The earliest biomolecules must have been able to both carry genetic information and copy themselves. Now researchers report that a type of RNA that helps spur the synthesis of nucleotides--the raw stuff of RNA and DNA--can evolve from random bits of RNA. The finding, which appears in today's Nature, strengthens a popular theory among origins-of-life researchers that early life used RNA alone to carry out these two functions.
The discovery in the early 1980s that some RNA molecules, called ribozymes, can catalyze chemical reactions led many biologists to speculate that RNA was the primordial biomolecule. One snag in this theory has been the fact that modern organisms need protein enzymes to copy their genetic information--substances that would not have existed in a purely RNA world. In particular, early life would have needed to synthesize nucleotides, the subunits of RNA and DNA, in order to reproduce itself. To see if ribozymes could evolve this ability, biochemists Peter Unrau and David Bartel at the Massachusetts Institute of Technology in Cambridge began with a pool of more than 1015 different RNA molecules.
The duo selected RNA molecules that showed even a slight talent for catalyzing the formation of nucleotides from precursor molecules. Next they copied the promising RNA molecules using a technique that caused small mutations in their nucleotide sequences and let these mutated RNA molecules have another go at the reaction. After only 11 "generations" of evolution, Unrau and Bartel had produced ribozymes highly effective at catalyzing nucleotide formation. Indeed, some of the ribozymes were 10 million times more efficient than the initial pool of RNA molecules.
These and other experiments "clearly reinforce the possibility of primitive RNA-based life during early evolutionary stages," says evolutionary biologist Antonio Lazcano of the National University of Mexico in Mexico City. However, he notes that even if the RNA world did once exist, researchers still have no clue how the RNA nucleotides became plentiful and stable enough to form the larger molecule.