Fear of Snakes May Have Driven Pre-Human Evolution

http://www.foxnews.com/story/0,2933,204977,00.html

Fear of Snakes May Have Driven Pre-Human Evolution

Friday , July 21, 2006

By Ker Than

An evolutionary arms race between early snakes and mammals triggered the development of improved vision and large brains in primates, a radical new theory suggests.

The idea, proposed by Lynne Isbell, an anthropologist at the University of California, Davis, suggests that snakes and primates share a long and intimate history, one that forced both groups to evolve new strategies as each attempted to gain the upper hand.

To avoid becoming snake food, early mammals had to develop ways to detect and avoid the reptiles before they could strike. Some animals evolved better snake sniffers, while others developed immunities to serpent venom when it evolved.

Early primates developed a better eye for color, detail and movement and the ability to see in three dimensions — traits that are important for detecting threats at close range.

Humans are descended from those same primates.

Scientists had previously thought that these traits evolved together as primates used their hands and eyes to grab insects, or pick fruit or to swing through trees, but recent discoveries from neuroscience are casting doubt on these theories.

"Primates went a particular route," Isbell told LiveScience. "They focused on improving their vision to keep away from [snakes]. Other mammals couldn't do that. Primates had the pre-adaptations to go that way."

Harry Greene, an evolutionary biologist and snake expert at Cornell University in New York, says Isbell's new idea is very exciting.

"It strikes me as a very special piece of scholarship and I think it's going to provoke a lot of thought," Greene said.

Isbell's work is detailed in the July issue of the Journal of Human Evolution.

A new weapon

Fossil and DNA evidence suggests that snakes were already around when the first mammals evolved some 100 million years ago. The reptiles were thus among the first serious predators mammals faced.

Today, the only other threats faced by primates are raptors, such as eagles and hawks, and large carnivores, such as bears, large cats and wolves, but these animals evolved long after snakes did.

Furthermore, these other predators can be safely detected from a distance. For snakes, the opposite is true.

"If you see them close to you, you still have time to avoid them," Isbell said. "Primate vision is particularly good at close range."

Early snakes killed their prey using surprise attacks and by suffocating them to death — the method of boa constrictors.

But the improved vision of primates, combined with other snake-coping strategies developed by other animals, forced snakes to evolve a new weapon: venom. This important milestone in snake evolution occurred about 60 million years ago.

"The [snakes] had to do something to get better at finding their prey, so that's where venom comes in," Isbell said. "The snakes upped the ante and then the primates had to respond by developing even better vision."

Once primates developed specialized vision and enlarged brains, these traits became useful for other purposes, such as social interactions in groups.

Seeing in 3D

Isbell's new theory could explain how a number of primate-defining traits evolved.

For example, primates are among the few animals whose eyes face forward (most animals have eyes located on the sides of their heads).

This so-called orbital convergence improves depth perception and allows monkeys and apes, including humans, to see in three dimensions.

Primates also have better color vision than most animals and are also unique in relying heavily on vision when reaching and grasping for objects.

One of the most popular ideas for explaining how these traits evolved is called the "visual predation hypothesis."

It proposes that our early ancestors were small, insect-eating mammals and that the need to stalk and grab insects at close range was the driving force behind the evolution of improved vision.

Another popular idea, called the "leaping hypothesis," argues that orbital convergence is not only important for 3D vision, but also for breaking through camouflage.

Thus, it would have been useful not only for capturing insects and finding small fruits, but also for aiming at small, hard-to-see branches during mid-leaps through trees.

But there are problems with both hypotheses, Isbell says.

First, there is no solid evidence that early primates were committed insectivores. It's possible that like many primates today, they were generalists, eating a variety of plant foods, such as leaves, fruit and nectar as well as insects.

More importantly, recent neuroscience studies do not support the idea that vision evolved alongside the ability to reach and grasp. Rather, the data suggest that the reaching-and-grasping abilities of primates actually evolved before they learned to leap and before they developed stereoscopic, or 3D, vision.

Agents of evolutionary change

Isbell thinks proto-primates — the early mammals that eventually evolved into primates — were in a better position compared to other mammals to evolve specialized vision and enlarged brains because of the foods they ate.

"They were eating foods high in sugar, and glucose is required for metabolizing energy," Isbell said. "Vision is a part of the brain, and messing with the brain takes a lot of energy, so you're going to need a diet that allows you to do that."

