Aquatic Ape Human Ancestor Theory

Aquatic Ape Theory - What is it?

A Brief Summary of AAT - key arguments

A Brief History and Key Proponents of AAT

Current Aquatic Evolution Theories


Alternative theories of human evolution

Wikipedia and the scientific community

. Anatomical Evidence
... Bipedalism
... Birth and babies
... Brain
... Breath control
... Fat
... Fingers, toes and feet
... Furlessness
... Hair and baldness
... Kidneys
... Menopause
... Nose
... Olfactory sense
... Pachyostosis
... Paranasal Sinuses
... Platycephaly
... Sexual features
... Surfer's ear
... Sweating
... Tears
... Underwater vision

. Diet
. Language & Song
. Sleep (USWS)
. Waterside environments
. Sea Gypsies

. Homo erectus - shallow diver

. Fossil evidence
. Paleoecological evidence

A call to scientists...

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Underwater vision

The eyes of aquatic animals work in a different way to those of terrestrial animals because they have to see in water rather than in air. Semi-aquatic animals have had to adapt so that they can see both in air and in water. The most radical example of a species with both air and water vision may be the four-eyed fish (Anableps) that lives at the water surface and has functionally two pairs of eyes, one above the water, and the other below [1].

Humans do not see very well underwater and this may lead some people to conclude that we could not have had a semi-aquatic evolutionary past. However, Erika Schagatay, who has researched human diving adaptations among the Moken, or Sea Nomads of Indonesia, has observed at first hand how the children there, who learn to swim and dive even before they can walk, have superior underwater vision as compared to European children who have had little or no diving experience. [2] [3] She says:

"In this ethnic group, the children start swimming before they can walk, and children aged 4-10 years were observed to collect small shells from a non-contrasting background with high precision (ES, unpublished observations). It was realized by the author that, unlike herself, they could see these shells despite not wearing goggles. This initiated detailed studies by an animal vision research group at Lund University [4-7]. Human underwater vision was an unexplored area at that time, and the first question was whether South-East-Asian Sea Nomads had a better underwater vision than humans in general. If such qualities were discovered, the next question would be how these were achieved." [8]

The mammalian eye is a bit like a camera: light hits the cornea and is directed through the lens which focuses the light onto the back of the eye, the retina, as an upside-down image, which then travels along the optic nerve to the brain where the image is interpreted. In terrestrial animals, the cornea is normally rounded or curved because there is air outside it. Air has a different density to the liquid in the eye so the cornea refracts the light, in order to ensure it hits the retina properly. The curvature of the cornea helps the image to focus properly. In water, the light does not refract to the same degree against the human eye, which is why we see everything blurry. When we use a diving mask, however, the air remains outside our cornea so the light is properly refracted and we see everything clearly. There are semi-aquatic spiders which achieve the same effect by creating a bubble of air around their eyes when they dive so that they can see clearly. [9]

Diagram of the focal plane in air and underwater

For animals that can't blow bubbles around their eyes, or wear masks, an alternative option is to change the shape of the cornea. A flat cornea does not bend the light like a curved one. A flat cornea is used by fish that live in intertidal waters or mudflats, and many amphibious birds have slightly flattened corneas, including penguins, albatrosses, dippers (Cinclus spp.) and Manx shearwaters (Puffinus puffinus) [10-15].

Another solution is to make the lens more spherical, a process known as accommodation, and it's a strategy commonly used by terrestrial animals that forage in water. Seals have flattened corneas and rather spherical lenses [16,17], suggesting that they have optically adapted more to an aquatic life than to a terrestrial life. Otters have an extraordinarily large accommodative range, as do walruses and sea lions. These amphibious animals have
developed stronger ciliary muscles to allow for this extensive accommodation [18].

Bajau Laut childrenFor the purposes of research, Anna Gislén and Erika Schagatay from Lund University in Sweden enlisted the help of 17 Moken children (age 7 - 13) and 18 European children of the same age. The children underwent a series of tests which involved diving down to look at a series of graded patterns to identify how well they could distinguish the patterns under water. The Moken children, who had already spent much of their lives diving underwater without any equipment, did more than twice as well as the European children. [19]

Other tests were carried out to try to ascertain how the children were able to do this. The curvature of the cornea was measured using something called a Placido's disk [20], and the range of their accommodation was measured using ophthalmological glasses with lenses of various optical power. They discovered that the Moken children had excellent terrestrial vision and normal corneas, which implied that the children were not adapting to underwater vision in the same way as the above mentioned amphibious species.

