July 20, 2006

Snake-spotting may have helped us evolve -study

WASHINGTON (Reuters) - Snakes may make people jump for a
good reason -- human close-up vision may have evolved
specifically to spot the reptiles, researchers reported on

Humans, monkeys and other primates have good color vision,
large brains, and use their vision to guide reaching and

But while some scientists believe these characteristics
evolved together as early primates used their hands and eyes to
pick fruit and other foods, Lynne Isbell, a professor of
anthropology at the University of California Davis, believes
they may have evolved to help primates evade snakes.

"A snake is the only predator you really need to see close
up. If it's a long way away it's not dangerous," said Isbell,
who has published her theory in the Journal of Human Evolution.

Neurological studies show the structure of the brain's
visual system seems to be well connected to brain structures
involved in vigilance, fear and learning, she said.

Mammals evolved about 100 million years ago and fossils of
snakes with mouths big enough to eat those mammals appear at
about the same time, she pointed out.

Other predators such as big cats, and hawks and eagles,
evolved later. And then venomous snakes evolved about 60
million years ago, which forced primates to get better at
detecting them.

"There's an evolutionary arms race between the predators
and prey. Primates get better at spotting and avoiding snakes,
so the snakes get better at concealment, or more venomous, and
the primates respond," Isbell said.

And there are no dangerously venomous snakes on Madagascar,
and lemurs, which only live on that large island and which have
poor eyesight, have not evolved much in other ways in the past
60 million years, either, Isbell added.