"Living Dinosaur" Is Fastest-Evolving Animal
Dave Hansford in Wellington, New Zealand
for National Geographic News
|March 31, 2008|
New Zealand's "living dinosaur," the tuatara, hasn't changed its look in millions of years. But the reptile is actually evolving faster than any other animal studied so far, new DNA analysis reveals.
Scientists recovered DNA from 8,000-year-old tuatara bones and compared it with DNA in blood samples from living tuatara. The modern species is the only surviving member of the order Sphenodontia, which flourished around 200 million years ago.
The results showed that tuatara evolve faster than bears, horses, and many other warm-blooded vertebrates.
(Related: "'Instant' Evolution Seen in Darwin's Finches, Study Says" [July 14, 2006].)
"Tuatara do most things slowly," said study lead author David Lambert of New Zealand's Massey University. He and colleagues published the findings in the March issue of the journal Trends in Genetics.
Tuatara "have a very low metabolic rate. So you would be forgiven for thinking that they haven't been doing very much over 200 million years of evolution."
But Lambert said the reptile's ancient anatomy hides the rapid evolution of DNA within the animals' cells.
"What [the research] is telling us is that the processes that govern anatomical evolution are quite different from those governing molecular evolution."
Axel Meyer, of the University of Konstanz in Germany, agreed.
"There can be a real disconnect" between an animal's physical and genetic evolution rates, said Meyer, who was not involved in the research.
"Fast [evolution] does not necessarily imply 'good' or 'adaptive,'" he added.
He also cautioned against an assumption that so-called living fossils are somehow backward.
For example Meyer's work on the coelacanth—a 300-million-year-old fish species thought to be extinct until 1938—showed that the fish do not evolve more slowly than "normal" species.
(See a picture of a coelacanth captured in 2007.)
Lead author Lambert said the tuatara's genetic vigor came from the structure of its mitochondrial genome. Mitochondria are the powerhouses of the cell.
"When the molecule actually replicates itself, the two DNA strands separate from one another—this region on the genome is single-stranded for lot of the time," Lambert said.
"And when you are single-stranded, you tend to accumulate more change."
However, the tuatara's dynamic DNA is not the secret of its extraordinary survival through millions of years, Lambert said.
"The tuatara of today is not some supercharged model of what was here 8,000 years ago. It's the same tuatara—its survival is related more to where it lives."
New Zealand split from the supercontinent of Gondwana 80 million years ago—before the advent of mammals. As a result, tuatara were not hunted by swift, smart predators until humans brought rats to the islands of New Zealand around a thousand years ago.
While the tuatara's evolutionary rate beat out a field of living and extinct animals, Lambert said the differences were not that huge.
"This is because the mitochondrial genome—and the processes that govern it—is very similar in a lot of those animals.
"The big surprise in all of this is that tuatara are evolving as quickly as they are."
New Survival Challenge?
The hardy reptile may face another survival test as human-induced climate change takes hold, experts say.
Conservationists are concerned that climate change may upset the sex ratios of hatchling tuatara, as warmer temperatures result in the birth of more males.
Lambert said the section of DNA his team studied would not help the lizard to adapt any better to global warming.
But he said tuatara populations had survived countless ice ages, possibly simply by shifting north or south as temperatures fluctuated.
"That's important, because it illustrates how behavior can control evolution—not to cause animals to change, but to remain the same throughout climate change [events]."
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