Free Power, Sex, Suicide: Mitochondria and the Meaning of Life by Nick Lane Page B

in profound ways from bacteria, ways that go well beyond the construction of their cell walls. We now know that about 30 per cent of archaeal genes are unique to the group. These unique genes code for forms of energy metabolism (such as the generation of methane gas) and cell structures (such as membrane lipids) that are not found in any other bacteria. The differences are important enough for most scientists to regard the archaea as a separate ‘domain’ of life. This means that we now classify all living things into three great domains—the bacteria, the archaea, and the eukaryotes (which, as we have seen, includes all multicellular plants, animals, and fungi). The bacteria and the archaea are both prokaryotic (lacking a cell nucleus) while the eukaryotes all do have a nucleus.
    Despite their love of extreme environments and unique characteristics, the archaea also share a mosaic of traits with both bacteria and eukaryotes. I say ‘mosaic’ advisedly, as many of these traits are self-contained modules, encoded by groups of genes that work together as a unit (such as the genes for protein synthesis, or for energy metabolism). These individual modules fit together like the pieces of a mosaic, to construct the overall pattern of an organism. In the case of the archaea, some pieces are similar to those used by eukaryotes, while others are more reminiscent of bacteria. It is almost as if they were selected at random from a lucky dip of cell characteristics. So, for example, even though the archaea are prokaryotes, easily mistaken for bacteria when viewed down the microscope, some of them nonetheless wrap their chromosome in histone proteins, in a very similar manner to eukaryotes.
    The parallels between archaea and eukaryotes go further. The presence of histones means that archaeal DNA is not easily accessible, so, like the eukaryotes, archaea need complicated transcription factors to copy or to transcribe their DNA (reading off the genetic code to construct a protein). The detailedmechanism of genetic transcription in the archaea parallels that in eukaryotes, albeit in a simpler fashion. There are also similarities in the way that the two groups construct their proteins. As we saw in the Introduction, all cells assemble their proteins using the tiny molecular factories called ribosomes. The ribosomes are broadly similar in all three domains of life, implying that they share a common ancestry, but they differ in many details. Interestingly, there are more differences between the bacterial and archaeal ribosomes than there are between archaeal and eukaryotic ribosomes. For example, toxins like diphtheria toxin block protein assembly on ribosomes in both the archaea and eukaryotes, but not in bacteria. Antibiotics like chloramphenicol, streptomycin, and kanamycin block protein synthesis in the bacteria, but not in the archaea or eukaryotes. These patterns are explained by differences in the way that protein synthesis is initiated, and in the detailed structure of the ribosome factories themselves. The ribosomes of eukaryotes and archaea have more in common with each other than either do with bacteria.
    All this means the archaea are about as close to a missing link between the bacteria and the eukaryotes as we are ever likely to find. The archaea and the eukaryotes probably share a relatively recent common ancestor, and are best seen as ‘sister’ groups. This seems to back up Cavalier-Smith’s view that the loss of the cell wall, possibly in the common ancestor of the archaea and the eukaryotes, was the catastrophic step that later propelled the evolution of eukaryotes. The earliest eukaryotes may have looked a little like modern archaea. Intriguingly, though, no archaea ever learnt to change shape to scavenge a living by engulfing food in the eukaryotic fashion. On the contrary, instead of developing a flexible cytoskeleton as the eukaryotes did, the archaea developed quite a stiff membrane system, and remained nearly as