Peculiar symmetry of DNA sequences and evidence suggesting its evolutionary origin in a primeval genetic code

Statistical analysis of the distribution of codons in DNA coding sequences of bacteria or archaea suggests that, at some stage of the prebiotic world, the most successful RNA replicating sequences afforded some tendency toward a weak form of palindromic symmetry, namely complementary symmetry. As a consequence, as soon as the machinery allowing translation into proteins was beginning to settle, we assume that primeval versions of the genetic code essentially consisted of pairs of sense-antisense codons. Present-day DNA sequences display footprints of this early symmetry, provided that statistics are made over coding sequences issued from groups of organisms and not only from the genome of an individual species, These fossil traces are proven to be significant from the statistical point of view. They shed some light onto the possible evolution of the genetic code and set some constraints on the way it had to follow.