By Kenneth Kidd
The need to detect snakes may have given humans an evolutionary nudge toward developing better vision.
When Eve encountered a certain serpent and was bedazzled into eating fruit from the Tree of Knowledge, nothing nice ensued. In the Old Testament, nothing nice ever ensued when God's wrath was aroused.
So while Eve's eyes were, indeed opened, the whole episode ensured a life of toil for Adam and billions of painful childbirths for all women to come.
It now turns out that key elements of that story – serpent, fruit, vision and the large-headed babies that are the proximate cause of painful births – may have played similarly crucial roles in key stages of human evolution, with somewhat more generous results.
Lynne Isbell, a professor of anthropology and animal behaviour at the University of California at Davis (UCD), contends that snakes lie at the root of what makes humans, well, human.
It's a complex thesis, relying on everything from fossil records and primate behaviour to palaeogeography and modern advances in neuroscience. But at the risk of doing violence to that complexity, the essential argument is this:
The earliest predators of our mammal ancestors were snakes, especially constrictors, and venomous snakes can be equally lethal, even when striking defensively. Over the course of millions of years, the need to detect snakes thus gave an evolutionary nudge toward developing better vision.
So for some animals, sight started to become more important than a sense of smell. Better vision, in turn, made it easier for our ancestors to pick out fruits that were ripe, and therefore rich in glucose. We can distinguish reds and oranges from greens.
Through neuroscience, we now know that a vast amount of the human brain is connected to vision, and we also know that glucose is crucial to brain development. Combine the two over millennia, and you end up with today's (painfully) big-headed babies.
Earlier this week, the Star spoke with Isbell, whose book, The Fruit, The Tree and The Serpent, has just been published. The following are edited excerpts from that conversation.
What made you suspect that snakes played such a key role in human evolution?
I was actually looking at a different question: Why is it that female primates in the New World (South America and Central America) are so willing to leave their home areas when there's a heavy cost to leaving. You're more susceptible to predation. But in the Old World, monkeys don't do that. The females stay put.
In my own experience (in Africa), leopards had just decimated the groups of monkeys that I studied in two different places. I started to look at, when did (large predatory) cats get to the New World compared with the Old World?
We know that constrictors are predators of primates, too, so when did constrictors get to the New World?
And it turns out that, not only did constrictors and venomous snakes evolve much earlier than leopards and raptors, they've co-existed with Old World primates for much longer.
They had this bio-geographical relationship with primates that sort of fit with what I knew then to be differences in their visual systems.
In that Old World primates have more advanced vision?
All I knew at the time was that there was a difference is colour vision and the degrees of visual acuity.
There have been other theories about why primates developed better vision, but yours seems to be the first to identify the key role of snakes and to rely heavily on neuroscience. Do you expect neuroscience to start playing a bigger role in answering other evolutionary questions?
It would be great if there could be dialogue back and forth. If people from different disciplines could get together and talk, I think we could make some pretty interesting advances in all fields.
It used to be that mammals were classed into orders based mostly on physical characteristics. Now molecular research is shuffling the deck based on DNA, so some of our closest relatives also include flying lemurs, treeshrews, rabbits and rats. Why didn't our non-primate cousins develop similar vision?
All mammals would have had to deal with constricting snakes, so vision would have been useful to them. But not all mammals eat the same things, to allow them to benefit from the tradeoff between vision and olfaction. If you have good vision, then you're going to lose your sense of smell to some degree.
Because of their diets, then, rabbits and rats had to rely much more on smell to locate greens and seeds?
They eat foods that plants don't want them to eat (so) plants don't evolve ways to make those types of foods smellier. It's the animals that eat fruits that wouldn't suffer any loss if their sense of smell started to get worse. They could afford to expand their vision.
You note how, whenever you're doing field research, it's always the monkeys who detect snakes before anyone else. They get very agitated and even have special alarm calls that only refer to snakes, not other predators.
Yeah. I was just talking with a grad student yesterday. She's worked in Costa Rica and she said that when she's not with the monkeys, she rarely sees snakes, but when she's there with them, she sees them almost every other day. The monkeys point them out.
How much of our fear of snakes is instinctive and how much is socialized?
That's a really good question. Since we are Old World primates ourselves, our ancestors had to deal with snakes for a long time. It's possible that if we're primed to be afraid of snakes, all it takes is for us to learn to be afraid.
Rhesus macaques are not necessarily afraid of snakes the first time they see them, but if they see another rhesus macaque reacting fearfully, then they learn to fear the snakes.
There's an evolutionary preparedness to be afraid of snakes that doesn't exist for more innocuous objects, like flowers. It's probably the same for us.
Is that why snakes figure so prominently in so many myths? Even the mighty Thor gets felled by a snake in Norse mythology.
That's what I was wondering. Why do we focus so much on snakes? Why, unless there's something deep within us, a long evolutionary association with snakes that brings it out in myths and religion.
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