杏吧原创

The Greek philosopher Diogenes, in response to Plato鈥檚 prematurely triumphant definition of Man as a 鈥渇eatherless biped鈥�, presented the latter鈥檚 disciples with a plucked chicken. 鈥淗ere,鈥� Diogenes exclaimed, 鈥渋s Plato鈥檚 man.鈥� Plato modified his definition 鈥� to a 鈥渇eatherless biped with broad nails鈥�.

Veteran science journalist Tim Friend provides a fascinating survey of microbial ecology in his search for extremophiles in the biosphere. But it falls short. Friend is deceived by the modern equivalent of Plato鈥檚 mistake: that the taxonomic group archaea 鈥� archaeobacteria, as many biologists still call it 鈥� is a unique and newly discovered form of life. This notion might provide the dramatic vision needed to launch a book onto the popular science stage, but it is as bogus as featherless bipeds.

In Friend鈥檚 world, the biotechnologists who use venture capital funds to hunt down archaea are the rock stars of science, dripping with wealth and power. The prime example is Ferrari-driving Karl Stetter, co-founder of the biotech company Diversa. Stetter and his peers sear through the molasses of academia, hot on the trail of the secrets of evolutionary history, biological pollution-treatments and life in space.

鈥淭he biologists who hunt down archaea are science鈥檚 rock stars, dripping with wealth鈥�

Friend globetrots and dines with these corporate adventurers with the excitement of a child released for recess. Archaeobacteria often inhabit extreme environments, so Friend peers into the bubbling hot springs of Yellowstone National Park (pictured), and descends to the ocean bottom in the . We share his close-up view of the sunken, half-digested Titanic, from which 鈥渞usticles鈥� spring forth. These icicle-like rocks were formed by microbes that get their carbon from carbon dioxide, using energy released by the oxidation of the wreck鈥檚 iron and manganese.

Friend is an inspired guide, but he is not always a reliable one. For example, he says that life 鈥渞esists entropy鈥�, citing bacterial strains that cooperate to increase the metabolic efficiency of the group. He doesn鈥檛 appreciate that these interactions reflect life鈥檚 tendency not to resist entropy, but to create it. A more efficient microbial consortium produces more entropy as it releases more heat.

Friend also believes that prokaryotes 鈥渄on鈥檛 exist鈥�, and that Carl Woese changed microbial ecology forever in 1977 with his discovery of the archaea. The prevailing five-kingdom classification of life recognised prokaryotes (bacteria) and four kingdoms of eukaryotes: protoctists such as slime moulds and amoebae; fungi; plants and animals. Prokaryotic cells lack nuclei and visible movement inside the cell. Eukaryotic cells evolved when prokaryotes fused as symbionts and generated a jump in complexity. The presence or absence of a cell nucleus therefore reflects life鈥檚 major gap: evolution by accumulation of mutations versus evolution by symbiogenesis.

Instead, Woese proposes just three fundamental domains: eukaryotes, bacteria and archaea. The back flap of Friend鈥檚 book informs us that the third domain, archaea, is 鈥渁 distinct form of life, unlike anything seen on Earth before鈥�. Hardly. Friend follows Woese, Stetter and the others by jettisoning prokaryote as a valid taxon, but this is misinformed.

Woese鈥檚 fantastic contribution to our understanding of life depends mostly on differences in a molecule required for protein manufacture in all cells: 16S rRNA. Though many accept the tree of life based only on such RNA sequences, I do not. The various types of archaea 鈥� methanogens, acid and sulphur-loving thermophiles, and halophiles 鈥� share some 16S rRNA sequences, but no foolproof criterion applies to all. As Friend chronicles, new archaea with signature 16S rRNA turn up in ordinary environments such as soil or the open ocean.

Bacteria swap genes all the time; they pick up useful traits or lose superfluous ones. This ability enables Friend鈥檚 scientific superstars to manipulate these genes. But it also means that the 16S rRNA sequence, unlike the presence of a nucleus, is not an inviolable genetic tracer. It is just one of thousands of gene products to consider in the reconstruction of evolutionary history.

In our new book, Kingdoms and Domains: An illustrated guide to the phyla of life on Earth (Academic Press, New York, 2008), my co-author Michael Chapman and I classify the archaea as a subkingdom within the more inclusive group of prokaryotes. Friend鈥檚 book shows us less a taxonomic breakthrough than an opportunity to refocus public attention on the miscegenous, gene-swapping, microbial phenomenon that has been life on Earth for nearly 4 billion years.