Aquatic ape hypothesis

Newborns float and hold their breath instinctively when submerged. This is argued to be one of many aquatic adaptations by proponents of the aquatic ape hypothesis.

A female gorilla wades across a body of water. Observed bipedalism in many simian species in shallow water is argued as illustrative of the origin of human bipedalism.

The aquatic ape hypothesis (AAH), often also referred to as aquatic ape theory (AAT), although it does not hold the status of a scientific theory, is a hypothesis about human evolution, which posits that the ancestors of modern humans spent a period of time adapting to a semiaquatic existence. AAH emerged from the observation that some traits that set humans apart from other primates have parallels in aquatic mammals. The hypothesis was first proposed by German pathologist Max Westenhöfer in 1942, and then independently by English marine biologist Alister Hardy in 1960. After Hardy, the most prominent proponent was Welsh writer Elaine Morgan, who wrote a series of books on the topic.

AAH is not accepted among the mainstream explanations of human evolution. The scientific consensus is that humans first evolved during a period of rapid climate fluctuation between wet and dry, and that most of the adaptations that distinguish humans from the great apes are adaptations to a terrestrial, as opposed to an earlier, arboreal environment. Few paleoanthropologists have explicitly evaluated AAH in scientific journals, and those that have reviewed the hypothesis have been critical. An extensive criticism appeared in a peer reviewed paper by John H. Langdon in 1997. Langdon states that the AAH is one of many hypotheses attempting to explain human evolution through a single causal mechanism, and that the evolutionary fossil record does not support such a proposal; that the hypothesis is internally inconsistent, has less explanatory power than its proponents claim, and that alternative terrestrial hypotheses are much better supported. AAH is popular among laypeople and has continued support by a minority of scholars. Langdon attributes this to the attraction of simplistic single-cause theories over the much more complex, but better-supported models with multiple causality.

History

The German pathologist Max Westenhöfer (1871–1957) can be said to have worded an early version of AAH, which he labeled "the aquatile man" (German: aquatile Mensch), which he described in several publications during the 1930s and 1940's. Westenhöfer was partially influenced by contemporary German National Socialism and disputed Charles Darwin's theory on the kinship between modern man and the great apes. As part of a complex and unique version of human evolution, he argued that a number of traits in modern humans derived from a fully aquatic existence in the open seas, and that humans only in recent times returned to land. In 1942, he stated: "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." Westenhöfer's aquatic thesis suffered from a number of inconsistencies and contradictions, and consequently he abandoned the concept in his writings on human evolution around the end of the Second World War.

Independently and ignorant of Westenhöfer's writings, marine biologist Alister Hardy (1896–1985) had since 1930 also hypothesized that humans may have had ancestors more aquatic than previously imagined, although his work, unlike Westenhöfer's, was rooted in the Darwinian consensus. As a young academic with a hypothesis belonging to a topic outside his field, and because he was aware of its inherent controversy, Hardy delayed reporting his idea for some thirty years. After he had become a respected academic and knighted for contributions to marine biology, Hardy finally voiced his thoughts in a speech to the British Sub-Aqua Club in Brighton on 5 March 1960. Several national newspapers reported distorted presentations of Hardy's ideas, which he countered by explaining them more fully in an article in New Scientist on 17 March 1960. Hardy defined his idea:

My thesis is that a branch of this primitive ape-stock {hominoids} 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.

The idea received some interest after the article was published, but was generally ignored by the scientific community thereafter. In 1967, the hypothesis was briefly mentioned in The Naked Ape, a book by Desmond Morris (1928–) in which can be found the first use of the term "aquatic ape".

While doing research for her book The Descent of Woman published in 1972, a book inspired by reading Morris' The Naked Ape, TV-writer Elaine Morgan (1920–2013) was struck by the potential explanatory power of Hardy's hypothesis. While elaborating on Hardy's suggestion, in the book Morgan primarily sought to challenge what she considered a masculine domination of the debate on human evolution, and the satirical book became an international bestseller, making Morgan a popular figure in feminist movements and on various TV talk shows in, for example, the United States. On the other hand, her scientific contributions, including her elaboration on Hardy's aquatic humans, were effectively ignored by anthropology. Consequently, Morgan became the leading proponent of Hardy's original idea, which after a number of publications culminated in 1997 with the book The Aquatic Ape Hypothesis, which, with its factual language and proper referencing, was aimed primarily at the academic community.