Modern primates are among the most frugivorous, or "fruit-loving," of all mammals, and this trend might have started with the proto-primates.

"Today there are primates that focus on leaves and things like that, but the earliest primates may have had a generalized diet that included fruits, nectar, flowers and insects," she said.

Thus, early primates not only had a good incentive for developing better vision, they might have already been eating the high-energy foods needed to do so.

Testing the theory

Isbell says her theory can be tested. For example, scientists could look at whether primates can visually detect snakes more quickly or more reliably than other mammals. Scientists could also examine whether there are differences in the snake-detecting abilities of primates from around the world.

"You could see whether there is any difference between Malagasy lemurs, South American primates and the African and Asian primates," Isbell said.

Anthropologists have tended to stress things like hunting to explain the special adaptations of primates, and particularly humans, said Greene, the Cornell snake expert, but scientists are starting to warm to the idea that predators likely played a large role in human evolution as well.

"Getting away from things is a big deal, too," Greene said in a telephone interview.

If snake and primate history are as intimately connected as Isbell suggests, then it might account for other things as well, Greene added.

"Snakes and people have had a long history; it goes back to long before we were people, in fact," he said. "That might sort of explain why we have such extreme attitudes towards snakes, varying from deification to ophidiphobia, or fear of snakes."

Mr Than states:

Today, the only other threats faced by primates are raptors, such as eagles and hawks, and large carnivores, such as bears, large cats and wolves, but these animals evolved long after snakes did.

Not true, several species of Python and Boa Constrictor actively hunt in the trees. And Primates are most definately on the menu.

Hmm I disagree, for those who do choose to believe in the theory of evolution, I think that preditation and preying was a big part of the evolution of every creature as well as environmental changes, but I don't see how snakes alone caused all mammals to change.

Also, thats a good point that rattlesnake made.

I agree with all of the above and want to say that predatation surely was one of the major driving forces in evolution; possibly the most important. It's hard to pass on your genes if you're something's lunch, but to think that snakes were the only predators to watch out for is ludicrous. I hardly believe that the world was solely covered in snakes and pre-monkeys at one time in history. I do, though, agree that vision was evolved for protection and self preservation, not for food or foraging, as many creatures still here today have very poor sight and seem to eat just fine. They are also the same creatures that are lower on the food chain and their survival is usually reliant upon reproduction in large numbers; the "they can't eat us all" method of survival of lower evolved species.

Once again, the media report on the issue manages to bungle it to the point that it's hard to make out the basis for the theory.

The actual abstract (sorry, I'm at home; I can get the full journal article tommorrow at lab):

Originally Posted by Abstract

Current hypotheses that use visually guided reaching and grasping to explain orbital convergence, visual specialization, and brain expansion in primates are open to question now that neurological evidence reveals no correlation between orbital convergence and the visual pathway in the brain that is associated with reaching and grasping. An alternative hypothesis proposed here posits that snakes were ultimately responsible for these defining primate characteristics. Snakes have a long, shared evolutionary existence with crown-group placental mammals and were likely to have been their first predators. Mammals are conservative in the structures of the brain that are involved in vigilance, fear, and learning and memory associated with fearful stimuli, e.g., predators. Some of these areas have expanded in primates and are more strongly connected to visual systems. However, primates vary in the extent of brain expansion. This variation is coincident with variation in evolutionary co-existence with the more recently evolved venomous snakes. Malagasy prosimians have never co-existed with venomous snakes, New World monkeys (platyrrhines) have had interrupted co-existence with venomous snakes, and Old World monkeys and apes (catarrhines) have had continuous co-existence with venomous snakes. The koniocellular visual pathway, arising from the retina and connecting to the lateral geniculate nucleus, the superior colliculus, and the pulvinar, has expanded along with the parvocellular pathway, a visual pathway that is involved with color and object recognition. I suggest that expansion of these pathways co-occurred, with the koniocellular pathway being crucially involved (among other tasks) in pre-attentional visual detection of fearful stimuli, including snakes, and the parvocellular pathway being involved (among other tasks) in protecting the brain from increasingly greater metabolic demands to evolve the neural capacity to detect such stimuli quickly. A diet that included fruits or nectar (though not to the exclusion of arthropods), which provided sugars as a neuroprotectant, may have been a required preadaptation for the expansion of such metabolically active brains. Taxonomic differences in evolutionary exposure to venomous snakes are associated with similar taxonomic differences in rates of evolution in cytochrome oxidase genes and in the metabolic activity of cytochrome oxidase proteins in at least some visual areas in the brains of primates. Raptors that specialize in eating snakes have larger eyes and greater binocularity than more generalized raptors, and provide non-mammalian models for snakes as a selective pressure on primate visual systems. These models, along with evidence from paleobiogeography, neuroscience, ecology, behavior, and immunology, suggest that the evolutionary arms race begun by constrictors early in mammalian evolution continued with venomous snakes. Whereas other mammals responded by evolving physiological resistance to snake venoms, anthropoids responded by enhancing their ability to detect snakes visually before the strike.

