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to direct
E. coli
slowly but reliably toward the serine. Once it gets to the source, it stays there by switching back to its aimless wandering.
    Scientists began piecing together
E. coli’
s system of sensing and swimming in the 1960s. They chose
E. coli’
s system because they thought it would be easy. They could take advantage of the long tradition of using mutant
E. coli
to study how proteins work. And once they had solved
E. coli’
s information processors, they would be able to take what they had learned and apply it to more complex processors, including our own brains. Forty years later they understand
E. coli’
s signaling system more thoroughly than that of any other species. Some parts of
E. coli’
s system turned out to be simple after all.
E. coli
does not have to compute barrel rolls or spiral dives. Its swim-and-tumble strategy works very well. Every
E. coli
may not get exactly where it needs to go, but many of them will. They will be able to survive and reproduce and pass the run-and-tumble strategy on to their offspring. That is all the success a microbe needs.
    Yet in some important ways,
E. coli’
s navigation defies understanding. Its microbial tongue can detect astonishingly tiny changes in the concentration of molecules it cares about, down to one part in a thousand. The microbe is able to amplify these faint signals in a way that scientists have not yet discovered. It’s possible that
E. coli’
s receptors are working together. As one receptor twists, it causes neighboring receptors to twist as well.
E. coli
may even be able to integrate different kinds of information at the same time—oxygen climbing, nickel falling, glucose wafting by. Its array of receptors may turn out to be far more than just a microbial tongue. It may be more like a brain.
    THE MYTH OF THE TANGLED SPAGHETTI
    E. coli’
s brainy tongue does not fit well into the traditional picture of bacteria as primitive, simple creatures. Well into the twentieth century, bacteria remained saddled with a reputation as relics of life’s earliest stages. They were supposedly nothing more than bags of enzymes with some loose DNA tossed in like a bowl of tangled spaghetti. “Higher” organisms, on the other hand—including animals, plants, fungi—were seen as having marvelously organized cells. They all keep their DNA neatly wound up around spool-shaped proteins and bundled together into chromosomes. The chromosomes are tucked into a nucleus. The cells have other compartments, in which they carry out other jobs, such as generating energy or putting the finishing touches on proteins. The cells themselves have structure, thanks to a skeletal network of fibers crisscrossing their girth.
    The contrast between these two kinds of cells—sloppy and neat—seemed so stark in the mid-1900s that scientists used it to divide all of life into two great groups. All species that carried a nucleus were eukaryotes, meaning “true kernels” in Greek. All other species—including
E. coli
—were now prokaryotes. Before the kernel there were prokaryotes, primitive and disorganized. Only later did eukaryotes evolve, bringing order to the world.
    There’s a kernel of truth to this story. The last common ancestor of all living things almost certainly didn’t have a nucleus. It probably looked vaguely like today’s prokaryotes. Eukaryotes split off from prokaryotes more than 3 billion years ago, and only later did they acquire a full-fledged nucleus and other distinctive features. But it is all too easy to see more differences between prokaryotes and eukaryotes than actually exist. The organization of eukaryotes jumps out at the eye. It is easy to see the chromosomes in a human cell, the intricately folded Golgi apparatus, the sausage-shaped mitochondria. The geography is obvious. But prokaryotes, it turns out, have a geography as well. They keep their molecules carefully organized, but scientists have only recently begun to discover the keys to that