The Evolutionary Origins of Primate Visual Systems: Insights from Snake Detection Theory
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Evolution has favoured the honing of vision in primates more than in other mammals. Primates have greater depth perception from having forward-facing eyes with extensively overlapping visual fields, sharper visual acuity, more brain areas involved with vision, and, in some primates, trichromatic vision (i.e. they can distinguish red from green). In fact, primates rely much more on vision than on the other senses – unlike other mammals who depend equally on multiple senses.
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The ‘snake detection theory’ posits that when large-gaped constricting snakes appeared about 100 million years ago and began eating mammals, this predation favoured modification of vision in one kind of prey, the lineage of monkeys destined to lose the tail: the ability to see predatory snakes before getting too close became a highly beneficial trait for them to have and pass on to their offspring. Then, about 60 million years ago, venomous snakes appeared in Africa or Asia, sharpening the natural selection of primates who could detect and avoid snakes.
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The degree of complexity in primate visual systems and the length of evolutionary time that primates have spent with venomous snakes match perfectly. The lineage that comprises African and Asian monkeys, apes and humans has the best vision of all mammals (i.e. fully trichromatic vision). These evolved roughly at the same time and in the same place as venomous snakes. They are also uniformly wary of snakes. On the other hand, Madagascar primates have the simplest visual systems. They have low visual acuity because their fovea, a depression in the retina that is responsible for our visual acuity wherever we focus our eyes, is poorly developed (when it’s present at all). Although Madagascar has constricting snakes, it has no venomous snakes, so primates on that island never had to face that particular selective pressure. In fact, they don’t all react fearfully toward snakes: some even walk on snakes or snake models, treating them as just another branch.
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The visual systems of North or South American monkeys are in the middle. They have better visual acuity than Madagascar primates but more variability in their visual systems than African or Asian monkeys. For example, American howler monkeys are all fully trichromatic, but in other American primate species, only some individuals are able to distinguish red from green hues. American primates were originally part of the anthropoid primate lineage in Africa that also includes African and Asian monkeys and apes, and so had to deal with venomous snakes for about 20-25 million years, but then, some 36 million years ago, they left Africa and arrived in South America where – though constrictors were there – venomous snakes were absent until roughly 15 million years later. By then, American monkeys had begun to diversify into different genera, and so each genus evolved separate solutions to the renewed problem caused by the arrival of venomous snakes.
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Several studies have shown that nonhuman and human primates, including young children and snake-naive infants, have a visual bias toward snakes compared with other animate objects, such as lizards, spiders, worms, birds and flowers; that we pick out images of snakes faster or more accurately than those of other objects, especially under cluttered and obscuring conditions or when we have to decide fast; that snakes also distract us from finding other objects as quickly; and our ‘primary visual area’ in the brain shows electrophysiological responses to images of snakes for 150-300 milliseconds longer than it does for those of lizards.