In 1987 a symposium was held in Valkenburg, the Netherlands, to debate the pros and cons of AAH. The proceedings of the symposium were published in 1991 with the title Aquatic Ape: Fact or Fiction?. The chief editor summarized the results of the symposium as failing to support the idea that human ancestors were aquatic, but there is also some evidence that they may have swum and fed in inland lakes and rivers, with the result that modern humans can enjoy brief periods of time spent in the water.

Weaker versions of the hypothesis suggesting littoral feeding and wading rather than strong aquatic adaptation have since been proposed. These weaker versions of the hypothesis have not yet been scientifically explored.

The context of the initial presentations of AAH (a popular essay and a political text) diverted attention away from the possible scientific merits of the hypothesis. It has never been seriously scrutinized and discussed within the field of paleoanthropology and most paleoanthropologists reject the AAH.

Hypothesis

Suggested aquatic adaptations in humans (click to enlarge). Note that the points listed are not facts, but hypothetical claims that require further scientific investigations to verify their accuracy, falsifiability, and relevance to human evolution.

AAH suggests that many features that distinguish humans from their nearest evolutionary relatives emerged because the ancestors of humans underwent a period when they were adapting to a semiaquatic existence, but returned to terrestrial life before having become fully adapted to the aquatic environment. Variations within the hypothesis suggests these protohumans to have spent time either wading, swimming or diving on the shores of fresh, brackish, alkaline or saline waters and feeding on littoral resources.

Key arguments have been developed and presented by Elaine Morgan since 1972, these based on the original suggestion of Alister Hardy. In later years, other contributors have further developed the aquatic ideas, some of which differ heavily from the original "aquatic ape" of Hardy et Morgan. The term waterside hypotheses of human evolution has been coined by AAH-proponent Algis Kuliukas to collectively represent this diversity, of which AAH is only one such hypothesis. Most traits perceived as aquatic are physiological and biochemical, while few are behavioral (ethological). The time frame for the origin and possible termination of such an aquatic existence also differs between proponents, or though the same time frame as anthropological consensus is generally followed. In most cases, this aquaticism is perceived as having been instigated by selective pressure around the split of the last common ancestor between humans and chimpanzees.

Anatomical parallels with those of the modern primate species that swim, wade, dive, or use aquatic environments for thermoregulation, display behavior, range, diet, or predation, but the behavioral parallels, e.g., between humans and the proboscis monkey, could be facilitated by anatomical adaptations without having been the basis for them.

The argued degree of human aquaticism varies amongst proponents, however the vast majority argues a semiaquatic ape on par with e.g. hippos and sea otters. Very few have argued a fully aquatic stage on par with e.g. whales or pinnipeds, and this is rejected by the majority, including Morgan. Some pseudoscientific and cryptozoologic speculations have made use of parts of the AAH argumentation, e.g. the claimed existence of mermaids, but this is also rejected by proponents, including Morgan.

While most proto-human fossil sites are associated with wet conditions upon the death of the hominins, this is not seen as unequivocal evidence for the AAH since being buried in waterside sediment is one of the rare situations in which fossilization is likely to occur; paleontologists are aware of this preservation bias and expect fossils to be located near such sediments.

Several theoretical problems have been found with the AAH, and some claims made by the AAH have been challenged as having explanations aside from a period of aquatic adaptation. Review of the individual claims used as evidence for the AAH generally does not support the hypothesis overall, and most of these traits have an explanation within conventional theories of human evolution. Other authors have suggested that wading, food gathering and other interactions with watery environments may have provided a less extreme but still present role in human evolution.

Specific claims

Proponents of the AAH have claimed that a number of specific features in hominin evolution suggests that water played a role in natural selection, and that specific traits shared by all humans can therefore be understood as adaptations to an aquatic environment. These claims have been criticized for only superficially supporting the hypothesis, and for not being supported by the fossil record. All of the features that proponents claim to explain by exposure to an aquatic environment have conventional explanations that are more accepted within the paleoanthropological community.

Physiological and biochemical claims

Bipedalism is argued as an adaptation to wading in shallow waters.

