When atoms were first proved to exist (a mere century ago), they were thought to be made only of electrons and protons. That explained a lot, but it did not quite square with other observations. Then, in 1932, James Chadwick discovered the neutron. Suddenly everything made sense-so much sense that it took only another 13 years to build an atomic bomb.

It is probably no exaggeration to say that biology is now undergoing its "neutron moment". For more than half a century the fundamental story of living things has been a tale of the interplay between genes, in the form of DNA, and proteins, which the genes encode and which do the donkey work of keeping living organisms living. The past couple of years, however, have seen the rise and rise of a third type of molecule, called RNA.

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Philosophers of science love this sort of thing. They refer to it as a paradigm shift. Living through such a shift is confusing for the scientists involved, and this one is no exception. But when it is over, it is likely to have changed people's views about how cells regulate themselves, how life becomes more complex, how certain mysterious diseases develop and even how the process of evolution operates.

Another consequence of RNA's rise to prominence is that researchers have a new source of ideas about how diseases might one day be treated.  The main hunt for new drugs centres on a technology called RNA interference, or RNAi.