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Aquatic ape hypothesis

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The aquatic ape hypothesis (AAH) is an alternative explanation of some characteristics of human evolution which hypothesizes that the common ancestors of modern humans spent a period of time adapting to life in a partially-aquatic environment. The hypothesis is based on differences between humans and other great apes, and apparent similarities between humans and some aquatic mammals. First proposed in 1942 and expanded in 1960, its greatest proponent has been the writer Elaine Morgan, who has spent more than forty years discussing the AAH.

While it is uncontroversial that both H. neanderthalensis and early H. sapiens were better suited to aquatic environments than other great apes,[1][2] and there have been conjectures suggesting protohumans underwent some adaptations due to interaction with water[3] the sort of radical specialization posited by the AAH has not been accepted within the scientific community as a valid explanation for human divergence from related primates. It has been criticized for possessing a variety of theoretical problems, for lacking evidentiary support, and for there being alternative explanations for many of the observations suggested to support the hypothesis. Morgan has also suggested that her status as an academic outsider has hindered acceptance of the hypothesis.

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."[4]

In 1930 marine biologist Alister Hardy 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, not expecting any attention, but it was reported in a national newspaper. This generated immediate controversy in the field of paleoanthropology. Consequently Hardy published the hypothesis in an article in New Scientist on 17 March 1960. He 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.[5]

The idea received some interest after the article was published,[6] 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".[7] 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.[8] The context of initial presentations of the idea (a popular work and a political text) prevented the AAH from garnering serious interest or an exploration of its scientific merit.[9]

Despite maintaining some popular and scientific interest over several decades, the aquatic ape hypothesis has not been accepted by a large majority of researchers within the field of paleoanthropology.[10] A small but active number of promoters working outside of mainstream paleoanthropology, non-anthropologists and the occasional professional still cite and bring attention to the AAH but it has never been completely discredited to its adherents nor fully explored by researchers.[9]

The hypothesis

The AAH suggests that many of the features that distinguish humans from their nearest evolutionary relatives can be explained through a period of aquatic adaptation in which protohumans spent time wading, swimming and feeding on the shores of fresh, saline or brackish waters (though there has been disagreement and modification of the theory regarding the salinity of the purported watery environment[11][12][13]) and suggests comparisons with other aquatic or semiaquatic species with similar characteristics. Some observations include:

  • Bipedalism out of water causes considerable problems for the back, knees and organs, while water would support the joints and torso and permit breathing[14][15]
  • Humans are relatively hairless compared to great apes, similar to the hairlessness of land-dwelling rhinoceros and elephant which both have aquatic ancestors;[16] what body hair humans do have also follows water flow-lines[17]
  • Increased subcutaneous fat for insulation, especially in human infants[5]
  • A descended larynx[17][18]
  • A hooded nose, muscular nostril aperture control and the philtrum preventing water from entering the nostrils[17]
  • Extensive coverage of the skin by sebaceous glands[19]
  • The requirement of the human brain for certain nutrients including iodine[20] and some essential fatty acids[21] which are most easily found and absorbed in seafood[22]
  • Voluntary breath control which allows diving and swimming,[14][23] and a more streamlined shape compared to other apes[17]
  • The mammalian diving reflex which occurs when the head is immersed in cold water[24]
  • Vestigial webbing between the fingers[25]
  • The waxy coating found on newborns[17]
  • Certain morphological adaptations within the kidney[26]

The timelines hypothesized for a period of adjusting to aquatic living vary from the Miocene about 6 million years ago,[5] to nearly 2 million years ago in the late Pliocene or early Pleistocene.[27][28] It is also theorized that the semi-aquatic phase occurred when protohumans migrated along the southern Asian coastline during a previous ice age when sea levels were considerably lower; this is also proffered as a reason why human fossils are not found in aquatic habitats, as those regions were inundated when the polar ice caps melted.[29]

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.[9] Other authors have suggested that wading and other interactions with watery environments may have provided a less extreme but still present role in human evolution.[3]

Criticisms

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.[9]

Theoretical considerations

The AAH has been criticized for containing multiple inconsistencies and lacking evidence from the fossil record to support its claims.[9][30] It is also described as lacking parsimony, despite purporting to be a simple theory uniting many of the unique anatomical features of humans.[9]

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; 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.[31] There is ultimately only circumstantial evidence to suggest, and no solid evidence to support the AAH.[32][33] 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.[10]

