All life on Earth is based on 20 amino acids, which are directed by DNA to form proteins. It might seem rather remarkable that it takes only 20 to create the diversity of life we see around us, but it turns out that we shouldn't even need this many. In order to make all the proteins we need, life should only require 10 to 13 of the essential amino acids. How, then, did life settle on 20?

"Researchers have been puzzled for decades why evolution has selected these 20 amino acids for genetic encoding," said Bernd Moosmann of the Institute of Pathobiochemistry at Johannes Gutenberg University Mainz, in a recent press release. "The presence of the last and newest seven amino acids is particularly hard to explain, because suitable and functional proteins can be assembled using just the first and oldest 10 to 13 amino acids."

Indeed, this conundrum is considered to be one of the great mysteries of biochemistry, and solving it could help to shed light on the origins of life itself.

Moosmann and fellow researcher Matthias Granold took a novel, unexpected approach to this problem; they ran a comparison of the quantum chemistry of amino acids from space (such as those brought in by meteorites), with all of the amino acids used by life on Earth.

Quantum chemistry is essentially a subfield that takes some of the principles of quantum mechanics (such as describing particles according to probabilistic, wave-like properties), and applies them to the ways that atoms behave in chemical reactions. It sounds convoluted, but this method allowed the researchers to not only look at the ways that amino acids are spatially structured, but also at how their chemical reactivity might impact their roles. And, it turns out, the findings were significant.

"The transition from the dead chemistry out there in space to our own biochemistry here today was marked by an increase in softness and thus an enhanced reactivity of the building blocks," explained Moosmann.

This added property — softness — is the key to why life requires an extra 7 to 10 amino acids. Researchers surmise that in the planet's early days, increases in oxygen levels would have posed a significant risk to evolving life. This is because oxygen promotes the formation of toxic free radicals, which can expose organisms and cells to massive oxidative stress. Conveniently, at least three of the amino acid add-ons that today form the fundamental building blocks for all life — methionine, tryptophan and selenocysteine — are "soft," in that they are more easily repairable after oxidation.

The quantum chemistry calculations revealed for the first time the unique property that these additional amino acids possess that life required.

"With this in view, we could characterize oxygen as the author adding the very final touch to the genetic code," stated Moosmann.

And so, it would seem the mystery is solved. The solution also ties in nicely with how conditions on the early Earth would have impacted early natural selection. It's a legacy that we still carry with us in each of our cells today.

The study was published in the Proceedings of the National Academy of Sciences.