By reconstructing ancient genes from long-extinct animals, scientists have for the first time demonstrated the step-by-step progression of how evolution created a new piece of molecular machinery by reusing and modifying existing parts.
The researchers say the findings, published today in the journal Science, offer a counterargument to doubters of evolution who question how a progression of small changes could produce the intricate mechanisms found in living cells.
"The evolution of complexity is a longstanding issue in evolutionary biology," said Joseph W. Thornton, professor of biology at the University of Oregon and lead author of the paper. "We wanted to understand how this system evolved at the molecular level. There's no scientific controversy over whether this system evolved. The question for scientists is how it evolved, and that's what our study showed."
Charles Darwin wrote in The Origin of Species, "If it would be demonstrated that any complex organ existed which could not possibly have formed by numerous, successive, slight modifications, my theory would absolutely break down."
Discoveries like that announced this week of a fish with limblike fins have filled in the transitions between species. New molecular biology techniques let scientists begin to reconstruct how the processes inside a cell evolved over millions of years.
Dr. Thornton's experiments focused on two hormone receptors. One is a component of stress response systems. The other, while similar in shape, takes part in different biological processes, including kidney function in higher animals.
Hormones and hormone receptors are protein molecules that act like pairs of keys and locks. Hormones fit into specific receptors, and that attachment sends a signal to turn on — or turn off — cell functions. The matching of hormones and receptors led to the question of how new hormone-and-receptor pairs evolved, as one without the other would appear to be useless.
The researchers found the modern equivalent of the stress hormone receptor in lampreys and hagfish, two surviving jawless primitive species. The team also found two modern equivalents of the receptor in skate, a fish related to sharks.
After looking at the genes that produced them, and comparing the genes' similarities and differences among the genes, the scientists concluded that all descended from a single common gene 450 million years ago, before animals emerged from oceans onto land, before the evolution of bones.
The team recreated the ancestral receptor in the laboratory and found that it could bind to the kidney regulating hormone, aldosterone and the stress hormone, cortisol.
Thus, it turned out that the receptor for aldosterone existed before aldosterone. Aldosterone is found just in land animals, which appeared tens of millions of years later.
"It had a different function and was exploited to take part in a new complex system when the hormone came on the scene," Dr. Thornton said.
What happened was that a glitch produced two copies of the receptor gene in the animal's DNA, a not-uncommon occurrence in evolution. Then, for reasons not understood, two major mutations made one receptor sensitive just to cortisol, leading to the modern version of the stress hormone receptor. The other receptor became specialized for kidney regulation.
Dr. Thornton said the experiments showed how evolution could and did innovate functions over time. "I think this is likely to be a very common theme in how complex molecular systems evolved," he said.
Christoph Adami, a professor of life sciences at the Keck Graduate Institute in Claremont, Calif. who wrote an accompanying commentary in Science, said the research showed how evolution "takes advantage of lucky circumstances and builds upon them."
Dr. Thornton said the experiment refutes the notion of "irreducible complexity" put forward by Michael J. Behe, a professor of biochemistry at Lehigh University.
Dr. Behe, a main advocate of intelligent design, the theory that life is so complicated that the best explanation is that it was designed by an intelligent being, has compared an irreducibly complex system to a mousetrap. Take away any piece, and the mousetrap fails to catch mice. Such all-or-none systems could not have arisen with incremental changes, Dr. Behe has argued.
Dr. Thornton said the key-and-lock mechanism of a hormone-receptor pair was "an elegant exemplar of a system that has been called irreducibly complex."
"Of course," he added, "our findings show that it is not irreducibly complex."
Dr. Behe described the results as "piddling." He wondered whether the receptors with the intermediate mutations would be harmful to the survival of the organisms and said a two-component hormone-receptor pair was too simple to be considered irreducibly complex. He said such a system would require at least three pieces and perform some specific function to fit his notion of irreducibly complex.
What Dr. Thornton has shown, Dr. Behe said, falls within with incremental changes that he allows evolutionary processes can cause.
"Even if this works, and they haven't shown that it does," Dr. Behe said, "I wouldn't have a problem with that. It doesn't really show that much."
