Origin of Life Chicken-and-Egg Problem Solved

A chicken-and-egg paradox at the foundations of life may finally be solved.

Scientists have wondered how the first simple, self-replicating chemicals could have formed complex, information-rich genetic structures, when replication was originally such an error-prone process. Every advance would soon be lost to copying errors.

According to a new study, the answer may lie in the fundamental nature of those chemicals. The errors may have triggered an automatic shutdown of replication. Such stalling would allow only error-free sequences to be completed, giving them a chance at evolving.

“A chemical system with this property would be able to propagate sequences long enough to have function,” wrote researchers led by Harvard University systems biologist Irene Chen. The study was published April 1 in the Journal of the American Chemical Society.


Scientists think life’s first spark came in the form of ribonucleic acid, or RNA. The single-stranded molecular forerunner of the DNA in every animal’s genes, RNA is the basis of the simplest self-replicating structures.

Estimates of error rates in early RNA replication run around 20 percent. For every pair of basic chemical units in a molecule of RNA, there was a one in five chance of getting the match wrong when a copy was made.

Strands of RNA longer than five units would be rare — and even simple RNA structures responsible for improving copy fidelity are 30 units long. Getting to that point would be practically impossible, and error-ridden copies would steal chemical resources from successful molecules.

But researchers have observed that DNA sometimes stalls when an error occurs during self-replication. If that could happen to RNA, then only accurate copies would continue to replicate, reasoned Chen’s team. The paradox would be solved.

RNA proved too unstable to work with, so Chen’s team used simple, short strands of DNA as a proxy. They put the strands in a mixture of organic compounds known to have existed on early Earth, and tagged them with fluorescent proteins that allowed reactions to be tracked.

As the researchers watched, errors caused DNA self-replication to slow. The model system was only an approximation of early Earth chemistry, but if such pauses existed for RNA, they would have allowed RNA to evolve into complicated forms.

“They’ve gone beyond the paradox,” said Bodo Stern, a Harvard University systems biologist who wasn’t involved in the study. “Whether that’s what happened, we don’t know, but it’s a conceptual leap forward.”

Stalling appears to be a natural function of DNA’s geometry. “Imagine that DNA is a zipper. The next piece is the incoming nucleotide. If the next piece is not exactly aligned with the remainder of the zipper, it will have a hard time getting into position,” Chen said.

According to Hans-Joachim Ziock, a protocellular researcher at Los Alamos National Laboratory, “anything that can help make correct copies would help, so base mismatches slowing the replication process would be beneficial.” But he said that even without an error-stalling function, nucleic acids may eventually have taken higher forms.