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Underwater vision
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:
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]
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
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:
Human Aquatic Color VisionWang-Chak Chan Conclusion 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. [27] Visual training improves underwater vision in childrenAbstract Children in a tribe of sea-gypsies from South-East Asia have been found to have superior underwater vision compared to European children. In this study, we show that the improved underwater vision of these Moken children is not due to better contrast sensitivity in general. We also show that European children can achieve the same underwater acuity as the Moken children. After 1 month of underwater training (11 sessions) followed by 4 months with no underwater activities, European children showed improved underwater vision and distinct bursts of pupil constriction. When tested 8 months after the last training session in an outdoor pool in bright sunlight—comparable to light environments in South-East Asia—the children had attained the same underwater acuity as the sea-gypsy children. The achieved performance can be explained by the combined effect of pupil constriction and strong accommodation. [Link] References: 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. 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. |
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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].
For 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]