• Bipedalism. Some proponents of AAH claim that bipedalism offers numerous advantages in water, including permitting deeper wading, improved balance and reduced strain on the back, hips and knees as well as improved blood circulation. But bipedialism also gives many advantages on land, particularly lower energy expenditure and the ability of long-distance running—which humans do better than most terrestrial mammals. Proponents of the AAH suggest that bipedalism is disadvantageous when comparing humans to medium-sized, terrestrial quadrupeds, but the fossil record shows that the evolution of humans from ape ancestors didn't include a period of quadrupedal locomotion. Instead, human evolution features mainly brachiation, suspension and climbing as the primary method of transportation, with a gradual increase in bipedal locomotion over time. In addition, the elongated lower limbs of humans, which is explained by AAH proponents as improving swimming speeds, appears only after the evolution of the genus Homo and biomechanical analysis indicates humans are far too poor swimmers to have derived from an ape ancestor that swam, and pre-human apes would face similar problems. There is no single accepted explanation for human bipedalism but freedom of the hands for tool use, carrying of infants, feeding adaptations, improved energy expenditure or some combination of these are suggested, with considerable diversity in pre-human skeletal adaptations that would assist in bipedalism.

Loss of fur in humans is argued to be convergent with some furless aquatic, semiaquatic and former semiaquatic (e.g. elephants) mammals.

The insulating layer of fat in the human skin (shown here as yellow beads) is perceived as a primitive layer of blubber.

• Hairlessness. Morgan claimed the relatively hairless skin of humans was due to comparable adaptations in aquatic mammals and land-dwelling mammals that have aquatic ancestors as well as those that currently spend much of their time in wet conditions, and what body hair humans do have follows the flow of water over the body. However, humans vary strongly in the amount and distribution of body hair and comparably sized mammals adapted to semi-aquatic lifestyles actually have dense, insulating fur or large, barrel-shaped bodies that retain heat well in water. Hairlessness is only an advantage for aquatic mammals such as whales and dolphins that have spent millions of years adapting to aquatic lifestyles involving diving, fast swimming and migration over long distances; such animals show considerable skeletal and cardiovascular adaptations to an aquatic environment. Though a variety of explanations have been proposed for human hairlessness, the best-supported hypothesis involves improved cooling through perspiration; while fur helps cool inactive animals, hairless skin that sweats vigorously is much better at cooling humans who generate body heat through activity. Langdon, in his 1997 critique of the hypothesis, stated that the streamlining features attributed to hair follicle distribution and direction would be more reasonably achieved through changes in the shape of the skeleton and soft tissues.

The inherent human diving ability far exceeds all other simians.

• Descended larynx. The human larynx is situated in the throat rather than the nasal cavity, a feature that is shared by some aquatic animals who use it to close off the trachea while diving, it also facilitates taking large breaths of air upon surfacing. However, other terrestrial mammals, such as the red deer, also have a permanently descended larynx. Humans also have a considerable amount of control over their breathing, which is an involuntary reflex for most terrestrial mammals. However, breath control is thought to be preceded by bipedalism, which frees up the muscles of the upper torso from locomotion and allows breathing independent of limb position. Both of these adaptations are thought to derive from improvements in vocalization and the evolution of the ability to speak and the human larynx is shaped differently from that of aquatic animals, predisposing humans to choking.

DHA and Iodine rich seafood, e.g. shellfish, is argued as a possible prehistoric dietary source to the expansion of the human brain.

• Encephalization: The human encephalization quotient, an expression of the size and complexity of the brain of a species, compared to its physical size and other factors, is considered the highest in the animal kingdom, followed by whales, in particular dolphins, other great apes, elephants, certain species of squid and some intelligent birds. It has been argued, that aquatic mammals more often develop large brains, and that particularly grassland mammals conversely stagnate in brain development. Morgan and other authors have suggested that the encephalization of the human brain was a response to increased consumption of seafood. A team lead by Canadian biochemist Stephen Cunnane has argued, that both developing and maintaining a healthy human brain is heavily dependent on a key series of micronutrients, most especially docosahexaenoic acid, DHA (an Omega 3 fatty acid) and iodine-ions. Both these have proven extremely rare in purely terrestrial food groups (including cereals, fruits, vegetables and husbandry meats), but are conversely abundant in fish, shellfish and other sea foods, particular from saline and alkaline waters. Critics have argued, that considerable human encephalization began quite late in the development of the genus Homo, particularly with Homo erectus, long after the development of bipedalism. Bipedalism had occurred already in the australopithecines (4.2–3.9 mya) and Ardipithecus (4.4 mya), and perhaps as early as in the species Sahelanthropus tchadensis (approx. 7 mya). On the other hand increase in cranial capacity occurs quite late in the fossil record: Homo habilis (approx. 2 mya) for example, while fully bipedal, had a brain size within the range of modern day gorillas. Counter to this, Cunnane et al. have argued, that a transition of semiaquatic Hominina-forms from fresh water habitats in the hinterland of Africa to more alkaline and saline habitats in Eastern Africa, e.g. in the then sea-flooded Afar-depression in modern Ethiopia, could have supported the increase in human brain size through an increased access to e.g. DHA and iodine rich foods. It is argued, that molluscs, e.g. clams and oysters, as found along the shores of East Africa and in alkaline lakes along the Great Rift Valley has an optimal composition to support the extant human brain's nutritional needs. Proponents point to archaeological finds of shellfish kitchen middens as far back as in middle pleistocene some 164,000 years ago, during the earliest days of archaic Homo sapiens. Conversely, critics argue that landlocked humans without access to seafood develop normal brains and that these nutritional requirements can be met with a specific terrestrial diet. The encephalization of early Homo species is also argued as having been possibly driven by the consumption of hunted or scavenged animal brains supplying large amounts of scarce nutrients including DHA.