A third test therefore was carried out to see if perhaps the Moken children were constricting their pupil size to better focus underwater, as it has been suggested that seals [21], dolphins [22, 23] and semi-aquatic snakes [24] use this method to improve depth of focus under water. The test showed that the Moken children indeed did constrict their pupils under water, whereas the European control group did not [25].

Gislén and Schagatay were not convinced that pupil constriction alone could explain their enhanced underwater acuity, and theoretical calculations indicated that if constricting the pupils was combined with accommodation, they would indeed achieve the higher degree of underwater vision they had demonstrated. In order to test this theory, and to see whether it was something that was acquired or inherited, they attempted to train European children to improve their visual acuity in an under water environment. The results showed that the children could learn to control their accommodation, and pupil constriction was clearly elicited as a result of this, in only about four to six months, depending on the individual. They even achieved the level of underwater acuity found in the Moken children. This shows that the human eye and visual system are very flexible and can even adapt to the blurry underwater environment [26]. Schagatay concluded:

"Adaptability of the eye is a key feature in a semi-aquatic mammal. Our results show an unexpected adaptability of the human eye to the underwater environment. Such an adaptation is central to food gathering and many other activities under the water surface. The mechanisms used by humans for achieving superior underwater vision included features previously observed in semi-aquatic mammals and birds. Although the main mechanism observed in the Moken children, pupil constriction, was not among the most typical ones, there seem to be other animals that use the same strategy of pupil constriction to improve depth of focus under water, including seals, dolphins, and sea snakes. This may be an interesting example of convergent evolution, and this solution might be more common in the animal kingdom than previously thought. The human eye has proven to be flexible and adaptable enough to function under water, and an explanation for this unexpected adaptation in a terrestrial mammal could be that it has evolved during a phase with close contact with and food gathering in water."

Human Aquatic Color Vision

Wang-Chak Chan
Department of Cognitive Sciences, Lund University, Sweden


Humans and chimpanzees share a last common ancestor presumably some five million years ago, so it is not surprising to find their color vision systems to have many characteristics in common. However, some clear differences are found, like shifts in visual pigment sensitivity and the prevalence of c olor blindness in humans, but current explanations are either absent or unsatisfactory. Within the framework of the AAH, it is possible to discuss these discrepancies in terms of semi-aquatic adaptations and convergent evolution with aquatic mammals, in addition to the usual logic of terrestrial adaptations and primate inheritance. This means that there might be two different groups of explanations for human features, in and outside the water, whereas the conventional literature only considers terrestrial life.

Assuming a semi-aquatic past for our species may also help to clarify some puzzling issues in linguistics, especially concerning the curious existence of fuzzy color terms. Although one must be careful in making too many assumptions, considering the culture dependency of color categorization, we provide an explanation that is based on a semi-aquatic past, which corresponds well with world survey findings, color psychology and etymological evidence.

The above analysis provides a preliminary outline of a new model, i.e., human aquatic color vision (HACV), which demonstrates the potential of AAH to better understand human physiology, perception and cognition. It may be noted that another feature of human perception, i.e., reduced olfactory sense, which already started in primates, but is strongly reduced in humans, can be considered as a semi-aquatic adaptation. This model can be refined, by gathering more data from primates and (semi-)aquatic mammals. Starting from the HACV model, we could also identify more aspects of color vision or of the visual system as a whole, like our excellent hue discrimination of blue colors in the light spectrum, and the prevalence of myopia and astigmatism.



Fifty Years after Alister Hardy Waterside Hypotheses of Human Evolution, Bentham Science Publishers, 2011, Mario Vaneechoutte, Algis Kuliukas and Marc Verhaegen (Eds). Anna Gislén1 and Erika Schagatay2, Chapter 10: Superior Underwater Vision Shows Unexpected Adaptability of the Human Eye, p.164-172.
Wang-Chak Chan, Chapter 11: Human Aquatic Color Vision, p.179-180

Their references quoted as follows:

[1] Sivak JG. Optics of the eye of the ‘four-eyed fish’ (Anableps anableps). Vision Res 1976; 16: 531-4.
[2] Schagatay E. The significance of the human diving reflex. In: Roede M, Wind J, Patrick J, Reynolds V, Eds. The aquatic ape: Fact or fiction? London: Souvenir Press 1991; pp. 247-54.
[3] Schagatay E. The human diving response - effects of temperature and training. Thesis, Department of Animal Physiology. Lund: Lund University, 1996.
[4] Gislén A, Dacke M, Kröger R, Abrahamsson, M, Nilsson, D, Warrant E. Superior underwater vision in a human population of sea gypsies. Curr Biol 2003; 13: 833-6.
[5] Gislén A, Gislén L. On the optical theory of underwater vision in humans. J Opt Soc Am A 2004; 21: 2061-4.
[6] Gislén A, Warrant E, Dacke M, Kröger R. Visual training improves underwater vision in children. Vision Res 2006; 46: 3443-50.
[7] Gislén A, Gustafsson J, Kröger R. The accommodative pupil responses of children and young adults at low and intermediate levels of ambient illumination. Vision Res 2008; 48: 989-93.
[8] Anna Gislén1 and Erika Schagatay2,*Superior Underwater Vision Shows Unexpected Adaptability of the Human Eye, Chapter 10 of "Was Man More Aquatic in the Past - Fifty Years after Alister Hardy", Bantham ebooks, p.164.
[9] Williams DS. The physiological optics of a nocturnal semi-aquatic spider, Dolomedes aquaticus (Pisauridae). Z Naturforsch 1979; 34c: 463-9.
[10] Graham JB, Rosenblatt RH. Aerial vision: Unique adaptation in an intertidal fish. Science 1970; 168: 586-8.
[11] Howland HC, Sivak JG. Penguin vision in air and water. Vision Res 1984; 24: 1905-9.
[12] Williams TD. The penguins. Oxford University Press: London 1995.
[13] Martin GR. Eye structure and amphibious foraging in albatrosses. Proc R Soc Lond B 1998; 265: 665-71.
[14] Goodge WR. Adaptations for amphibious vision in the Dipper (Cinclus mexicanus). J Morphol 1960; 107: 79-91.
[15] Martin GR, de L Brooke M. The eye of a Procellariiform seabird, the Manx shearwater Puffinus puffinus: Visual fields and optical structure. Brain Behav Evol 1991; 7: 65-78.
[16] Renouf D. Sensory function in the harbour seal. Sci Am 1989; April: 62-7.
[17] Sivak JG, Howland HC, Weerheim J. The eye of the hooded seal, Cystophora cristata, in air and water. J Comp Physiol 1989; 165: 771-7.
[18] West JA, Sivak JG, Murphy CJ, Kovacs KM. A comparative study of the anatomy of the iris and ciliary body in aquatic animals. Can J Zool 1991; 69: 2594-607.
[19] Anna Gislén1 and Erika Schagatay2,*Superior Underwater Vision Shows Unexpected Adaptability of the Human Eye, Chapter 10 of "Was Man More Aquatic in the Past - Fifty Years after Alister Hardy", Bantham ebooks, p.167.
[20] [32] Rand RH, Howland HC, Applegate RA. Mathematical model of a Placido disk: Keratometry and its implications for recovery and corneal topography. Optom Vis Sci 1997; 74: 926-30.
[21] Sivak JG. A survey of vertebrate strategies for vision in air and water. In: Sensory ecology. Ali MA, Ed. New York: Plenum Press 1978; pp. 503-19.
[22] Herman L, Peacock MF, Yunker MP, Madsen CJ. Bottle-nose dolphin: Double-slit pupil yields equivalent aerial and underwater diurnal activity. Science 1975; 189: 650-2.
[23] Rivamonte LA. Eye model to account for comparable aerial and underwater acuities of the bottlenose dolphin. Netherlands J Sea Res 1976; 10: 491-8.
[24] Schaeffel F. Underwater vision in semi-aquatic European snakes. Naturwissenschaften 1991; 78: 373-5.
[25] Gislén A, Dacke M, Kröger R, Abrahamsson, M, Nilsson, D, Warrant E. Superior underwater vision in a human population of sea gypsies. Curr Biol 2003; 13: 833-6.
[26] Gislén A, Warrant E, Dacke M, Kröger R. Visual training improves underwater vision in children. Vision Res 2006; 46: 3443-50.
[27] Wang-Chak Chan, Fifty Years after Alister Hardy Waterside Hypotheses of Human Evolution, Bantham ebooks, 2011. Chapter 11: Human Aquatic Color Vision, p.179-180

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