evolution does not design, it tinkers. It fiddles around with stuff to hand, and as long as it works, [right now] that’s all that matters…If biochemical activity is produced by much of our DNA, it is indistinguishable from random noise.We know that only 5% of the human genome encodes proteins (when Francis Crick was working on the meaning of the genetic code in the 1950s, he assumed that’s all that a gene would ever do). We now know that another 5-10% is regulatory DNA, which produces RNA that regulates the activity of other genes. As to the remaining 85% – around 2.7 billion base pairs – it appears mainly to be ‘junk’, which has no apparent function – if it were deleted, it would not affect the fitness of the organism at all.
Because of factors such as duplication, the variation in genomic size that we see between species resists any overall functional explanation. This is highlighted by what is known jocularly as the onion test: the onion genome contains around 16 billion base pairs, or five times that of a human.