Here is Why Evolution is Irreversible

PORTLAND – By resurrecting ancient proteins, University of Oregon researchers have found that evolution is irreversible, and can only go forward.

The University of Oregon research team found that evolution can never go backwards, because the paths to the genes once present in our ancestors are forever blocked.

The team used computational reconstruction of ancestral gene sequences, DNA synthesis, protein engineering and X-ray crystallography to resurrect and manipulate the gene for a key hormone receptor as it existed in our earliest vertebrate ancestors more than 400 million years ago.

They found that over a rapid period of time, five random mutations made subtle modifications in the protein’s structure that were utterly incompatible with the receptor’s primordial form.

According to Joe Thornton, a professor in the UO’s Center for Ecology and Evolutionary Biology and the Howard Hughes Medical Institute, “Evolutionary biologists have long been fascinated by whether evolution can go backwards, 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.”

“We solved those problems by studying the problem at the molecular level, where we can resurrect ancestral proteins as they existed long ago and use molecular manipulations to dissect the evolutionary process in both forward and reverse directions,” he said.

Thornton’s team focused on the evolution of a protein called the glucocorticoid receptor (GR), which binds the hormone cortisol and regulates the stress response, immunity, metabolism and behavior in humans and other vertebrates.

In previous work, Thornton’s group showed that the first GR evolved more than 400 millions ago from an ancestral protein that was also sensitive to the hormone aldosterone.

They then identified seven ancient mutations that together caused the receptor to evolve its new specificity for cortisol.

Once Thornton’s team knew how the GR’s modern function evolved, they wondered if it could be returned to its ancestral function.

So they resurrected the GR as it existed soon after cortisol specificity first evolved-in the common ancestor of humans and all other vertebrates with bones – and then reversed the seven key mutations by manipulating its DNA sequence.

“We expected to get a promiscuous receptor just like the GR’s ancestor, but instead we got a completely dead, non-functional protein,” Thornton said.

“Apparently other mutations that occurred during early GR evolution acted as a sort of evolutionary ratchet, rendering the protein unable to tolerate the ancestral features that had existed just a short time earlier,” he added.

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