Way back when, snakes may well have helped primates develop our visual acuity. Excerpt:
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.
What is it about snakes that makes them so attention-grabbing to us? Naturally, we use all the cues available (such as body shape and leglessness) but it’s their scales that should be the most reliable, because a little patch of snake might be all we have to go on. Indeed, wild vervet monkeys in Africa, for instance, are able with their superb visual acuity to detect just an inch of snake skin within a minute of coming near it. In people, electrophysiological responses in the primary visual area reveal greater early visual attention to snake scales compared with lizard skins and bird feathers. Again, the primary visual area is highly sensitive to edges and lines of different orientations, and snake skins with their spades offer these visual cues in spades.
Speaking as a guy who grew up in rattlesnake country, yeah, we do tend to notice snakes, especially the ones with sharp bits that inject poison – and a big timber rattler can kill you if it strikes too close to your chest or head.
Primates do have an unusual visual acuity as mammals go. They can see more color, our daytime vision is much better than most mammals who sacrifice some color vision for improved nighttime vision. But then, lots of mammals are nocturnal or crepuscular.
Primate color vision, by the way, is also really useful for determining which fruits are ripe and therefore good to eat.
Now, if only they could find a way to reverse-engineer my farsightedness.