Challenging Assumptions About the Origin of Life
Now, research from UNC School of Medicine biochemist Charles Carter, PhD, appearing in the September 13 issue of the Journal of Biological Chemistry, offers an intriguing new view on how life began. Carter’s work is based on lab experiments during which his team recreated ancient protein enzymes that likely played a vital role in helping create life on Earth. Carter’s finding flies in the face of the widely-held theory that Ribonucleic Acid (RNA) self-replicated without the aid of simple proteins and eventually led to life as we know it.
In the early 1980s, researchers found that ribozymes — RNA enzymes — act as catalysts. It was evidence that RNA can be both the blueprints and the chemical catalysts that put those blueprints into action. This finding led to the “RNA World” hypothesis, which posits that RNA alone triggered the rise of life from a sea of molecules.
But for the hypothesis to be correct, ancient RNA catalysts would have had to copy multiple sets of RNA blueprints nearly as accurately as do modern-day enzymes. That’s a hard sell; scientists calculate that it would take much longer than the age of the Universe for randomly generated RNA molecules to evolve sufficiently to achieve the modern level of sophistication. Given Earth’s age of 4.5 billion years, living systems run entirely by RNA could not have reproduced and evolved either fast or accurately enough to give rise to the vast biological complexity on Earth today.
“The RNA world hypothesis is extremely unlikely,” said Carter. “It would take forever.”
The finding also suggests that Urzymes evolved from even simpler ancestors — tiny proteins called peptides. And over time those peptides co-evolved with RNA to give rise to more complex life forms.
In this “Peptide-RNA World” scenario, RNA would have contained the instructions for life while peptides would have accelerated key chemical reactions to carry out those instructions.
“To think that these two Urzymes might have launched protein synthesis before there was life on Earth is totally electrifying,” Carter said. “I can’t imagine a much more exciting result to be working on, if one is interested in the origin of life.”
The study leaves open the question of exactly how those primitive systems managed to replicate themselves — something neither the RNA World hypothesis nor the Peptide-RNA World theory can yet explain. Carter, though, is extending his research to include polymerases — enzymes that actually assemble the RNA molecule. Finding an Urzyme that serves that purpose would help answer that question.
The study’s co-authors include Li Li of UNC and Christopher Francklyn of the University of Vermont, Burlington. Read the study at The Journal of Biological Chemistry.
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