In a kind of evolutionary bridge-burning, once a gene has morphed into its current state, the road back gets blocked, new research suggests. So there's no easy way to turn back.
"Evolutionary biologists have long been fascinated by whether evolution can go backwards," said study researcher Joe Thornton of the University of Oregon's Center for Ecology and Evolutionary Biology and the Howard Hughes Medical Institute. "But the issue has remained unresolved, because we seldom know exactly what features our ancestors had, or the mechanisms by which they evolved into their modern forms."
Thornton's team solved this problem by looking at evolution at the molecular level, where they could figure out the steps taken between the ancestral form of a protein and its successor.
Their results, detailed in the Sept. 24 issue of the journal Nature, reveal that over long time scales certain genetic blockades arise that make it nearly impossible to transform a modern protein into its ancestral state, even if ancient environmental pressures were to exist.
"This is the best demonstration of the molecular foundations of evolutionary irreversibility that I have ever read," said Michael Rose, a professor of ecology and evolutionary biology at the University of California, Irvine, who was not involved in the current study.
Turning back the genetic clock
The team looked at the so-called glucocorticoid receptor, a protein that binds with the hormone cortisol and regulates stress responses, immunity and other bodily processes in humans.
They knew that during a relatively short stint more than 400 million years ago, that receptor gained its current abilities from its ancestral state, which was sensitive to another hormone.
So Thornton's team created both forms of the protein. "We resurrected the first protein to have the modern function and from just before that the last protein to have the ancestral function," Thornton said.
They found seven key mutations that together gave the ancient protein its updated function. To figure out if they could coax the modern protein into its former function, the researchers reversed those seven key mutations.
"We expected to get the ancestral function back out of it," Thornton said during a telephone interview. "But instead we got a dead protein. It didn't work at all. It was completely non-functional."
Burning bridges
Here's what they suggest is behind the phenomenon: As the ancient protein evolved, five other mutations made subtle changes in the protein's structure that were incompatible with the primordial form.
"Suppose you're redecorating your bedroom — first you move the bed, then you put the dresser where the bed used to be," Thornton said. "If you decide you want to move the bed back, you can't do it unless you get that dresser out of the way first."
He added, "The restrictive mutations in the GR (glucocorticoid receptor) prevented evolutionary reversal in the same way."
This same restrictive process might not occur over shorter time scales, as Rose has found in his research.
"What this new Nature publication shows is that on a much longer time-scale (more than a million generations), it is harder to get evolution to reverse itself," Rose told LiveScience. "This is how evolutionists explain things like the failure to reverse-evolve gills in whales or dolphins. Too many generations have elapsed since the ancestors of the Cetaceans had functional gills as adults."
Thornton hopes to study the reversibility of evolution in other proteins. "I expect that this will be a fairly general observation that other proteins and other traits will often be irreversible," he said.
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