Asymmetry of Life

The prevalence in living organisms of certain kinds of molecules over their mirror images may have originated in space

One of the mysteries of living organisms is that many molecules crucial to life are asymmetric. Many of them exist in two forms which are mirror images of each other, but of these two forms only one is used by living organisms. Most amino acids crucial to life are of one type, denoted as the L type, while the other type, D amino acids, are not used in living organisms. These amino acids come together to form proteins vital to the functioning of life; proteins made of L amino acids are called left-handed proteins.

This particular preference for a certain handedness has been a mystery. The two competing explanations are that either this was a result of random selection on earth among equally likely alternatives and once one type prevailed it came to be prevalent in all living organisms, or, as the alternate argument goes, this is a result of pre-selection for L amino acids well before life originated;  that is, for some reason one form of molecules was more prevalent than the other in space and was then seeded on earth.

Now in work published online on the website of The Astrophysical Journal Letters, French researchers have shown that under conditions that duplicate those in space, an excess of one kind of asymmetric molecules can be produced. The researchers worked with substances replicating interstellar and cometary ice and subjected these to ultraviolet light of the kind found in space. When they analysed the resultant mixture, they found that instead of a fifty-fifty mixture, one form of the amino acid alanine exceeded the other by 1.3 per cent, suggesting that the asymmetry of life may be a result of conditions in outer space. This excess, significantly, is comparable to the excess of such molecules found on meteorites. This suggests the origins of asymmetry of molecules in living organisms is not a random process, but is something that originates from the Universe’s laws of physics.

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Hartosh Singh Bal