Arrival of the Fittest: Solving Evolution's Greatest Puzzle by Andreas Wagner

of organisms alive today. Some of these texts appeared less than a hundred years ago—a mere moment in evolutionary time.
    Consider pentachlorophenol, a nasty molecule that humans first produced in the 1930s. It is used in antifouling paint to coat ships’ hulls, and also as an insecticide, fungicide, and disinfectant—in short, to kill life. Pentachlorophenol also damages our kidneys, blood, and nervous system, and it causes cancer. But despite its noxious nature, life has found ways not only to tolerate pentachlorophenol but to thrive on it. The aptly named bacterium
Sphingobium chlorophenolicum
can extract both energy and carbon from it, using pentachlorophenol as its only food source. To do so, its genome encodes four enzyme-catalyzed reactions that convert pentachlorophenol into molecules that are as digestible as glucose—the equivalent of transforming a chemical weapon into a chocolate bar. 3
    The combination of these reactions is unique to
S. chlorophenolicum,
but the reactions themselves are not. Each of them occurs in hundreds if not thousands of other organisms. Two of them help recycle superfluous amino acids in some bacteria, whereas the other two disarm toxic molecules produced by some fungi and insects—molecules that happen to resemble pentachlorophenol. 4 Like a garage mechanic building a sprinkler system out of an alarm clock, a bicycle pump, and some PVC pipe, evolution has created in
S. chlorophenolicum
a new arrangement of chemical reactions catalyzed by enzymes that individually exist in other organisms. In other words, metabolic innovation is combinatorial.
    Innovations that allow organisms to feed on highly toxic, man-made molecules are not rare. The bacterium
Burkholderia xenovorans
happily
feeds on the now outlawed polychlorinated biphenyls, which were widely used in making plastics and in the electrical industry. 5 Other bacteria readily digest chlorobenzene, a toxic organic solvent used in chemical laboratories. 6 And even more striking are the bacteria that feed on the very antibiotics designed to kill them. 7 Some of these antibiotics are man-made, so bacteria did not encounter them until recently.
    Just as nature can convert poisons into food, it also came up with ingenious ways of managing its waste. Ammonia (NH 3 ), for example, isn’t just the gas in household cleaners with the sharp, unpleasant odor that makes your eyes burn, but a highly toxic waste product of animal metabolism. Because ammonia dissolves in water, fish can just excrete it into the water surrounding them and forget about it—the fish equivalent of peeing in the swimming pool. But when animals first conquered land more than three hundred million years ago, they did not have this luxury. They needed to prevent toxic ammonia gas from poisoning their blood.
    The solution lay in a metabolic text that contains the instructions for converting ammonia into the less toxic molecule urea, which we secrete to this day in our urine. This metabolic innovation involves five common chemical reactions, each one independently useful to organisms long before the need to detoxify ammonia appeared. 8
    When exactly this innovation appeared is unknown, but clues are easy to find. Even though modern
teleosts
—bony fish—have no need to detoxify ammonia,
their
ancestors already harbored a chemical blueprint for making urea, still seen in cartilaginous fish like sharks and rays that swam through the oceans long before modern fish appeared. However, the title character of
Jaws
uses urea for a different purpose than the humans hunting it—for nitrogen storage, buoyancy, or as a counterweight to the salt in seawater. (You might think that the DNA of bony fish should contain some remnant of this innovation, if it had already originated in their distant ancestors. And that’s indeed the case: The text for the urea cycle still exists in bony fish, even though they rarely express its chemical meaning. They are a bit like adults who learned a