The term "primordial soup" was coined nearly 100 years ago to describe the mysterious chemical broth that was home to Earth's very first self-replicating organisms. Ever since, scientists have been trying to cook that soup up in a lab, searching for the exact combination of compounds that gave rise to life as we know it some 4 billion years ago.

It's not a simple recipe. But researchers at Georgia Tech may have identified one key ingredient: thickener.

In a study published Monday in the journal Nature Chemistry, scientists describe how dissolving strands of genetic material in a thick, sludgy solution keeps them moving slowly, giving them time to start the replication process.

"Chemical evolution in the origins of life is a field that definitely has more questions than answers," said doctoral student Christine He, a lead author of the study. But the viscous soup could be one contribution to the answer column — and a pretty "simple and elegant" one at that, He said.

According to biochemist Nicholas Hud, one of He's advisers at Georgia Tech and a co-author of the study, there are two big questions about how life got started on Earth. First, how did long nucleic acids such as RNA and DNA first form? And second, how did they manage to reproduce themselves?

But each of those questions unpacks into several smaller quandaries, Hud explained. "You take a question like 'How do you replicate?' and you have to look at the different steps that are necessary to replicating a nucleic acid." (Scientists think that self-replicating nucleic acids such as RNA and DNA are what eventually evolved into cells). 

The paper by He and Hud attempts to solve one of these smaller problems. When cells reproduce in organisms today, specialized proteins called enzymes are used to tease apart the DNA double helix, assemble a matching string of nucleotides for each strand, and stitch together the copies to produce two new, identical sets of DNA.

But in a prebiotic world, there would have been no enzymes around to do all this work. Instead, the environmental conditions had to be just right to allow each step to occur spontaneously.

Hud thinks that energy from the sun would have heated the primordial soup enough to break the bonds between the two strands of the DNA double helix — something he was able to replicate in the lab using DNA from a bacterium and a polymerase chain reaction, or PCR, machine.

But in a watery "soup," those strands snapped back together as soon as the solution cooled down, leaving no time for making copies.

"We overcame that challenge using a really simple and elegant idea," He said. They just made the solution thicker.

Using a combination of glycerol and choline chloride, she and her team produced a soup slightly more viscous than maple syrup. In those conditions, the long nucleic-acid strings that make up DNA moved much more sluggishly. Before each could reunite with its other half, smaller, more nimble nucleotides (the sub-units of DNA) were able to assemble alongside the longer strands. Soon a new string of genetic material formed to match half of the old one — the first step in DNA replication.

He and Hud did not achieve true self-replication; they needed an enzyme borrowed from a cell to stitch the nucleotides together into a complimentary strand.

"The way you can think about it is that we've figured out the mechanical step of separating the strands and lining them up, and now we need the chemical one to make those bonds," Hud said. But, like everything in origins-of-life research, "the chemistry of how do you make that bond in a prebiotic context is really complicated."

Niles Lehman, a chemist at Portland State University in Oregon and the editor of the Journal of Molecular Evolution, called the paper a "significant advance in prebiotic chemistry." It expands the range of chemical reactions that could have taken place on early Earth, he wrote in an email.

The solution the Georgia Tech team used to model the primordial soup might not be a perfect reproduction of the conditions that existed on early Earth; scientists aren't sure whether choline chloride, an organic salt, could have existed before life. But He said that she and her team tested several other viscous combinations and that the experiment worked with all of them.

"We are using these specific molecules in this paper," she said, "but really we want to present a really general method for overcoming this problem."

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