What the livescience reporter took 3 pages to say can be said in a 6 lines: Primates have forward facing eyes, numerous visual specializtions, and big brains.
Previously this was explained as mechanisms for climbing and leaping in trees.
However, primates with forward-facing eyes don't always have brains suited to climbing in trees, and vice-versa.
On the other hand, primates do have big portions of the brain associated with fear and predators, and these are tied to visual areas of the brain in primates.
On top of that, the variation in brain expansion matches the degree of prehistoric experience with snakes.
As a test of this, raptors which eat snakes also have a higher degree of binolcular vision than other raptors.

Personally, I think it's interesting, but I'm skeptical of throwing away the theory that arboreal habitats lead to primate brain and eye specializations; plenty of arboreal animals have little binocular vision (anoles, squirrels, etc), and there's no reason both selective pressures couldn't be acting together to produce the same end-result.

I'm also very curious about the prosimians of madagascar, such as the lorises and pottos, both of which lack extensive experience with venomous reptiles, but have highly binocular vision.

Originally Posted by rattlesnake

Not true, several species of Python and Boa Constrictor actively hunt in the trees. And Primates are most definately on the menu.

If you read carefully, the article states "Today, the only other threats faced by primates are..." (emphasis mine), which states that snakes are still a major primate predator.

I think that preditation and preying was a big part of the evolution of every creature as well as environmental changes, but I don't see how snakes alone caused all mammals to change.

Well, in the aftermath of the KT extinction (the one that killed the dinosaurs), all the remaining mammals were small, and snakes were still around. It's a pretty fair bet that snakes were a major predator of early mammals. Prior to the extinction, terrestrial mammals had to contend with small predatory dinosaurs (especially the high-performance coelurosaurs), but early proto-primates, already arboreal, did not, though they would likely have faced snakes. So basically, primates have been on the menu for a long, long time, even before the dinosaurs died.

That said, I'm unsure just how much selective pressure snakes would put on primates. On one hand, even small selective pressures can rapidly produce large changes, but on the other, it's overly simplistic to consider snakes the only game in town, especially since, as ectotherms, they don't eat nearly as many monkeys per snake than a hawk of the same mass.

I do, though, agree that vision was evolved for protection and self preservation, not for food or foraging, as many creatures still here today have very poor sight and seem to eat just fine.

It's typically a bad idea to assume complex features evolved for a singel reason. Hands, for example, are great for walking on, but also for picking off mites, climbing trees, probing hard-to-reach spaces, and using tools. All of those examples can be found in primates: gorillas, baboons, gibbons, aye-ayes, and humans respectively.

Simple, clean-cut theories are rarely right in biology; things are almost always more complex than we'd like them to be, and a single trait such as hands, eyes, hair, or the digestive system may be acted upon by dozens of different, sometimes conflicting, selective pressures, and the relative intensity of those pressures can vary through the organism's evolutionary history.

I'll read the full text of the paper tommorrow and post if there's anything more that's either strongly supportive or dubious.

Henry

Is there a link to the journel article?
Not the news article ,the actual article that was mentioned above.
Thanks.

The good news is, I found the journal article. The bad news is that it's not publically availible, though I'll save it and can send the pdf to you if you PM me your email address.

It's also 35 pages long, so it's not exactly light reading.

Henry

Well, I finished reading the article over break, and I must say, I'm actually quite impressed with it, and it makes a very strong arguement. What I was able to follow of the neurobiological arguement (not my area, unfortunately) seemed solid, as did the behavioral, anatomical, paleobiological and comparative portions. It even addressed my objection concerning the highly binocular nocturnal prosimians (tarsiers; apparently I mis-remembered the name) by pointing out that they actuall *eat* snakes, thus need the visual system for spotting them.

I'm not entirely convinced, but it does seem like a viable hypothesis with a fair body of empirical evidence that can be marshalled to support it.

Henry