Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [74]
But enough about you. A different route travels from the common ancestor to E. coli. The journey is just as long and no less interesting.
The last common ancestor of all living things was probably much simpler than E. coli. While each species today carries some unique genes, it also shares genes found in all other species. These universal genes probably are the legacy of the last common ancestor. A simple search for universal genes brings up a pretty short list, about 200 genes long. The common ancestor probably had a bigger genome, because many genes have been lost over the history of life. Christos Ouzounis and his colleagues at the European Bioinformatics Institute in Cambridge estimate that its full genome contained somewhere between 1,000 and 1,500 genes. Even if Ouzounis is right, however, the last common ancestor of all living things had only a third or a quarter of the genes that a typical strain of E. coli has today.
That last common ancestor did not have early Earth all to itself. It shared the planet with an uncountable number of other microbes. Over time the other branches on the tree of life became extinct while our own survived. The world on which these early microbes lived was profoundly different from our own. Four billion years ago, Earth was regularly devastated by gigantic asteroids and miniature planets. Some of the impacts may have boiled off the oceans. As the water slowly fell back to Earth and grew into seas again, life may have found refuge in cracks in the ocean floor. It may be no coincidence that on the tree of life some of the deepest branches belong to heat-loving species that live in undersea hydrothermal vents.
Once Earth became more habitable, the descendants of the common ancestor fanned out. They spread across the seafloor, growing into lush microbial mats and reefs. Continents swelled up, and early organisms moved ashore, forming crusts and varnishes. Along the way they evolved new ways to feed and grow. Some bacteria and archaea consumed carbon dioxide and used iron or other chemicals from deep-sea vents as a source of energy. They built up a supply of organic carbon that other microbes began to feed on.
E. coli may descend from those ancient scroungers. Its ancestors certainly could not have been living inside humans 3 billion years ago, or inside any other animal for that matter. Some of E. coli’s closest living relatives (a group collectively known as gamma-proteobacteria) offer some clues to what E. coli’s ancestors might have been doing then. Some eat oil that oozes from cracks in the seafloor. Others live on the sides of undersea volcanoes, where they glue themselves to passing bits of proteins. E. coli may have acquired its metabolism from such carbon-scrounging ancestors.
E. coli’s complex social life—forming biofilms, waging wars with colicins, and so on—may have also had its origins in free-living ancestors in the ocean. Aquatic microbes today have intensely social lives, living mainly in biofilms rather than floating alone as individuals.
About 2.5 billion years ago, the ancestors of E. coli were rocked by a planetwide catastrophe: oxygen began to build up in the atmosphere. To us oxygen is essential to life, but on the early Earth it was poison. Initially the planet