Thursday, 26 September 2013

How Early Cells May Have Got Complex DNA

DNA-grabbing bacteria hint at early phase of evolution - life - 26 September 2013 - New Scientist

How did the simplest cells early in the evolution of life build their DNA?

It has always been assumed that DNA evolved by three separate processes:
  1. DNA transfer between living organisms.
  2. Sexual reproduction where DNA is shuffled with that of a partner to produce descendants with different combinations.
  3. Random mutation with natural selection sieving out the less fit mutations and favouring the more fit ones.

Now we can add a fourth: mopping up pieces of DNA from dead and decaying organisms and possibly viruses and incorporating them into the genome where they will be replicated in future generations.

DNA is notoriously stable as a molecule, which is why it can be recovered from long-dead bodies, dried up smears of body fluids and the partially fossilised remains of Neanderthals, mammoths and ancestral horses. Genetic material would have been more abundant in the environment in which early cells were evolving than in an adolescent boy's bedroom, especially since, prior to the evolution of photosynthesis, there would have been very low levels of free oxygen.

Søren Overballe-Petersen of the Natural History Museum of Denmark in Copenhagen found that when he fed fragments of DNA to a culture of Acinetobacter baylyi they were passively absorbed. They even absorbed fragments of 43,000 year-old mammoth DNA. The fact that this was a passive process suggests it may be a very primitive ability. By contrast, assimilating pieces of DNA passed across from another microorganism takes at least 40 genes.

So we have the possibility that very early cells acquired pieces of DNA from that sloshing about in their environment and that natural selection did the rest, sieving out those new combinations which were less successful at surviving to replicate and allowing through those which gave most descendants. With possibly billions of cells taking part in this process, it would have very quickly led to the accumulation of more and more complex and successful genomes. And of course the most successful organisms left more DNA fragments for others to mop up when they eventually died, so 'success' was not only inherited by descendants but could be picked up by the descendants of less successful organisms.

References:
Michael Marshall, DNA-grabbing bacteria hint at early phase of evolution, New Scientist Magazine issue 2936, 26 September 2013.

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