Anthropology at the Australian National University in Canberra, Australia with co-author David W. Cameron stated that

...nor can we exclude the Aquatic Ape Hypothesis (AAH). Elaine Morgan has long argued that many aspects of human anatomy are best explained as a legacy of a semiaquatic phase in the proto-human trajectory, and this includes upright posture to cope with increased water depth as our ancestors foraged farther and further from the lake or seashore. At first, this idea was simply ignored as grotesque, and perhaps as unworthy of discussion because proposed by an amateur. But Morgan's latest arguments have reached a sophistication that simply demands to be taken seriously (Morgan 1990, 1997). And although the authors shy away from more speculative reconstructions in favour of phylogenetic scenarios, we insist that the AAH take its place in the battery of possible functional scenarios for hominin divergence.^[24]

Despite proponents claiming the theory has not had a fair hearing, a variety of scholars have criticized the AAH. Specific criticisms include inconsistencies within the theory, the failure of the fossil record to support the claims^[23][25] and the theory is not parsimonious despite claims that it is.^[23] To Morgan's statement that wading into shallow water would help protohumans 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,^[26] while lacking the fangs, claws or size to defend themselves.^[27] 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.^[27] 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.^[26] Humans also lack immune system defences to protect against the waterborne parasites.^[27] 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.^[28] The loss of body hair is also explainable through a lower parasite load and maintained through sexual selection^[29]

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. (Morgan's later books addressed this.) Zilman also says that Morgan ignores the fossil record, skirts the absence of evidence that australopithecine underwent any adaptations to water, making the theory impossible to validate from fossils.^[25] The AAH is thought by some anthropologists to be accepted readily by popular audiences, students and non-specialist scholars because of its simplicity and ease of communication.^[23]

[edit] See also

• Aquatic adaptation
• Human evolution

[edit] References

[edit] External links

• Aquatic ape hypothesis at the Open Directory Project
• Scars of Evolution (online series of two BBC radio programmes about AAH/AAT)