Habitat

Morgan presented the AAH as an alternative to the "savanna model", which uses very vague descriptive statements portraying protohumans as moving out from forested environments and into a hot dry savanna. However, this idea has been called a caricature of the actual environments in which protohumans are thought to have evolved, and presents a false dichotomy as more recent theories propose a tree or forest-based habitat providing the driving forces for adaptation,[32] and a straw man of the actual theories and arguments used in the study of paleoanthropology. Morgan further criticized scientists for admitting they were uncertain regarding the reasons for the development of hairlessness, bipedalism, brain size and speech. This ignores the fact that science legitimately admits ignorance when it is unclear and that a lack of "final answers" does not legitimize a competing theory by default.[9]

The belief that wading into shallow water would help proto-humans avoid dry-land predation discounts the risks presented by aquatic animals such as crocodiles and hippopotamuses that present a current risk to Africans living near bodies of water,[34] and that protohumans lacked the fangs, claws or size to defend themselves from these threats.[35]

The susceptibility of humans to waterborne parasites have been suggested as evidence against the AAH,[35] though the presence of certain parasites that appear to co-exist with humans has also been presented as evidence for the AAH.[3][36]

Anatomical and physiological claims

  • Hairlessness – Most aquatic mammals that are comparably sized to humans are not hairless, but have dense, insulating fur and swim very well, with fatty layers beneath the skin.[34] Aquatic mammals do not vary greatly in their body hair, while humans do.[10] Hairless skin is also only an advantage for fully-aquatic mammals that dive, swim quickly or migrate long distances such as whales and dolphins,[35] and only appears and is an advantage for extremely large aquatic mammals who would overheat with large amounts of body hair, who are fully-aquatic and have evolved as an aquatic species for millions of years. The loss of body hair is also explainable through a lower parasite load, and maintenance through sexual selection.[37] Furthermore, while shaving human swimmers to eliminate the little body hair that remains results in a minor decrease in drag,[38][39] this cannot be extrapolated to a beneficial effect of loss of a full coat of fur, which has been shown to have superior drag reduction ability.[40] While relative hairlessness and hair direction is cited as an adaptation to swimming and diving, there is no evidence of similar skeletal or soft tissue adaptations that are expected to accompany such adaptations.[9]
  • Breath control – The position, evolutionary timing of changes, and size of the nerve openings in the vertebrae suggest that breath control in humans improved because of the increased complexity and use of speech rather than an aquatic phase of evolution.[41] In addition, breath control is thought to be preceded by bipedalism, which frees the muscles around the upper torso from locomotion and allows breathing rates to occur independent of locomotion. Voluntary speech is thought to be a sufficient evolutionary pressure to explain breath control, independent of other explanations. The vocalizations of dolphins and other aquatic species are not thought to be comparable to humans. In addition, certain birds have speech and breath control comparable to humans, without a phase of aquatic adaptation.[9]
  • Diet – a broad terrestrial diet would ensure sufficient access to required essential fatty acids without a high consumption of seafood[42] and the "best" fats found in fish are from cold water fish that did not occupy the same coastal environments as humans. In addition, the requirements of these fats are very minimal, with no evidence that extra fats would result in an evolutionary pressure towards a larger brain. Humans without access to shoreline foods also develop normal brains.[9][43]
  • Diving reflex – the mammalian diving reflex is exhibited by terrestrial mammals as well as aquatic ones and humans have not been compared to other living hominoids; there is not enough information on for this reflex for it to be used to support the AAH.[9]
  • Body fat – the subcutaneous fat distribution in humans is more similar to a domesticated animal than an aquatic one, and is nearly identical to that of other primates. The subcutaneous fat of aquatic mammals and humans also seems to serve different uses – it forms the streamlined shape of seals, while in humans it is used for sexual selection.[44] In addition, the distribution of fat and blood vessels allows for improved thermoregulation, as hot blood from the body can bypass the fat to radiate heat through the skin.[9]
  • Bipedalism – the disadvantages cited for bipedalism within the AAH are often the result of comparing humans to medium, terrestrial quadrupeds, but human evolution never included 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 as improving swimming speeds, appears only after the evolution of the Homo genus.[9]
  • Descended larynx – the human larynx is not shaped like the larynxes of aquatic animals; it forms and descends as an infant begins to speak, making it easier to aspirate water and drown. Additionally, a descended larynx is not unique to aquatic animals, and permanently or temporarily descended larynxes are seen in dogs, pigs, goats, monkeys,[45] big cats,[46] deer,[47] and young chimps.[48] Mainstream anthropology explain the descended larynx as an adaptation to improve vocalizations by increasing the number of pronounceable vowels and improving the ability of humans to control their speech.[9]
  • Nose shape – the shape of the human nose is extremely variable within the species, and believed to be related to climatic adaptations and the warming and moistening of air before it enters the respiratory tract, not to prevent water entry while swimming. In addition, the muscles surrounding the nose show no evidence of having been previously more developed, but are part of a complex of muscles that are specially developed in humans to show emotion and aid in communication.[9]
  • Interdigital webbing – Morgan's claims for syndactylism, the presence of webbing between the fingers, were based on the purported "rareness" of birth defects "adding" features normally thought absent from an evolutionary order. Interdigital webbing is not the "addition" of new tissue, it results from the failure to eliminate skin cells connecting the fingers, a process common to all tetrapods.[9]
  • Sebaceous gland – many aquatic animals have rudimentary or no sebaceous glands. In humans, sebaceous glands become active during puberty with men having far more than women while women have much better scent receptors. This suggests the glands are sexually dimorphic for sexual selection rather than waterproofing. In seals that use sebaceous glands for waterproofing, the glands are active from birth and are secreted by hard, keratinized skin that is very different from human skin.[citation needed]
  • Swimming – modern humans are inefficient swimmers, with shapes that are not well suited to rapid travel through water.[49] Swimming is also a learned trait, and though newborns are able to propel themselves inefficiently through water, they are unable to lift their faces to breathe.[50]

