Aquatic ape hypothesis

The aquatic ape hypothesis (AAH) is a hypothesis about human evolution, which posits that the ancestors of modern humans spent a period of time adapting to life in a wet environment. AAH emerged from the observation that some traits that set humans apart from other primates have parallels in aquatic mammals. It was first proposed by German pathologist Max Westenhöfer in 1942, and then independently by British marine biologist Alister Hardy in 1960. After Hardy, the most prominent proponent has been Welsh screenwriter Elaine Morgan, who has written several books on the topic.

AAH has not been accepted among the mainstream explanations of human evolution. Scientific consensus is that humans first evolved in East Africa in a period when the climate fluctuated 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 theory have been critical and an extensive criticism appeared in a peer reviewed paper in 1997.^[1] The article argued 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. It also argued that the hypothesis is internally inconsistent, has less explanatory power than its proponent claims, and that alternative terrestrial hypotheses are much better supported. The hypothesis is popular among laypeople and has continued support by a minority of scholars—Langdon (1997) attributes this to the attraction of simplistic single-cause theories over the much more complex, but better supported models with multiple causality.

History

In a 1942 book, the German pathologist Max Westenhöfer published the idea of humans evolving in proximity to water with the statement "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."^[2]

From 1930, marine biologist Alister Hardy had hypothesized that humans may have had ancestors more aquatic than previously imagined. Because it was outside his field and he was aware of the controversy it would cause, Hardy delayed reporting his hypothesis. 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. A national newspaper reported a distorted interpretation 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 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.^[3]

The idea received some interest after the article was published,^[4] 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 in which can be found the first use of the term "aquatic ape".^[5] Writer Elaine Morgan read about the idea in Morris' book and was struck by its potential explanatory power, becoming its main promoter and publishing six books over the next 40 years.^[6] 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?".^[7] 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.^[8]

Weaker versions of the hypothesis suggesting littoral feeding and wading rather than strong aquatic adaptation have since been proposed.^[9] These weaker versions of the hypothesis have not yet been scientifically explored.^[10]

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^[1] and most paleoanthropologists reject the AAH.^[11][12]

The hypothesis

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 an aquatic or semiaquatic way of life, 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 or saline waters and feeding on littoral resources.^[13] Various traits that have been proposed to indicate past adaptation to aquatic conditions and the return to land,^[6] but generally the evidence provided for the AAH is equally well accounted for by land-based adaptations without needing to posit an aquatic phase of human development. Parallels made by proponents of the AAH between humans and the proboscis monkey, which shows mainly behavioral adaptations to a water-based habitat, contradicts any claims of anatomical evidence for the hypothesis.^[1] Many species of modern primates demonstrate some sort of aquatic behaviors (such as swimming, wading or diving) and use of aquatic environments (for thermoregulation, display behavior, range, diet and predation) but many do not display the traits posited by AAH, suggesting the traits posted as evidence for the AAH may facilitate aquatic behavior rather than evolving as a result of it.^[14][11]

While most proto-human fossil sites are associated with wet conditions upon the death of the hominin, this is not seen as evidence for the AAH since being buried in waterside sediment is one of the rare occasions where fossilization is likely to occur; paleontologists are aware of this preservation bias and expect fossils to be located near such sediments.^[11] There is no fossil evidence to support the AAH.^[15]

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.^[1] 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.^[1] 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.^[10][16][17]

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 theory, 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.

• Bipedalism. Proponents of AAH argue 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.^[18][19][10] But bipedialism also gives many advantages on land, particularly lower energy expenditure and the ability of long distance running - in 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 ^[1] and biomechanical analysis indicates humans are far too poor swimmers to have derived from an ape ancestor that swam,^[20] and pre-human apes would face similar problems.^[21] 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.^[22]
• 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.^[23][24] However, humans vary strongly in the amount and distribution of body hair^[25] and comparably-sized mammals adapted to semi-aquatic lifestyles actually have dense, insulating fur^[21][26] or large, barrel-shaped bodies that retain heat well in water.^[21] 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.^[21][1] Though a variety of explanations have been proposed for human hairlessness, the best-supported theory 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.^[21] Langdon, in his 1997 critique of the theory, 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.^[1]
• Descended larynx. The human larynx is situated in the throat rather than the nasal cavity, a feature shared by some aquatic animals who use it to close off the trachea while diving and facilitates taking large breaths of air upon surfacing.^[24] However, other terrestrial mammals, such as the red deer, also have a permanently descended larynx.^[27] Humans also have a considerable amount of control over their breathing, which is an involuntary reflex for most terrestrial mammals.^[18][24] 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^[1][28] and the human larynx is shaped differently from that of aquatic animals, predisposing humans to choking.^[1]
• Encephalization. Morgan^[24] and several other authors^[29][30][31] have suggested that the encephalization of the human brain was a response to increased consumption of fatty acids found in fish. However, considerable human encephalization began quite late in the development of the genus Homo, 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. Critics have also pointed out that landlocked humans without access to fish develop normal brains^[1] and these nutritional requirements are easily met with a land-based diet.^[32][33] The encephalization of early Homo species predated the appearance of humans exploiting waterside food sources by over 400,000 years, driven by the consumption of hunted or scavenged animal brains supplying large amounts of scarce nutrients including docosahexaenoic acid.^[34][33] Further, the "best fats" for human brains are found in oily fish swimming in cold waters very distant from the tropical climate where humans evolved.^[35]

Theoretical considerations

The AAH has been criticized for containing multiple inconsistencies and lacking evidence from the fossil record to support its claims^[1][36][15] (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).^[1] It is also described as lacking parsimony, despite purporting to be a simple theory uniting many of the unique anatomical features of humans.^[1] 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.^[37]

Ellen White describes Morgan's work as failing to be empirical, not addressing evidence that contradicts the theory, 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 theory 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.^[38] Others have similarly noted the AAH "is more an exercise in comparative anatomy than a theory supported by data."^[39]

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^[15]); 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.^[40] There is ultimately only circumstantial evidence to suggest, and no solid evidence to support the AAH.^[41][42]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.^[25]

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."^[10]

Marc Verhaegen has also challenged the AAH as expressed by Morgan. He argues that the ancestors of apes as well as humans may have had their evolutionary history influenced by exposure to flooded forest environments ^[16], 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. ^[17]

Reception

The AAH has received little serious attention or acceptance from mainstream paleoanthropologists,^{[11][12][43][44]} has been met with significant skepticism^[45][44] and is not considered a strong scientific hypothesis.^[11][39] 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.^[38] The AAH is thought by some anthropologists to be accepted readily by popular audiences, students and non-specialist scholars because of its simplicity.^[1] In 1987 a symposium was held in Valkenburg, the Netherlands, titled "Aquatic Ape: Fact or fiction?", which published its proceedings in 1991.^[7] 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.^[36]

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.^[46]

Morgan's critics have claimed that the appeal of AAH can be explained in several ways:^[1]

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 is true.
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^[47] and fellow ScienceBlogs paleontologist Greg Laden^[25] recommend the website "Aquatic Ape Theory: Sink or Swim?" by Jim Moore as a resource on the topic.^[48]

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^[49] and Carsten Niemitz has found more recent, weaker versions of the hypothesis more acceptable, approaching some of his own theories on human evolution.^[10]

See also

• Endurance running hypothesis

Footnotes

External links

• Presentation by Elaine Morgan at TED July, 2009; Comment on ScienceBlogs by paleoanthropologist Greg Laden
• Aquatic ape hypothesis at the Open Directory Project - other web resources