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from the first members of the species. Scientists have a rough idea of when those earliest E. coli lived. In 1998, Jeffrey Lawrence of the University of Pittsburgh and Howard Ochman of the University of Arizona estimated when the ancestors of E. coli and the ancestors of its close relative Salmonella enterica split off from each other. Lawrence and Ochman tallied the differences in the species’ DNA. When two species branch off from a common ancestor, they acquire mutations at a roughly regular rate. Lawrence and Ochman estimated their common ancestor lived about 140 million years ago. In 2006, Ochman and several other colleagues tackled E. coli’s origins from another direction: they surveyed E. coli strains and estimated when their common ancestor lived. They concluded that the species was already well established 10 million to 30 million years ago. E. coli is much older than the English bog man, in other words, but it is not a living fossil. It is about as primitive as a primate.

E. coli’s ancestors split from those of Salmonella at a time when dinosaurs dominated the land. Pterosaurs flew overhead, along with birds that still had teeth in their beaks and claws on their wings. The typical mammal at the time was a squirrel-like creature. Around 65 million years ago this picture began to change dramatically. Pterosaurs and the big dinosaurs became extinct, probably in part thanks to an asteroid that crashed into the Gulf of Mexico. After the crash, mammals diversified into flying bats, enormous elephant-like browsers, cat-and doglike carnivores, seed-gnawing rodents, tree-scampering primates. Birds took on their modern forms as well. Mammals and birds share more than survival, however. Their ancestors independently evolved the ability to control their body temperature. Their guts became a desirable habitat for bacteria, including the ancestors of E. coli. Warm-blooded animals need to eat a lot of food to fuel their metabolism, and that rich diet can support a menagerie of microbes. The constant warmth of their guts allows the enzymes of microbes to work quickly and efficiently. It may be no coincidence that the rise of E. coli coincides with the rise of its current hosts.

The early E. coli produced the vast diversity of lineages that live inside us today, some harmless, some even beneficial, some that ravage the brain or ruin the kidneys, and some that are adapted to life outside warm bodies altogether. Life has often exploded into this sort of diversity when it has gotten the opportunity. But E. coli’s explosion is different: scientists can dissect it gene by gene.

VENN GENOMES

Two strains, K-12 and O157:H7, are enough to provide a sense of how diverse E. coli is as a species. K-12 is so harmless that scientists make no efforts to protect themselves from it; instead, they have to protect it from fungi and bacteria. If K-12 is a lapdog, O157:H7 is a wolf. It injects molecules into our cells, disrupts our intestines, makes us bleed, loads us with toxins, shuts down our organs, and sometimes kills us. Each microbe relies on a network of genes and proteins to thrive in its particular ecological niche, and those networks are very different from one another. As different as they are, though, K-12 and O157:H7 have a common ancestor, which scientists estimate lived 4.5 million years ago—at a time when our ancestors were upright-walking apes.

In 2001, scientists got their first good look at how a single microbe could give rise to two such different organisms. It was in that year that two teams of scientists—one Japanese, the other American—independently published the complete genome of O157:H7. Scientists could then compare it, gene for gene, with the genome of K-12, which had been published four years earlier. No one could quite predict what would be found.

In the 1970s, scientists had begun comparing small fragments of DNA from different strains of E. coli. The fragments were nearly identical from strain to strain, both in their genetic sequence and in their position on the chromosome. Scientists could even find the corresponding