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. In addition, the AAH is contradictory in several places; the AAH theorizes humans developed some unique skin features due to adaptation to water, but other features emerged after leaving the habitat, and the specialization that is hypothesized for an aquatic life are uneven, with humans lacking many truly specialized features of aquatic species (such as head shape, repositioned nostrils and streamlining of the body). 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.[9] 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 listed above facilitate aquatic behavior rather than evolving as a result of it.[51]

Reception

The AAH has received little serious scrutiny from mainstream paleoanthropologists[52] and has been met with significant skepticism.[53] The AAH is thought by some anthropologists to be accepted readily by popular audiences, students and non-specialist scholars because of its simplicity.[9] In 1987 a symposium was held in Valkenburg, the Netherlands, titled "Aquatic Ape: Fact or fiction?", which published its proceedings in 1991.[54] 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.[55] The results of the conference were reported in the anthropological press as having rejected the hypothesis.[9] 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.[30]

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.[56] However, anthropologist Colin Groves has stated that Morgan's theories are sophisticated enough that they should be taken seriously as a possible explanation for bipedalism.[57]

The appeal of the hypothesis has been explained in several ways:[9]

  1. The hypothesis appears to offer absolute answers while orthodox science is qualified and reserved, a situation which has great appeal to students and the public
  2. Unusual ideas challenge the authority of science and scientists, which appeals to antiscience 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.