• Reproduction: Morgan and others point to the increased adiposity of human infants, a marked difference from the offspring of other great apes. This is suggested as an adaptation to increased insulation and buoyancy in water for human babies. It is pointed out, that vernix caseosa, a cheesy varnish coating the skin of newborn babies, apart from humans so far has only been observed on the cups of a few pinniped species, e.g. harbour seals. It is pointed out, that infant humans cannot walk upright, until as much as one year of age, completely unknown among simian offspring, e.g. grassland-dwelling baboons. Morgan also claims, that newborns are adequately suited to swim along with their mother, while being able to hold their breath upwards of 45 seconds. Historically, women throughout the world have experienced a series of potentially life-threatening circumstances delivering above water, while Morgan and others, e.g. the French physician Michel Odent, point to recent decades of studies into baby swimming and water birth, which have become common practices in modern obstetrics and pediatrics to relieve stress and pain effects for both mother and child, with no corresponding observed drowning risks for the child. Morgan also points to unique features of both men and women's genitals, and the woman's protruding, fat-filled bosom as possible aquatic adaptations, with alleged convergence observed in sirenia.Presented criticism to these claims include the infant's increased risk of drowning if parting from its mother, coupled with observations of both young children as well as adults developing aquaphobia, Presented criticism to these claims include the infant's increased risk of drowning if parting from its mother, coupled with observations of both young children as well as adults developing aquaphobia, while baby swimming and water birth are being rejected as fads.

Ethological claims

• Habitat: Proponents of AAH argue, that the concentration of the contemporary world population of more than 7 billion people illustrates a trend for Homo sapiens to cling together in coastal regions alongside river valleys, lakes and seashores, in modern times particularly in India and China. Critics argue that this cannot disclose a similar trend in the eon long development of Homo, as modern human urbanization is said to be dependent on easier transit routes, this including rivers and seas. Conversely, Morgan argues, that humans have a similar preference for recreation sites along said coastal regions, e.g. tropical beaches.

Human bathing behavior is argued as a remnant of recent semiaquaticism.

• Bathing behavior: Across the globe, humans employ hygiene bathing, this regardless of faction, ethnicity, gender, etc. This is argued as a behavioral relic of a semiaquatic stage. As an offshoot to the argument that other traditional terrestrial mammal groups may also be past semiaquatics, noted are similar behavioral traits in both African and Asian elephants, which are also observed bathing and swimming regularly with its whole body submerged.

Other claims

Rarely presented AAH-arguments point to the human tendency to produce watery psychic tears, and the production of sweat as a cooling mechanism. Morgan has withdrawn previous arguments along this line, given that horses also sweat profusely. It is occasionally argued that humans compared to other apes have reduced olfaction, with claimed convergences observed in other aquatics, e.g. whales; that the protuding human nose would be adapted to keep splashes out of nasal cavities, arguing the semiaquatic proboscis monkey or semiaquatic tapirs as possible convergences; the tendency of partial to full baldness in men; the tendency for human obesity; and that human kidneys are better suited for excretion of salt than other apes. Such arguments are generally considered more speculative and are often heavily criticized.

Theoretical considerations

Critics of AAH note, that the descended larynx is not exclusive to humans in terrestrials, but also found in red deer.

Elaine Morgan retracted an early AAH-argument of human sweating being an aquatic indicator after critics cited, that also horses sweat.

