April 06, 2009
A New Thermodynamic Analysis
Suggests That 10 of Life's 20 Amino Acids Must Me Common
Throughout The Cosmos
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
synthesize 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
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
"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."