Aquatic ape hypothesis
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The aquatic ape hypothesis (or a common misnomer: aquatic ape theory) is the proposal that wading, swimming and diving for food had a large evolutionary effect on the ancestors of humans and is in part responsible for the split between the common ancestors of humans and other great apes.
The hypothesis is in contradiction to the more established hypothesis that the environment of evolutionary adaptiveness for hominids was the savannas of the Rift Valley and the more recent hypotheses that the environment was forests or even that it rapidly fluctuated.
The hypothesis that the morphological divergence of hominids was dictated by an aquatic environment was first formulated in 1923 by the German physician Max Westenhöfer and British marine biologist Sir Alister Hardy. Hardy was inspired after returning from an Antarctic expedition where he noticed the subcutaneous blubber fat layers of whales and seals and saw parallels to humans. He was also impressed by the human "exceptional ability" to swim. Fearing it would jeopardize his scientific career, Hardy did not publish his ideas until 1960.
Elaine Morgan took up the fight in 1972 when she discussed the aquatic ape hypothesis in her book The Descent of Woman. Morgan has been the primary proponent of the hypothesis, and culminated her argument in 1997 with the publication of The Aquatic Ape Hypothesis.
The major divergence between the great apes and hominids can be placed somewhere between 5-7 million years ago (mya) based on the fossil record. During this time significant differences in anatomy developed including:
- Bipedalism, which the theory claims was assisted by the supporting effects of water.
- Hairless skin, with the exception of the head which still needed to be shielded from the sun. Meanwhile primates such as baboons and vervet monkeys which live on the savanna have neither lost their hair nor developed an upright posture.
- Increased subcutaneous fat
- Greatly expanded brain size, which was promoted by the protein-rich fish diet.
- A hooded nose, which prevents water from entering the nostrils.
- Greasy skin with an abundance of sebaceous glands, which can be interpreted as a waterproofing device.
- Expressive faces, since communicating through body language is difficult when wading in deep water or swimming but communicating through facial expression is still possible.
Elaine Morgan, the primary proponent of this hypothesis, claims that these traits are common in aquatic mammals. She claims that these demonstrate convergent evolution of mammals living in an aquatic environment. The aquatic stage would have been about 5 mya until about 2.5 mya when the grassland expansion begun.
Objections to the hypothesis
There are several problems with this idea. The first is that convergent evolution predicts adaptations serving similar function, not necessarily similar structure. A platypus bill looks like a duck bill because it solves the precise same problem (sucking bugs out of mud); but the wings of a bat, a bird, and a bumblebee are products of convergent evolution in solving the problem of flight, but are structurally very different. Morgan's convergent evolution adaptations focus on structure.
The second problem is that evidence provided for the AAH is equally well accounted for by savanna-based adaptations without needing to posit an aquatic phase of human development.
- Loss of body hair: lower parasite load (early hominids appear to have been disgusting skanks) and maintenance through sexual selection. Furthermore, many aquatic mammals have actually kept their body hair. Several other savanna species have also lost much of their hair including elephants, rhinoceroses, and naked mole rats. Although it is interesting to note that both elephants and rhinoceroses are theorised to have evolved from aquatic ancestors so they may not be the best counter-examples, though the only modern rhinoceros that spends much of its time in the water is the hairy rhinoceros.
- Diet: the fatty acids for the brain were present in sufficient quantities on the savanna.
- Diving reflex: terrestrial mammals show this too; there has been insufficient study of other apes to use this as evidence that humans are special in this regard.
- Body fat: distribution is similar to that of other primates. The quantities seen in present-day humans accord with those seen in domesticated mammals, i.e. we're quite well fed compared with wild dumb beasts. Humans have relatively more fat below the waist which serves as insulation when wading. 
- Bipedalism: the AAH compares humans to quadrupeds, but we went from quadrupedal locomotion to brachiation; bipedalism follows quite readily from that.
- Descended larynx: again, speech. The human larynx is not shaped like that of aquatic mammals, and descended larynxes are also seen in dogs, pigs, goats, monkeys and young chimps.
- Nose shape: again, speech. Speech was rather important in human development, oddly enough.
- Sebaceous glands: not actually found in aquatic mammals. Appear to be sexually dimorphic (highly active in adolescent human males, scent receptors highly active in adolescent human females).
- Swimming: humans really aren't that good at swimming. Dogs and cats do about as well.
A third major problem is there are actual characteristics indicative of aquatic mammals which humans notably lack. Aquatic mammals are testicond and have internal testes to insulate the gonads; humans are not testicond. Aquatic mammals also store large amounts of oxygen in the blood rather than the lungs; a trait lacked by humans.