The AAH has been criticized for containing multiple inconsistencies and lacking evidence from the fossil record to support its claims (Morgan, for instance, failed to discuss any fossils found after 1960 and much of her analysis is by comparing soft tissues between humans and aquatic species). It is also described as lacking parsimony, despite purporting to be a simple theory uniting many of the unique anatomical features of humans. Anthropologist John D. Hawks expresses the view that rather than explaining human traits simply and parsimoniously, it actually requires two explanations for each trait - first that proximity to water drove human evolution enough to significantly change the human phenotype and second that there was significant evolutionary pressure beyond mere phylogenetic inertia to maintain these traits (which would not be adaptive on dry land) and points out that exaptation is not an adequate reply. Hawks concludes by saying:

In other words, the Aquatic Ape Theory explains all of these features, but it explains them all twice. Every one of the features encompassed by the theory still requires a reason for it to be maintained after hominids left the aquatic environment. Every one of these reasons probably would be sufficient to explain the evolution of the traits in the absence of the aquatic environment. This is more than unparsimonious. It leaves the Aquatic Ape Theory explaining nothing whatsoever about the evolution of the hominids. This is why professional anthropologists reject the theory, even if they haven't fully thought through the logic.

Ellen White describes Morgan's work as failing to be empirical, not addressing evidence that contradicts the hypothesis, relying on comparative anatomy rather than selection pressure, not predicting any new evidence and failing to address its own shortcomings. White stated that while the hypothesis had the scientific characteristics of explanatory power and public debate, the only reason it has received any actual scholarly attention is due to its public appeal, ultimately concluding the AAH was unscientific. Others have similarly noted the AAH "is more an exercise in comparative anatomy than a theory supported by data."

Though describing the hypothesis 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 (a comment made by others as well); 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. There is ultimately only circumstantial evidence to suggest, and no solid evidence to support the AAH. ScienceBlogs author Greg Laden has described the AAH as a "human evolution theory of everything" that attempts to explain all anatomical and physiological features of humans and is correct in some areas only by chance. Laden also states that the AAH was proposed when knowledge of human evolutionary history was unclear, while more recent research has found that many human traits have emerged at different times over millions of years, rather than simultaneously due to a single evolutionary pressure.

Evolutionary biologist Carsten Niemitz states that he believes the AAH as expressed by Morgan didn't fulfill the criteria of a theory or a hypothesis, merely "[listing] analogies of features of savannah type mammals on the one hand and of aquatic mammals and man on the other, asking the scientific community for explanations other than a common aquatic ancestor of extant man."

Marc Verhaegen has also challenged the AAH as expressed by Morgan, believing the ancestors of apes as well as humans may have had their evolutionary history influenced by exposure to flooded forest environments, and that based on the hominin fossil record, regular part-time underwater foraging began in the Pleistocene rather than the early Pliocene as Morgan’s model proposes.

In 2012 Langdon reviewed an e-book published by Bentham Science Publishers collecting 50 years of theorizing about the AAH. In his review, Langdon noted the lack of a single "aquatic ape hypothesis", instead there are multiple hypotheses with a common theme of evolutionary pressure due to dependence on an aquatic habitat. While original versions thought to explain an apparently substantial gap between humans and closely related common ancestors, more recent variants of these hypotheses have had to adjust to the fact that the gap was more apparent than real and the significant commonalities found between humans and other African apes. Three main strands of thought now exist regarding the AAH, varying according to when the theorized aquatic phase occurred - from the Middle Miocene to approximately three million years ago (Hardy's original model, which was based on a large gap in the fossil record that has since been filled in), from the Early Miocene when ancestral hominids were thought to wade in costal swamps and from which Homo species were thought to split off and adapt to swimming and diving (associated with the work of Marc Verhaegen), and from 200,000 years ago when exploitation of costal resources led humans out of Africa and resulted in the evolution of modern humans (associated with the work of Algis Kuliukas). Langdon notes the strong associations of humans with water, as well as the adaptability of the species to incredibly diverse ecological niches (including costal and wetland regions), both within and across lifetimes. Whether these associations define humans as "semiaquatic" or not "represents a fundamental point of departure between anthropologists and the [Aquatic Hypothesis] community." Langdon notes the three lines of evidence cited to support the AAH (comparative anatomy between humans and other semiaquatic species; hypothetical situations in which evolutionary pressure might have produced convergent evolution between humans and semiaquatic species; the ability for humans to perform various activities in the water) and concludes about these lines of evidence,

These rhetorical strategies create long lists of claims, but until each hypothesis is independently established, it does not constitute evidence for an aquatic scenario. At best it shows consistency with a prior assumption. Evolutionary convergence – structural similarity – by itself is a metaphor for functional similarity. Metaphors are useful, but they demand that we examine points of resemblance closely in order to learn whether they are meaningful. Like metaphors, evolutionary convergences have their limits: eventually differences will emerge. Dolphins and humans are similar in the loss of body hair, relatively large brains, and complex vocal capacities; but these similarities do not make us dolphins. Nor is it clear which, if any of these similarities are related to water. Each trait must be investigated and resolved as a separate functional and evolutionary question. Unproven suppositions cannot serve as evidence for other hypotheses.

