A new theory on how snakes helped evolution of primates
As a herpetologist, I love this.
Fear of Snakes Drove Pre-Human Evolution
By Ker Than
LiveScience Staff Writer
posted: 21 July 2006
12:59 am ET
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
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
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
A new weapon
Fossil and DNA evidence suggests that the 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.
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
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 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,"
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
If snake and primate history are as intimately connected as Isbell
suggests, then it might account for other things as well, Greene
"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.