Compared to your four-legged friends who follow their noses around, or the wild animals who can hear a hiker from three miles away, we primates and our powerful peepers sure seem unique- and we are! Primate brains have intensely intricate visual centers compared to other mammals' (Kaas 2013). It's evidence that some pretty crazy evolutionary forces have been driving our sense of sight to get better and better for millions of years. The question is: why?
For a long time, scientsits hypothesized that our eyes improved to aid our ability to grasp and manipulate food items at close ranges. However, in-depth experiments on primate brain structure in 2004 found that the changes in our brain that caused improvement in our sight were not correlated with the visual centers of the brain related to reaching and grasping (Barton, 2004). With the community all but stumped, one primate socioecologist named Dr. Lynne Isbell was gearing up to drop one of human evolution's most contenteous hypotheses: it's all about the snakes.
In a seminal 2006 paper (Isbell, 2006), Dr. Isbell presents the Snake Detection Hypothesis (SDH), which proposes that early primates with more atuned visual systems would be more able to notice venemous vipers lurking in the grass. As a result, our early primate ancestors with better sight than their peers would be less likely to die of snake bites. Isabell also points out that the primate lineages with better vision, like old world monkeys, are the ones that have spent the most evolutionary time around venomous snakes. On the other hand, primates with the least honed eyesight, like Malagasy prosimians, have never co-existed with venomous snakes.
And indeed, in the 15 years since it was proposed, Snake Detection Theory has gotten a lot of scientific traction. Psychological experiments on humans routinely find that our behaviour in response to images of snakes is exceedingly distinct from how we react to images of other dangerous animals. Not only do humans and monkeys show more intensely negative reactions to pictures of snakes than they do to pictures of, say, spiders (Van Strein et al, 2014)(Van Strein and Isbell, 2017), but humans even have straight up physiological reactions to snakes. Videos containing snakes increase the heart rate of the viewer, even if the viewer doesn't consciously notice the snake (Jensen and Caine, 2021). On top of that, people can recognize pictures of camoflauged snakes better than camoflauged images of any other animal (Kawai and He, 2016). In 2013, Isbell and her colleagues even identified a set of snake-response neurons in the simian brain (Van Le et al, 2013). It is all but a certainty that humans and our primate cousins have unique, innate, powerful reactions to the sight of snakes.
So, if there's so much apparent evidence backing up the SDH, why is it such a hotly debated concept? Well, no research has been able to definitively show that our reaction to snakes actually drove our evolution. Many still argue that snake detection is simply a consequence of our visual system improving. Indeed, one big hole in the current evidence for snake detection theory is that most studies use pictures or videos instead of real snakes, and that in natural environments our ability to detect snakes might not actually be that good (Coelho et al, 2019). Additionally, neither primate species that spend a lot of time around venemous snakes, nor those that are more likely to be preyed upon by serpents, have an increased amount of orbital convergence: the type of brain development that the SDH predicts increases snake detection abilities (Wheeler et al, 2011).
After 15 years, Snake Detection Theory is no more proven or disproven than it was the day Dr. Isbell's first paper came off the presses. Perhaps we'll never known with certainty whether we have the venomous vipers of yore to thank for our incredible eyesight. Nonetheless, it's still cool to imagine that, without snakes, the way you interact with the world, and maybe even the world itself, might've been fundamentally different