containing just one 2 0 G E N O M E
gene each, which could be lost or gained quite easily. If this was so, Carl Woese points out, the organism was not yet an enduring entity.
It was a temporary team of genes. The genes that ended up in all of us may therefore have come from lots of different 'species' of creature and it is futile to try to sort them into different lineages. We are descended not from one ancestral Luca, but from the whole community of genetic organisms. Life, says Woese, has a physical history, but not a genealogical one.10
You can look on such a conclusion as a fuzzy piece of comforting, holistic, communitarian philosophy - we are all descended from society, not from an individual species - or you can see it as the ultimate proof of the theory of the selfish gene: in those days, even more than today, the war was carried on between genes, using organisms as temporary chariots and forming only transient alliances; today it is more of a team game. Take your pick.
Even if there were lots of Lucas, we can still speculate about where they lived and what they did for a living. This is where the second problem with the thermophilic bacteria arises. Thanks to some brilliant detective work by three New Zealanders published in 1998, we can suddenly glimpse the possibility that the tree of life, as it appears in virtually every textbook, may be upside down. Those books assert that the first creatures were like bacteria, simple cells with single copies
The very first modern organisms were not like bacteria; they did not live in hot springs or deep-sea volcanic vents. They were much more like protozoa: with genomes fragmented into several linear chromosomes rather than one circular one, and 'polyploid' — that is, with several spare copies of every gene
As Patrick Forterre has long argued, it now looks as if bacteria came later, highly specialised and simplified descendants of the Lucas, long after the invention of the DNA-protein world. Their trick was to drop much of the equipment of the R N A world specifically to L I F E 2 1
enable them to live in hot places. It is we that have retained the primitive molecular features of the Lucas in our cells; bacteria are much more 'highly evolved' than we are.
This strange tale is supported by the existence of molecular 'fossils' - little bits of R N A that hang about inside the nucleus of your cells doing unnecessary things such as splicing themselves out of genes: guide R N A , vault R N A , small nuclear R N A , small nucleolar R N A , self-splicing introns. Bacteria have none of these, and it is more parsimonious to believe that they dropped them rather than we invented them. (Science, perhaps surprisingly, is supposed to treat simple explanations as more probable than complex ones unless given reason to think otherwise; the principle is known in logic as Occam's razor.) Bacteria dropped the old R N A s when they invaded hot places like hot springs or subterranean rocks where temperatures can reach 170 °C — to minimise mistakes caused by heat, it paid to simplify the machinery. Having dropped the R N A s , bacteria found their new streamlined cellular machinery made them good at competing in niches where speed of reproduction was an advantage - such as parasitic and scavenging niches. We retained those old R N A s , relics of machines long superseded, but never entirely thrown away.
Unlike the massively competitive world of bacteria, we — that is all animals, plants and fungi - never came under such fierce competition to be quick and simple. We put a premium instead on being complicated, in having as many genes as possible, rather than a streamlined machine for using them.11
The three-letter words of the genetic code are the same in every creature. C G A means arginine and G C G means alanine - in bats, in beetles, in beech trees, in bacteria. They even mean the same in the misleadingly named archaebacteria living at boiling temperatures in sulphurous springs thousands of feet beneath the surface of the Atlantic ocean or in those microscopic capsules of deviousness called viruses. Wherever you go in the world, whatever animal, plant, bug or blob you look at, if it is alive, it will use the same dictionary and know the same code. All life is one. The genetic code, bar a few tiny local aberrations, mostly for unexplained reasons in the ciliate 2 2 G E N O M E
protozoa, is the same in every creature. We all use exactly the same language.
This means - and religious people might find this a useful argument - that there was only one creation, one single event when life was born. Of course, that life might have been born on a different planet and seeded here by spacecraft, or there might even have been thousands of kinds of life at first, but only Luca survived in the ruthless free-for-all of the primeval soup. But until the genetic code was cracked in the 1960s, we did not know what we now know: that all life is one; seaweed is your distant cousin and anthrax one of your advanced relatives. The unity of life is an empirical fact.
Erasmus Darwin was outrageously close to the mark: 'One and the same kind of living filaments has been the cause of all organic life.'
In this way simple truths can be read from the book that is the genome: the unity of all life, the primacy of R N A , the chemistry of the very earliest life on the planet, the fact that large, single-celled creatures were probably the ancestors of bacteria, not vice versa.
We have no fossil record of the way life was four billion years ago.
We have only this great book of life, the genome. The genes in the cells of your little finger are the direct descendants of the first replicator molecules; through an unbroken chain of tens of billions of copyings, they come to us today still bearing a digital message that has traces of those earliest struggles of life. If the human genome can tell us things about what happened in the primeval soup, how much more can it tell us about what else happened during the succeeding four million millennia. It is a record of our history written in the code for a working machine.
C H R O M O S O M E 2
S p e c i e s
Man with all his noble qualities still bears in his bodily frame the indelible stamp of his lowly origin.
Sometimes the obvious can stare you in the face. Until 1955, it was agreed that human beings had twenty-four pairs of chromosomes.
It was just one of those facts that everybody knew was right. They knew it was right because in 1921 a Texan named Theophilus Painter had sliced thin sections off the testicles of two black men and one white man castrated for insanity and 'self-abuse', fixed the slices in chemicals and examined them under the microscope. Painter tried to count the tangled mass of unpaired chromosomes he could see in the spermatocytes of the unfortunate men, and arrived at the figure of twenty-four. 'I feel confident that this is correct,' he said.