The Aquatic Ape Hypothesis cannot easily explain why these features were maintained after our ancestors supposedly left the water and moved to savannas. 
Further, there's good evidence that many of the traits claimed to fall under the umbrella of the aquatic ape hypothesis evolved at radically different points in human evolutionary history. Fossils from 4.4 mya display bipedalism, while still possessing small brains long after, and not evolving sparse body hair until later still.
One of the largest problem with the hypothesis is that since the 1960s, rather than improving and gathering evidence, the hypothesis has become more nebulous and vague. Rather than some specific aquatic phase, modern aquatic ape hypothesis supporters cite anything from fresh water millions of years ago, salt water habitation more recently, several different aquatic phases or a general proximity to water to a greater degree than other apes. Incidentally other apes and even some monkeys sometimes gather food from water and may stand bipedally to do this.
There are also anti-science objections lodged against the creation of the hypothesis because Morgan had the idea first and then simply found differences between humans and other apes and composed ad hoc reasons associated with water. One could cherry pick data for many such pet theories without any merit behind them, while making them sound good on paper.
The aquatic ape hypothesis has mostly been ignored by palaeoanthropologists, and its strongest reception has been in popular literature and magazines. Very little research has been published in scientific journals directly supportive or critical of the hypothesis. Morgan blamed the poor reception to the idea on people's perception of herself personally, since she had no advanced degree.
Supporters of the hypothesis claim that this lukewarm reception resulted from the fact that Elaine Morgan was not a member of the palaeoanthropological establishment. Detractors maintain that the lack of convincing evidence may have played a role.
Modern aquatic primates
- Japanese macaque (Macaca fuscata) — since they live in a cold climate, during the winter, they frequent certain hot springs to keep warm.
- Certain bonobos spend a certain amount of time in water, either in the shallows or crossing it.
- Chimpanzees. Apparently not as much as bonobos, but this link shows a swimming one as does this one which shows a chimp walking bipedally in water. Apparently the chimps use a kind of breaststroke.
- Orangutans have been known to spend time in water and swim.
- Some monkeys in Thailand have also been filmed diving into water. The Proboscis monkey loves swimming too as can be seen here.
And, of course, Homo sapiens. Certain humans live in deserts etc, but some humans, especially those in the Indian Ocean islands, spend a lot of time in the sea, pearl diving, etc. Others are keen swimmers across distances of sea. While many humans are not currently "aquatic", a notable minority are.
- Elaine Morgan's web site.
- Elaine Morgan quoted on the aquatic ape theory.
- A strong critic of Elaine Morgans's quoting techniques.
- Wikipedia's article on Elaine Morgan.
- An interview with... Elaine Morgan!
- Wikipedia's article on the aquatic ape hypothesis.
- Elaine Morgan talking about the hypothesis at TED, 2009.
- Big brains, no fur, sinuses … are these clues to our ancestors' lives as 'aquatic apes'?
- Vaneechoutte, M. 2000. 'Report of the Symposium ‘Water and Human Evolution’. Human Evolution 15(3) 243-251.
- A Theory on the Evolution of the Habitual Orthograde Human Bipedalism — The "Amphibisce Generalistheorie": Niemitz, C
- The Aquatic Ape: Elaine Morgan
- Was man more aquatic in the past? (NewScientist): Hardy, A.
- Evidence for the unique function of docosahexanoic acid (DHA) during the evolution of the modern hominid brain (Lipids): Crawford, M et al
- Kingdon, Jonathan. (2003) Lowly Origin Princeton University Press, 242
- Pond, Caroline M. 1991 "Adipose Tissue in Human Evolution", pp. 193-220. The Aquatic Ape: Fact or Fiction? Edited by Machteld Roede, Jan Wind, John M. Patrick and Vernon Reynolds. Souvenir Press: London.
- aquatic elephant evolution
- aquatic rhinoceroses evolution
- The last suggestion could make sense because evolving humans being more intelligent than other apes would better find ways of getting to food in any lake, river, stream, coastal environment where they happened to be. Perhaps there was no specific aquatic phase. Perhaps there were evolving pre-humans/humans making use of water when there happened to be water based resources in their area but able to manage without when there was relatively little water. After all that is how modern humans behave. This in itself could perhaps have enabled us to evolve some limited adaptations to water. Perhaps there was some limited aquatic phase and the readiness of evolving hominids after this aquatic phase to use water opportunistically was sufficient to maintain some aquatic adaptations.
- First documented report of swimming and diving in apes — ScienceDaily