I’ve bashed bad science reporting in the mainstream media, so to be fair I should also praise good science reporting when I see it (and for yet another example of atrocious science reporting, see this post by PZ about one more breathless “this changes everything” story full of nonsense). So today, I come to praise Kenneth Chang, not to bury him, for this New York Times article on a recent paper in Science about reconstructing the precise mutational path that transformed one ancient protein with one function into another protein with a different function. I’ll post a long excerpt below the fold that explains the research and its implications well:
In work published last year, Dr. Thornton reported how his group reconstructed an ancestral protein of two hormone receptors found in humans. The two, once identical, diverged along different evolutionary paths. One is now part of the stress response system; the other is involved in different biological processes, including kidney function in many animals.
In the new study, the researchers determined the exact positions of more than 2,000 atoms in the ancestral hormone receptor. The receptor existed in animals that lived more than 440 million years ago, before the last common ancestor of people and sharks. Then the researchers examined what occurred during the next 20 million years — before another split of the evolutionary tree that led to bony fish. “That’s the ancestor of you and a salmon,” Dr. Thornton said.
In that time, one hormone receptor changed so that it bound most strongly to cortisol, a stress hormone. Bony fish and people have this version, called a glucocorticoid receptor. Sharks do not.
Of the glucocorticoid receptors that have been looked at in different species, five specific mutations are always present and distinguish them from the ancestral receptor. When the scientists introduced the five changes into the ancestral protein, they expected that it would be transformed into a glucocorticoid receptor.
Instead, the protein broke, unable to bind to any hormone.
On further investigation, the scientists found that two other mutations, which had negligible effects by themselves, strengthened some of the protein’s folds so it could withstand the other five mutations. The researchers were also able to show several sequences in which the seven mutations could have occurred without the protein’s functionality ever deteriorating.
Biologists often point out that evolution does not proceed through random chance. Rather, the process of natural selection — survival of the fittest — ruthlessly weeds out mutations detrimental to the survival of a species.
But the findings of Dr. Thornton and his colleagues show that some aspects of evolution do occur solely by chance. The two mutations that reinforced the protein did not directly help the organism. So natural selection did not particularly favor them. It was only by chance that they occurred and persisted to set the groundwork for the other mutations.
For those with access to Science, you can find the full paper here. It’s quite an interesting bit of research, the kind of painstaking investigative work that really helps our understanding of evolutionary processes and how they work.