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Theory questions RNA-only evolution of complex organisms

By Sam Harris

The origins of life on Earth, and how single cells evolved into complex organisms such as humans, is a hotly debated topic among scientists and academics. One long-standing theory, known as the “RNA world hypothesis,” is that life originated from RNA – ribonucleic acid. However, some doubt that RNA molecules could have replicated themselves without help; they believe there may have been other molecules involved.

New research, published in September in the Journal of Biological Chemistry, offers a different theory as to how RNA molecules were able to replicate and become the plants and animals we know today.

In the paper, Charles Carter Jr., professor of biochemistry and biophysics at UNC-Chapel Hill, and his colleagues show how two enzymes they call “urzymes” may have helped RNA molecules replicate, beginning the evolution of life as we know it.

Enzymes are the biological molecules, composed mostly of proteins, that speed up the reactions necessary for life.

“I believe that these urzymes were among the very earliest enzymes and that there are many families of enzymes that have descended from them,” Carter said.

Carter said the theory, which challenges the RNA world hypothesis, has been met with interest and skepticism from others studying the origins of life. “Many people doubt the pure form of the RNA World hypothesis. Yet others become angry if anyone suggests that other molecules might have played an essential role in replicating RNA.

“I’ve spoken about this work only twice in public – once last August and again in June of 2013. Eugene Koonin, a wonderfully bright and reflective scientist who spoke after me, began a talk by saying that he was about to tell the audience that the RNA World Hypothesis was ‘inevitable.’” According to Carter, Koonin then said, “Professor Carter had just shown that it was ‘evitable.’” In other words, capable of being avoided.

If Carter’s theory is correct, and life began not just from RNA molecules copying themselves but from a combination of RNA molecules and “urzymes,” this opens up the door to further research: where these urzymes came from, and what actually kickstarted the processes by which they replicated RNA, is still unknown.

One idea Carter wants to explore is that the two urzymes share a common gene – a gene that would have essentially been the “precursor” gene to life.

“Additional motivation for my work comes from a bizarre hypothesis about the two classes of enzymes, published in 1994 by two important theoretical biologists. I call it the Rodin-Ohno hypothesis after the two authors,” Carter said.

“Their hypothesis, which I’ve been trying to test, is that the original instructions for one enzyme were complementary to those for the second enzyme, and hence a single gene was the ancestor to both enzymes. The two urzymes described in our paper come quite close to realizing this paradigm.”

As to whether his theory may become more widely accepted, Carter said that “much remains to be seen.” In the meantime, he plans to continue to research the urzymes and the idea that other molecules contributed to the origins of life.

“In order of priority, I hope to characterize additional urzymes, I hope to understand the origin of genetic coding, and I’d like to subject my original hypothesis to experimental tests that are now possible because of the explosion of biotechnology.”

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