See also

Footnotes

  1. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1038/35011048, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1038/35011048 instead.
  2. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1073/pnas.0805474105, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1073/pnas.0805474105 instead.
  3. ^ a b c Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1007/s00114-009-0637-3, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1007/s00114-009-0637-3 instead.
  4. ^ Westenhöfer, M. (1942). Der Eigenweg des Menschen. Mannstaedt & Co.
  5. ^ a b c Hardy, A. (1960). "Was man more aquatic in the past" (pdf). New Scientist. 7: 642–645. Cite error: The named reference "Hardy1960" was defined multiple times with different content (see the help page).
  6. ^ Sauer, C O. (1960). "Seashore – Primitive home of man?". Proceedings of the American Philosopical Society. 106 (1): 41–47.
  7. ^ Morris, Desmond (1967). The Naked Ape. McGraw-Hill. p. 29. ISBN 0 09 948201 0. {{cite book}}: Cite has empty unknown parameter: |month= (help)
  8. ^ Morgan's books on the topic include:
  9. ^ a b c d e f g h i j k l m n o p q r s t Langdon JH (1997). "Umbrella hypotheses and parsimony in human evolution: a critique of the Aquatic Ape Hypothesis". J. Hum. Evol. 33 (4): 479–94. doi:10.1006/jhev.1997.0146. PMID 9361254.
  10. ^ a b c Laden, G (4 August 2009). "Musings on the Aquatic Ape Theory". ScienceBlogs. Retrieved 2 September 2009.
  11. ^ Ellis DV (1993). "Wetlands or aquatic ape? Availability of food resources". Nutrition and health. 9 (3): 205–17. PMID 8183488.
  12. ^ Cunnane SC, Plourde M, Stewart K, Crawford MA (2007). "Docosahexaenoic acid and shore-based diets in hominin encephalization: a rebuttal". Am. J. Hum. Biol. 19 (4): 578–81. doi:10.1002/ajhb.20673. PMID 17546620.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  13. ^ Cunnane, S (2010). Environmental Influences on Human Brain Evolution. John Wiley & Sons. ISBN 978-0-470-45268-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  14. ^ a b Niemitz C (2002). "A Theory on the Evolution of the Habitual Orthograde Human Bipedalism – The "Amphibisce Generalistheorie"". Anthropologischer Anzeiger. 60: 3–66.
  15. ^ Verhaegen M (1987). "Origin of hominid bipedalism". Nature. 325 (6102): 305–6. doi:10.1038/325305d0.
  16. ^ Morgan, E (1982). The Aquatic Ape. Stein & Day Pub. ISBN 0-285-62509-8.
  17. ^ a b c d e Morgan, Elaine (1997). The Aquatic Ape Hypothesis. Souvenir Press. ISBN 0-285-63518-2.
  18. ^ Crelin, Edmund S (1987). The Human Vocal Tract: Anatomy, Function, Development, and Evolution. New York: Vantage Press. ISBN 0 533 06967 X.
  19. ^ Kingdon J (2003). Lowly origin: where, when, and why our ancestors first stood up. Princeton, N.J: Princeton University Press. pp. 242. ISBN 0-691-05086-4.
  20. ^ Venturi, S (2010). "Thyroid Hormone, Iodine and Human Brain Evolution". Environmental Influences on Human Brain Evolution. John Wiley & Sons. pp. 105–124. ISBN 978-0-470-45268-4. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |editors= ignored (|editor= suggested) (help)
  21. ^ Crawford MA (2010). "Long-Chain Polyunsaturated Fatty Acids in Human Brain Evolution". Environmental Influences on Human Brain Evolution. John Wiley & Sons. pp. 13–32. ISBN 978-0-470-45268-4. {{cite book}}: Unknown parameter |editors= ignored (|editor= suggested) (help)
  22. ^ Ellis DV (1993). "Wetlands or aquatic ape? Availability of food resources". Nutrition and health (Berkhamsted, Hertfordshire). 9 (3): 205–17. PMID 8183488.; Cunnane, S., Plourde, M., Stewart, K., Crawford, M (2007). "Docosahexaenoic Acid and Shore-Based Diets in Hominin Encephalization: A Rebuttal". American Journal of Human Biology. 19 (4): 578–591. doi:10.1002/ajhb.20673. PMID 17546620.{{cite journal}}: CS1 maint: multiple names: authors list (link); Crawford, M (2000). "Evidence for the unique function of docosahexanoic acid (DHA) during the evolution of the modern hominid brain". Lipids. 34: S39–S47. doi:10.1007/BF02562227. PMID 10419087. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ Patrick, John (1991). Human Respiratory Adaptations for Swimming and Diving. Souvenir Press. ISBN 0-285-63033 4. {{cite book}}: Cite has empty unknown parameter: |month= (help)
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  28. ^ Verhaegen M; Munro S; Vaneechoutte M; Bender R; Oser N (2007). "The original econiche of the genus Homo: Open Plain or Waterside?" (pdf). SI Muñoz ed. Ecology Research Progress: 155–186.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  29. ^ Verhaegen M & Munro S (2002). "The continental shelf hypothesis". Nutrition & Health. 16: 25–28.
  30. ^ a b Zihlman, A (19 January 1991). "Review: Evolution, a suitable case for treatment". New Scientist. Archived from the original on 30 December 2008. Retrieved 3 May 2009. {{cite web}}: |archive-date= / |archive-url= timestamp mismatch; 23 January 2008 suggested (help)
  31. ^ Gee, H (2001). In search of deep time: beyond the fossil record to a new history of life. Cornell University Press. pp. 100–101. ISBN 0801487137.
  32. ^ a b Meier, R (2003). The complete idiot's guide to human prehistory. Alpha Books. pp. 57–59. ISBN 0028644212.
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  37. ^ Pagel M; Bodmer W (2003). "A naked ape would have fewer parasites" (pdf). Proc. Biol. Sci. 270 Suppl 1: S117–9. doi:10.1098/rsbl.2003.0041. PMC 1698033. PMID 12952654. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  38. ^ Sharp, RL (1989). "Influence of body hair removal on physiological responses during breaststroke swimming". Medicine and Science in Sports and Exercise. 21 (5): 576–580. PMID 2691818. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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  42. ^ Carlson BA, Kingston JD (2007). "Docosahexaenoic acid biosynthesis and dietary contingency: Encephalization without aquatic constraint". Am. J. Hum. Biol. 19 (4): 585–588. doi:10.1002/ajhb.20683. PMID 17546613.
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  47. ^ McElligott, AG (2006). "Retraction of the mobile descended larynx during groaning enables fallow bucks (Dama dama) to lower their formant frequencies". Journal of Zoology. 270 (2): 340–345. doi:10.1111/j.1469-7998.2006.00144.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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  51. ^ Attention: This template ({{cite doi}}) is deprecated. To cite the publication identified by doi:10.1159/000252586, please use {{cite journal}} (if it was published in a bona fide academic journal, otherwise {{cite report}} with |doi=10.1159/000252586 instead.
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