The focus of this post will be primarily on Chapter 11 of Wild Life of Our Bodies. Dr. Nelson expressed interest in my take on the connection between our past and anxiety disorders, which is something I’m definitely willing to talk about in class. However, Chapter 11 struck me as particularly problematic.
Dunn references a theory (apparently proposed by Isbell) of the acuity of primate vision and ultimately the emergence of our intelligence. In short, he points to an apparent relationship between the presence of venomous snakes and the ability of primate species to detect a wider range of colors. In particular, Dunn points to the fact that primates in the Americas and Madagascar cannot detect oranges and reds while those in mainland Africa can. He seems to also suggest that venomous snakes are linked to better vision altogether (e.g. resolution, depth perception, etc.) without giving any specifics.
The theory predicts that venomous snakes will be less prevalent where primates have poorer vision. And that’s about it. Unfortunately, robust theories are ones that make an amalgam of precise predictions–ones without alternative explanations–that hold up to testing and observations. It seems Isbell has just a single broad prediction that turns out to be true; this is not enough for any serious scientist to take the idea seriously.
The statistics Dunn gives are compelling. Many thousands of people in Africa are bitten by snakes every year. It seems likely that primates encounter snakes quite frequently, enough that they may have exerted selective pressures on primates. But here’s where I have problems…
Acute vision is one of the most effective adaptations that exists in animals. Almost all of them have it, and many species have vision far more acute than ours. There are, quite literally, a million reasons to have good vision, and snakes are just one of them. Don’t forget our horrendous night vision, something that I imagine would be useful for detecting snakes at night. And finally, correct me if I’m wrong, but my understanding is that there are very few snake species in African forests whose scales contain orange/red pigments. If the primary deficit of South American primate vision is their inability to see these colors, my intuition is that this would not pose a great disadvantage in Africa where the majority of snakes, I imagine, have evolved a degree of camouflage coloring in the form of browns, greens, blacks, etc.
The poorer vision of lemurs in Madagascar and other primates in South America may be simply attributable to various happenstances in their environments. Dunn admits Lemurs have much more acute sense of touch and smell. Given that humans have among the weakest senses of smell (correct me if I’m wrong), it is self-evident that having stronger versions of these senses provides advantages to many other species that may be comparable to the advantage added by better vision. In other words, lemurs may have developed their other senses in ways that negated the need to develop better vision. And again the correlation between snake prevalence (which is low in Madagascar and America) and vision is not strong enough evidence of anything.
Dunn goes further to suggest that snakes may be responsible for the emergence of our brains in general. Dunn seems to ignore two things about the power of brains:
1.) What advantage does an unarmed genius have over an unarmed idiot against a snake? Snakes are frightening and elusive animals, and my intuition tells me that intellect does very little to combat an angry snake until a species evolves sophisticated tools, something that was done long after the development of our vision and of the size of our brains.
2.) There are, just with a vision, a million reasons to develop large brains. In a competition for best natural invention ever, what wins? Our brains, hands down. We may have the most sophisticated thing in the universe sitting inside our skulls. So my point here is that snakes may have been just a single selective pressure to develop large brains and acute vision–maybe even a relatively strong one–but once our brains were put on the path of growing larger at the expense of other parts of our bodies, everything in our environments was selecting for higher intelligence, because our minds can bend and adapt in real time and sharp claws, for instance, can’t.