How seeing snakes in the grass helped primates to evolve
volution has favoured the modification and expansion of primate vision. Compared with other mammals, primates have, for example, greater depth perception from having forward-facing eyes with extensively overlapping visual fields, sharper visual acuity, more areas in the brain that are involved with vision, and, in some primates, trichromatic colour vision, which enables them to distinguish red from green hues. In fact, what separates primates from other mammals most is their much greater reliance on vision as the main sensory interface with the environment.
Vision is a window onto the world, its qualities determined by natural selection and the constraints of both animals’ bodies and the environments in which they live. Despite their long, shared evolutionary history, mammals don’t all see the world in the same way because they inhabit a variety of niches with different selective pressures. What were those selective pressures for primates, our lineage, that led to their having visual systems more expansive and more complex than those of other mammals?
In 2006, I published a new idea that could answer that question and more: the ‘snake detection theory’. I hypothesised that when large-gaped constricting snakes appeared about 100 million years ago and began eating mammals, their predatory behaviour favoured the evolution of changes in the vision of one kind of prey, the lineage that was to become primates. In other words, the ability to see immobile predatory snakes before getting too close became a highly beneficial trait for them to have and pass on to their descendants. Then, about 60 million years ago, venomous snakes appeared in Africa or Asia, adding more pressure on primates to detect and avoid them. This has also had repercussions on their visual systems.
There is a consistency between the degree of complexity in primate visual systems and the length of evolutionary time that primates have spent with venomous snakes. At one extreme, the lineage that comprises Old World monkeys, apes and humans has the best vision of all primates, including excellent visual acuity and fully trichromatic colour vision. Having evolved roughly at the same time and in the same place as venomous snakes, these primates have had continuous coexistence with them. They are also uniformly wary of snakes.
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