Here's a time line.
Mark Ridley believes that many of the delays in life's tangled and protracted history are due to the problems of overcoming DNA copying errors. This is what he has to say in the preface to his book "The Cooperative Gene: How Mendel's Demon Explains the Evolution of Complex Beings".
"The DNA in a human being is 6600 million letters long and codes for about thirty thousand genes. In contrast, the DNA of a bacterium is two or three million letters long and codes for two or three thousand genes. Copying mistakes will have become more numerous as the DNA grew longer, for much the same reason as they happen when we are copying written text.Ridley's book is a tribute to how much can be explained by starting with one fundamental engineering-type puzzle - how can you faithfully copy DNA, an essential step in evolutionary replication? - and use it to account for many apparently-diverse phenomena:
"The evolution of complex life required mechanisms to deal with copying mistakes in the DNA. The first mechanisms improved the accuracy of the copying itself. The earliest life forms probably made about one copying mistake in 100 letters, but bacteria had reduced the rate to less than one mistake in 1,000,000,000 letters.
"This huge improvement is due to the use of DNA for the master-copy — it is an impressively error-proof molecule — and a molecular machinery for proofreading and repairing mistakes. But the possibilities for improving the accuracy of copying seem to have been exhausted by the bacterial stage. The basic DNA copying machinery has remained much the same since then, and we make copying mistakes at a similar rate per letter of DNA as bacteria do.
"Our total error rate is much higher, however, because we use so many more letters of DNA code. Between bacteria and us the length of the DNA molecule has increased 1000-fold and the DNA has come to be copied 100 times per generation, against the once per generation of a bacterial cell.
"Our total error rate has gone up 100,000 times, and whereas a bacteria makes a mistake once in every 1000 offspring, we make over 100 mistakes in every offspring. It is something of a paradox how we can persist, while making so many copying mistakes in our DNA. The solution is uncertain, but is probably — sex. Sex can act to concentrate the copying errors in some of a parent's offspring, leaving other offspring relatively error-free. Sexual life forms could evolve to be more complex than clonal life forms."
- the stretched timeline of evolution itself
- the reason for sex
- the strange process of meiosis ( 2 => 4 => 1 chromosome)
- ageing (we can't accurately clone our own DNA over too many cellular generations)
The New York Times reviewer had the same problem that I guess all readers experienced with this book: it's really badly written. Professor Silver's thorough review attempted to be diplomatic:
"The title of Ridley's book suggests an alternative perspective on the gene's role in evolution from that presented by Dawkins in ''The Selfish Gene.'' But Ridley's ideas are not that different, and they've been expressed in more lively prose by others. Like the exodus of Israel from Pharaoh's Egypt, the story of evolution must be told anew to each generation. In both cases it is probably best to rely on earlier texts."Here are some of my own words for Ridley's writing: stodgy, indigestible, use of inappropriate student humour, gratuitous (and adolescent) attacks on the Bible.
It is possible to plough through the dense and indigestible prose, and the persevering reader is rewarded with some truly paradigm-changing insights. Professor Silver is not quite right: Dawkins didn't already write this material - one just wishes he had.