One of the great outstanding questions in biology involves the evolution of the genetic code and the fact it relies on 20 amino acids. How did this system evolve and why use 20 amino acids and not some other number? Today, Paul Higgs and Ralph Pudritz at McMaster University in Hamilton, Canada, put forward an answer which has profound implications for the nature of life on other planets.
We know that amino acids are common in our solar system and beyond. Various first experiments to recreate the conditions in the Earth’s early atmosphere have produced 10 of the amino acids found in proteins. Curiously, analyses of meteorite samples have found exactly these same 10 amino acids. Various researchers have noted this link but none have explained it.
Now we know why, say Higgs and Pudritz. They have ranked the amino acids found in proteins according to the thermodynamic likelihood of them forming. This turns out to match the observed abundances in meteorites and in early Earth simulations, more or less exactly.
That’s a neat piece of work. They go on to argue that the first genetic codes must have evolved to exploit these 10 prebiotic amino acids. The other amino acids which are all bigger and generally more difficult to synthesise must have been incorporated later. At any rate, Nature had settled on the full 20 we see today by the time the earliest common ancestor of all organisms on the planet first emerged, at least 3.5 billion years ago.
Higgs and Pudritz are not the first to suggest that the first genetic code consisted of 10 prebiotic amino acids but all previous arguments have differed in various ways. What’s impressive about their argument is that it is underpinned by the powerful theoretical machinery of thermodynamics.
The implications of this are huge. Thermodynamic arguments are as valid on Earth as they are in interstellar gas clouds, where evidence of amino acids has already been seen. What’s the betting that these amino acids are the same as the prebiotic 10 that Higgs and Pudritz finger?
These same thermodynamic arguments should also hold on Earth-like planets elsewhere in the cosmos. And if that’s the case, then ET may not be so alien after all, as Higgs and Pudritz imply with the extraordinary conclusion to their paper:
“The combined actions of thermodynamics and subsequent natural selection suggest that the genetic code we observe on the Earth today may have significant features in common with life throughout the cosmos.”
: A thermodynamic Basis for Prebiotic Amino Acid Synthesis and the Nature of the First Genetic Code