Evolution: A One-Way Street
Since Darwin, evolution has been in vogue. Most scientists take it on principle that accumulation of mutations in DNA over million of years leads to new life forms.
But a question that has intrigued researchers for some time is whether organisms can go back to their ancestral forms. Is evolution is reversible? Conventional wisdom—known in the sphere of evolutionary biology as “Dollo’s law” after pre-eminent dinosaur researcher Louis Dollo—says no. A recent study published in the journal Nature has elegantly confirmed that evolution is a one-way street by studying the process at the molecular level.
As evolution occurs, the changes are so intricate that it becomes nearly impossible for the organism to go back to its original form. Freshwater fishes that live in a dark cave will lose their eyesight over generations. Even if a landslide creates an opening in the cave and lets in some sunshine, it is highly unlikely that the eye will reform.
Dollo’s theory has remained mostly unchallenged, except for a few works that say that evolution is reversible. In 2003, a team of scientists said that they’d found a species of snail that had regained its ability to coil into a loop after having lost the trait in previous generations.
“But their methods were unreliable,” said Boris Igic, a professor of biology at the University of Illinois in Chicago who was not affliated with the study. The snails could either have gone back to their original genetic makeup, or they could have gained new proteins that give it the old coiling.
The problem was that there was no real test to prove these theories. Evolution of organisms takes millions of years, making it difficult for scientists to make any direct observations.
Joseph Thornton, a biology professor at the University of Oregon, and his colleagues went around the problem by examining a single protein that helps humans and vertebrates cope with stress.
Millions of years ago, a fish existed that lacked bones. It is the ancestor to most life forms on earth today. That fish contained a small protein called the glutocorticoid receptor, which became active in the presence of two distinct hormones.
Over the course of the next 40 million years, the receptor evolved and became more specific such that it activated in the presence of only a single hormone—cortisol. It had accumulated 37 changes, but only seven were necessary to make it into the new receptor.
The researchers wanted to find out whether evolution could reverse at the level of the protein. To do so, they reversed the seven changes.
“But to our surprise, we got a dead receptor when we reversed,” said Thornton.
They found that the only way to make the protein reverse completely was to make five extra changes. These five fine-tuned the receptor, but did not give it a new function.
The probability of all five of these changes getting reversed is highly unlikely since they don’t confer a new advantage to the organism. They act like brakes that have to be removed to make evolution a two-way street.
Once the scientists fixed these five, they found that making the seven key changes reversed the protein to the ancestral form. They called the five brakes “ratchets” that prevented reverse evolution.
“This is not to say that the ancestral function cannot be re-acquired,” said Igic. But the function will come from forward evolution rather than a reversal. When whales evolved from a four-legged terrestrial ancestor, they evolved new proteins that resulted in fins. It was a reversal in function toward an ancestor of tetrapods that could swim, but in biochemistry, it was a movement forward.
Natural selection can take numerous paths during evolution but once those paths are chosen, reversal is highly unlikely. This is the first study of its kind, but Thornton does not expect this to be a rare case.
That an experiment at the molecular level can deliver a decisive conclusion about higher-order evolution is testament to the elegance of life.
“Everything that makes us who we are is stored in DNA,” said Ortlund, a biochemistry professor at Emory University, and co-author of the study. “Changes at the macroscopic scale have to start at the molecular level.”