Inside your gut are maybe a hundred trillion cells. The number is an interesting one, because these cells sitting in your digestive tract outnumber the neurons, muscle cells, and other cells that make "you" by ten to one. In other words, by the numbers, your own cells are a machine that exists to keep a huger number of cells alive in your intestines. Among those trillions of cells is a small population of _Escherichia coli_, one of the world's most important and most studied bacteria. They may be tiny, but they are numerous and they are not simple, and the lessons within _Microcosm: E. coli and the New Science of Life_ (Pantheon) by science writer Carl Zimmer are that there is a complex cosmos of activity within _E. coli_, and there are relationships between one _E. coli_ and its fellow _E. coli_ and the other microbes churning in our guts, and there are relationships between _E. coli_ and the bigger animals that carry it. It is all as complicated as can be; we have come a long way in understanding some of these mysteries, but mysteries still abound. Zimmer's wonderful book keeps us from taking these humble bacteria for granted; as products of the same evolutionary processes that produced us, they have much in common with us.
Scientists make _E. coli_ a particular subject of investigation; it was one of the first microbes whose genome was fully mapped (1997). A few strains have toxins, but usually our own _E. coli_ are quietly going about their business and are a help to us. The intricacies of just one cell are astounding. An _E. coli_ has sixty million molecules which have to act just so to keep the bacterium living, and Zimmer examines a few of the intricate feedback systems involved. A team of microbiologists has succeeded in programming a computer with information on 1,260 of its genes and 2,077 of its chemical reactions; a huge program can predict what _E. coli_ will do, for instance, if starved for oxygen, and the model gets it right. But little _E. coli_ has been getting it right for ages. One of its pieces Zimmer pays special attention to is its flagellum, its means of mobility. Zimmer, in several pages devoted to flagella and Intelligent Design, tells again the story of the Dover, Pennsylvania, court decision that Intelligent Design had only religion going for it, not science, and thus could not be taught in public schools. A lawyer at the trial said, "We could probably call this the Bacterial Flagellum Trial", since the flagellum was discussed in detail, and was shown not to be "irreducibly complex", the supposed hallmark of designed systems that cannot be made any simpler and still remain operational. The ID proponents have only an "It's too wonderful not to have a designer" attitude, not experiments or evidence. Zimmer shows how there is within _E. coli_ molecular evidence that flagella are related to other bacterial systems, and that hypotheses built on this evidence show how natural selection was indeed sufficient to build flagella. Scientists can't say for sure that flagella were built in one certain way, but if the proposed steps of building reasonably come from the data, there is no reason to think that a deity somehow took pity on immobile bacteria and miraculously equipped them with motors.
"I look at life through a lens made of _E. coli_," writes Zimmer, and writes convincingly about how biologists are doing the same. Not only was the _E. coli_ genome among the first to be completely deciphered, they have been used to help understand how genes switch on and off. They are a foundation point for the study of molecular and now synthetic biology. They do a lot of the things we do. They sense nutrition molecules and go for them; they sense unattractive chemicals and run from them. They cooperate with other _E. coli_ and have a social life; they are not the loners scientists had originally thought, but can build their own microbial city. They have a type of chemical warfare that they deploy against enemies. They have a sex not in the way we do, but in their own way. They fight viruses and have virus-injected coding on their DNA just as we do. Zimmer frequently refers to the famous remark of biologist Jacques Monod, who said, "What is true for _E. coli_ is true for the elephant." There is hyperbole there, of course, but in one example after another, Zimmer's clear and enthusiastic prose beautifully demonstrates a biological and evolutionary universality.