Designed to scale: the hissstory of snakes
First it lost its limbs, then the snake embarked on a bizarre evolutionary journey that has made it one of nature’s miracles of re-engineering, writes Bob Holmes
SNAKE! Just the thought is enough to trigger a spasm of fear in many of us.
Snakes make biologists’ hearts beat faster, too, but for a different reason: in evolutionary terms, they may be the most surprising group of vertebrates on Earth.
Their long, legless bodies, it turns out, are the least remarkable thing about them. It’s on the inside that snakes have made extraordinary changes. They have pared down their internal organs, mostly eliminating one lung and all but one lobe of the liver.
They have evolved a heat-detecting sense organ, boast the most sophisticated venom system of any animal and can turn their metabolism up and down more dramatically than any other vertebrate. This re-engineering extends even to the molecular level – proteins that have remained unchanged across other vertebrates have been rebuilt in snakes.
“It looks like evolutionarily, snakes are a kind of redesigned organism,” says Stephen Mackessy, of the University of Northern Colorado, in the United States, whose research focuses on snakes and their venom. And with the help of the first two snake genomes to be sequenced, we are beginning to piece together their remarkable evolutionary journey.
The story of how snakes evolved begins just over 100 million years ago, with a lizard or lizard-like reptile. Biologists are still debating as to which group the ancestor of snakes belonged to. A few think snakes are descended from the marine reptiles known as mosasaurs, but they are in the minority.
“I think the great bulk of the evidence points to a terrestrial origin for snakes, and even a burrowing or secretive origin,” says Harry Greene, an evolutionary biologist at Cornell University in Ithaca, New York.