THE BIOLOGICAL BASIS of ageing can be traced to its beginning four billion years ago in a gene circuit in the first life forms that provided a survival advantage by turning off cellular reproduction while DNA was being repaired. One gene turns off reproduction; another makes a protein that turns off the first gene when it is safe to reproduce. When DNA breaks, the protein made by the second gene leaves to repair the DNA. As a result, the first gene is turned on to halt reproduction until that repair is complete.

The fossil record in our genes goes a long way to proving that every living thing that shares this planet with us still carries this ancient genetic survival circuit, in more or less the same basic form. It is there in every plant. It is there in every fungus. It is there in every animal.