While we don't know precisely how life got its start on Earth, most scientists accept what's called the "RNA world" hypothesis, which posits that RNA molecules kickstarted the process by acting as both catalysts and genetic material. This does, however, leave an awkward question: how did proteins get involved?

Proteins are long chains of chemicals called amino acids, and it's not clear how or why these molecules would have become associated with RNAs. Now, some research published in Angewandte Chemie is suggesting that chains of amino acids could have formed spontaneously, driven by nothing more than a cyclical dry period.

Amino acids are just what their name implies: they have an acidic group on one side of the molecule and a nitrogen-containing amino group on the other. It's possible to link these two groups together in a reaction that releases a water molecule. Once linked, they're stable, but the reaction that links them isn't energetically favorable. So, people pondering the origin of life have wondered whether there was a pathway in which the bond could form spontaneously.

One possible method for getting it to form would be for a solution of amino acids to dry out. As the solution becomes ever more concentrated, a reaction that produces a water molecule could become favorable even if it's expensive in purely energetic terms. But so far, the reaction conditions to get this to work have been rather extreme.

The researchers involved in the new work figured that the amino acids might be alone in these puddles as they dried out. A related chemical, called lactic acid, is thought to have been present on the early Earth. And that can undergo a polymerization reaction that releases water, very similar to that of the amino acids. The difference is that this reaction is more energetically favorable. Simply putting lactic acid on its own through four wet/dry cycles allowed chains of four or more units to form, all connected by what's called an ester bond.

The impressive part is what happens when you mix lactic acid with an amino acid: you start forming mixed chains of molecules. While the amino acid won't normally participate in these reactions, they can break the ester bond, essentially replacing one of the lactic acids. So, a few wet/dry cycles produces a chain that's a mix of lactic acids and amino acids.

But this reaction is like a one-way street. Since the bond between amino acids is more stable, the percentage of amino acid present in these chains keeps going up with the number of wet/dry cycles. By 20 cycles, 65 percent of the chains in the reaction mix were purely amino acid. These chains were still shorter than just about every protein that life uses. But, if formed from a mixture of amino acids, they could still be capable of some pretty complex chemistry.

As with all origin-of-life research, it's important to note that this doesn't mean that it's what actually happened. What it can tell us is what sort of reactions could have occurred in an environment like the early Earth's, and thus which potential paths for life getting its start are more likely.

Angewandte Chemie, 2015. DOI: 10.1002/ange.201503792 (About DOIs).