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By Root 1268 0

’s atmosphere was a smoggy mix of molecules, including heat-trapping methane produced by bacteria and archaea. Free oxygen was rare, in part because the molecules rapidly reacted with iron and other elements to form new molecules. Life changed the planet’s chemistry when some bacteria evolved the ability to capture sunlight. They gave off oxygen as waste, and after 200 million years it began to build up in the atmosphere. Unless an organism can protect itself, oxygen can be lethal. Thanks to its atomic structure, oxygen is eager to attack other molecules, wresting away atoms to bond with. The new oxygen-bearing molecules can roam through a cell, wrecking DNA and other molecules they encounter.

For the first billion and a half years of life, the planet had been mercifully free of the oxygen menace. And then, 2.5 billion years ago, oxygen levels rose tenfold. The oxygen revolution may have driven many species extinct, while others found refuge in places where oxygen levels remained low—deep inside mudflats, for example, or at the bottom of the ocean. But some species, including the ancestors of E. coli, adapted. They acquired genes that protected them from oxygen’s toxic effects. Once shielded, their metabolism evolved to take advantage of oxygen, using it to get energy out of their food far more efficiently than before. Today E. coli can still switch back and forth between its ancient oxygen-free metabolism and its newer network, depending on how much oxygen it senses in its environment.

The other major revolution that E. coli’s ancestors experienced was delivered by our own ancestors. Early eukaryotes, biologists suspect, were the predators of the early Earth. They were much like amoebas today, which prowl through soil and water in search of prey they can engulf. Bacteria that could defend themselves against these predators were favored by natural selection. Today bacteria have an impressive range of defenses against amoebas and other eukaryote predators. They can produce toxins that they can inject with microscopic needles into the amoebas. Their mucus-covered biofilms are difficult for predators to penetrate. Even when ingested, bacteria can avoid destruction.

In some cases, bacteria may have turned the tables on their predators. Amoebas today get sick with bacterial infections caused by species that have evolved the ability to infect and thrive inside hosts. Some bacteria are more polite lodgers, providing single-celled protozoans with life-giving biochemistry. Early eukaryotes acquired oxygen-breathing bacteria this way, and those bacteria are still part of our own cells today. Algae acquired photosynthesizing bacteria, and among their descendants are the plants that make the land green. Thanks to these bacterial partners, the continents could begin to support a massive ecosystem, with forests and grasslands and swamps becoming home to animals of all sorts, from insects to mammals.

These animals, the descendants of the predatory eukaryotes that harassed bacteria billions of years earlier, now became a new ecosystem for bacteria to invade. Thousands of species of microbes, including the ancestors of E. coli, adapted to the food-rich realm of the animal gut. They brought with them their abilities to break down organic carbon, communicate with one another, and cooperate. They had come a long way from the common ancestor of all living things. But as they took up residence inside humans and other animals, they had in their own way brought some branches of the tree of life together again.

E. COLI GOES TO COURT

The federal courthouse in Harrisburg, Pennsylvania, is a nondescript box of dark glass. Its judges deal mostly in humdrum conflicts over funeral-parlor regulations, liquor-store licenses, airport parking lots. But in 2005 a surge of people—reporters, photographers, and onlookers—hit the courthouse like a rogue wave. One case had drawn them all: Kitzmiller v. Dover Area School District. Eleven parents from the small town of Dover had taken their local board of education to court. They charged that the board was