Langdon criticizes the alleged "parsimony" of the AAH irrelevant as it is used to generate hypotheses about human adaptation – but does not prove them. The AAH is, like many Just So Stories in anthropology, ignored less because of prejudice than because of a lack of empirical evidence to support it, because it engages only with supporting evidence in the relevant scientific literature while ignoring the larger body of unsupporting evidence, and because its hypotheses are portrayed as "compatible with" more accepted hypotheses and thus unable to distinguish between or provide explicit evidence for the AAH. Langdon concludes his review:

It is now incumbent upon both authors and critics to clarify the assumptions with which they are working and, where possible, to make empirically testable predictions. Similarly, the many gloating references in this book to the collapse of the Savannah Hypothesis should not suggest that all terrestrial models have been challenged. Possibly the time has come to bring the “paradigms” together; to step out of the “us vs. them” mentality held by both sides of this debate; and simply to recognize that dozens of speculative hypotheses for human evolution exist in the literature that may or may not discuss a relationship with water.

The authors of the volume published a reply.

Reception

The AAH has received little serious attention or acceptance from mainstream paleoanthropologists, has been met with significant skepticism and is not considered a strong scientific hypothesis. The AAH does not appear to have passed the peer review process, and despite Morgan being praised by various scholars, none of her work has appeared in any academic journals of anthropology or related disciplines. The AAH is thought by some anthropologists to be accepted readily by popular audiences, students and non-specialist scholars because of its simplicity. In 1987 a symposium was held in Valkenburg, the Netherlands, titled "Aquatic Ape: Fact or fiction?", which published its proceedings in 1991. A review of Morgan's book The Scars of Evolution stated that it 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. The review also stated that Morgan ignored the fossil record and skirted the absence of evidence that australopithecine underwent any adaptations to water, making the hypothesis impossible to validate from fossils.

Morgan has claimed the AAH was rejected for a variety of reasons unrelated to its explanatory power: old academics were 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 hypothesis and occasional forays into scientific conferences, the AAH has neither been accepted as a mainstream theory nor managed to venture a genuine challenge to orthodox theories of human evolution.

Morgan's critics have claimed that the appeal of AAH can be explained in several ways:

1. The hypothesis appears to offer absolute answers, which appeals more to students and the public than the qualified and reserved explanations offered by mainstream science.
2. Unusual ideas challenge the authority of science and scientists, which appeals to anti-establishment sentiments.
3. The AAH as developed by Morgan has a strong feminist component, which particularly appeals to a specific, feminist audience.
4. The AAH can be explained simply and easily, lacking the myriad details and complicated theorizing involved in dealing with primary sources and materials.
5. The AAH uses negative arguments, pointing to the flaws and gaps in conventional theories; though the criticisms of mainstream science and theories can be legitimate, the flaws in one theory do not automatically prove a proposed alternative.
6. The consensus views of conventional anthropology are complicated, require specialized knowledge and qualified answers, and the investment of considerable time to understand.

John D. Hawks, along with PZ Myers and fellow ScienceBlogs paleontologist Greg Laden recommend the website "Aquatic Ape Theory: Sink or Swim?" by Jim Moore as a resource on the topic. Conversely, both Morgan and Algis Kuliukas have accused Moore of distorting Morgan and other AAH-proponents presentations from the debate, using only little referencing.

Anthropologist Colin Groves has stated that Morgan's theories are sophisticated enough that they should be taken seriously as a possible explanation for hominin divergence and Carsten Niemitz has found more recent, weaker versions of the hypothesis more acceptable, approaching some of his own theories on human evolution.

In a 2012 paper, anthropologist Philip Tobias noted that rejection of the AAH led to stigmatization of a spectrum of topics related to the evolution of humans and their interaction with water. The result of this bias, in his and co-authors opinions, was an incomplete reconstruction of human evolution within varied landscapes.

See also

• Endurance running hypothesis
• Aquatic mammal

External links

• Template:DMOZ – other web resources
• Scars of Evolution, podcast of two part BBC Radio 4 feature on AAH, broadcast in 2005
• Presentation by Elaine Morgan at TED July, 2009; Comment on ScienceBlogs by paleoanthropologist Greg Laden