In 1977, microbiologist Carl Woese at the University of Illinois discovered a new life form, right here on Earth. Not a new species – no, much more momentous than that. Woese had identified a new branch of organisms that would sit alongside bacteria and the plant–animal–fungi branch of eukaryotes. The discovery implied that the ‘Tree of Life’ – the branching representation of how living things have evolved – had not two, but three major limbs or ‘domains’. Freshly minted, Archaea would join Bacteria and Eukarya. Woese's discovery involved a novel investigative technique. He cut up ribosomal RNA from the methane-producing microbe Methanobacterium thermoautothrophicum. The collection of fragments he found were so different from the pieces one got from chopping up bacterial RNA that Woese concluded that methanogens were not bacteria at all. They were something different. On realizing the significance, Woese's colleague recalled that ‘he just went nuts. He ran into my lab and told me we had just discovered a new form of life’ (p. 106).
In Kin, microbiologist John L. Ingraham takes the reader on a tour of twentieth-century microbiology. He is especially interested in how microbial genetics offers information useful for taxonomy. (Although Kin claims to trace Aristotle's ladder of living things through to the modern explication of the ‘Tree of Life’, this is misleading. The book is not about the development of classification systems for living organisms, which it discusses only briefly.) Ingraham is plenty knowledgeable about his topic, having worked in microbiology throughout the period he writes of and having known some of the figures whose work the book covers. The author's occasional personal recollections of microbiologists are enlivening touches.
Ingraham includes some fascinating titbits in the book – the discovery of a new antibiotic, teixobactin, with the unusual ability to block a bacterium's ability to repair its cell walls, is one such story. But the author does not have the easy explanatory style that others have brought to popular histories of biology. Paragraphs are sometimes strung-together dot points; heavy going en masse and confusing to the narrative. The reader feels less like a fellow adventurer in the microbial world than an undergraduate in a lecture.
The first section of the book gives an overview of microbes and the use of genetic tools to investigate relatedness. The structure of this section is awkward. The section about Archaea comes before the identification of Archaea, which in turn comes before pre-Archaea classification schemes. It's hard to convey that identifying Archaea was groundbreaking without having first established the ground. Similarly, the idea that tracers, such as rRNA fragments, can be used to estimate relatedness seems predicated on already having an idea of relatedness to compare with – something that the book does not fully address.
The second section deals with prokaryotes’ ability to acquire genes laterally from their fellow microbes through a process termed ‘transformation’. (Prokaryotes are single-celled organisms whose cell lacks a nucleus.) Genetic inheritance, Ingraham explains, is not confined to the parent-to-child, vertical inheritance we are most familiar with. While some prokaryotes acquire foreign DNA naturally, Ingraham explains that many cells can be forced to transform artificially. Forcing cells to accept foreign DNA is at the core of recombinant DNA technology.
Kin goes on to describe other mechanisms by which cells transfer DNA horizontally, including Lynn Margulis's 1967 theory that animal and plant cells acquired their mitochondria and chloroplasts by engulfing bacteria which could, respectively, metabolize oxygen or photosynthesize. Symbiotically, the cell and its enslaved bacterium evolved to become cell and organelles.
Ingraham raises the issue of whether all this horizontal churning of DNA might throw a spanner in the taxonomic works. Transformation, he explains, occurs most readily between fellow bacteria, but also happens across kingdoms with animals and plants pinching genetic material from bacteria as well. If lifeforms can just swap bits and pieces of DNA, does the whole project of mapping out evolutionary relationships between organisms fall apart? The ‘Tree of Life’ would be a knotted mass of interconnected, DNA-trading species. Ingraham thinks this messy vision is not the case. He feels that horizontal DNA transfer is not widespread enough, nor does it involve enough core cellular functions, to scupper branching classification.
The final part rounds out the story in a more speculative, philosophical mode. These are short discussions on the future potential of microbiology and some of its possibly unanswerable questions. What might that topic du jour, the human body's microbiome, have in store for us? Can microbiology illuminate the origins of life? Kin gestures to tantalizing pathways.
Microbes, as Ingraham reminds us, have remarkable characteristics and their investigation using genetic tools can help establish evolutionary relationships, a project begun with Charles Darwin's Origin of Species. Kin is both an intriguing and a frustrating guide to twentieth-century microbiology.
