The Science of Discworld IV Judgement Da - By Terry Pratchett, Ian Stewart Page 0,61

though not too often, can be creative, in the context of a sufficiently rich system.

The RNA world is not the only game in town. The latest proposals for the origin of life hinge upon viruses. Viruses are long DNA or RNA chains, usually surrounded by a protein coat that contrives to inject them into other organisms, especially bacteria and animal and plant cells. Most viruses rely on the DNA/RNA copying machinery of the organism they infect to replicate them. Then the new copies are usually sprayed out into the environment when that cell, or the organism, dies.

Since the work of Carl Woese in 1977, taxonomists – scientists who classify life into its innumerable related forms – have recognised three basic kinds of life form, the largest and earliest branches of the tree of life. These ‘domains’ comprise bacteria, archaea and eukaryotes. Creatures in the first two domains are superficially similar, being micro-organisms, but each domain had a very distinctive evolutionary history. Archaea may trace back to the earliest organisms of them all; many live in strange and unusual environments: very hot, very cold, lots of salt. Bacteria, you know about. Both types of organism are prokaryotes, meaning that their cells do not clump their genetic material together inside a nucleus, but string it from the cell wall, or let it float around as closed loops called plasmids.

The third domain, eukaryotes, is characterised by having cells with nuclei. It includes all complex ‘multicellular’ creatures, from insects and worms up to elephants and whales. And humans, of course. It also includes many single-celled organisms. RNA sequences imply that the first big split in the tree of life occurred when bacteria branched away from ancestral archaea. Then that branch split into archaea and eukaryotes. So we are more closely related to archaea than we are to bacteria.

Viruses are not part of that scheme, and it is controversial whether they are a form of life because most of them can’t reproduce unaided. It used to be thought that viruses had two different origins. Some were wild genes that escaped from their genomes and made a living by parasitising other creatures and hijacking their gene-copying equipment. The others were desperately reduced bacteria or archaea; in fact, they were reduced so far in their pursuit of a parasitic lifestyle that all they had left were their genes. Occasionally it was supposed by lay people, physicists or maverick biologists – who should have known better – that, being so simple, they were relics of ancient life. This incorrect line of thinking stems from the same, mistaken, principle as the one that considers Amoeba to be ancestral because it looks simple. Actually, there are many kinds of amoeba, and some have 240 chromosomes, bodies in the cell that contain the genes. We have a mere 46 chromosomes. So in that sense an amoeba is more complex than we are. Why so many? Because it takes a lot of organisation, in a very small space, to get all of an amoeba’s functions to work.

Brüssow, in 2009, wrote a review called ‘The not so universal tree of life or the place of viruses in the living world’.fn2 In it he pointed out that Darwin’s tree of life, a beautiful idea that derives from a sketch in The Origin of Species and has become iconic, gets very scrambled around its roots because of a process called horizontal gene transfer. Bacteria, archaea and viruses swap genes with gay abandon, and they can also insert them into the genomes of higher animals, or cut them out. So a gene in one type of bacterium might have come from another type of bacterium altogether, or from an archaean, or even from an animal or a plant.

The major agents of this swapping are viruses, and there are lots of them on the planet, probably ten times as many virus particles as all other forms of life added together. Now, it might appear that with all this swapping, it would be virtually impossible to work out lineages, the heredity of individual bacteria. Even more so, it would seem impossible to work out the lineages of the viruses doing the swapping. Surprisingly, this is not the case; well, not altogether. There are clues in the order in which specific genes appear in many viruses, and there are useful clues as to which organisms the viruses parasitise. Some parasitise both bacteria and archaeans, suggesting strongly that they have been doing so since before these groups diverged. Moreover, these