Microcosmos

One of the quiet revelations of Microcosmos is just how inventive bacteria are compared with everything that came after. Larger organisms reproduce slowly and trade genes only within their own species. Bacteria do neither thing by our rules. They divide in minutes, and they swap genes sideways — passing stretches of DNA between unrelated strains almost like sharing notes. A trait that arises in one lineage can spread across the whole microbial world without waiting for the slow lottery of inheritance.

This is why bacteria are, in the authors' phrase, a kind of planetary web of shared information rather than a collection of separate species. Resistance to a new toxin, the ability to digest a new food, a trick for surviving heat or cold — once discovered anywhere, it can travel everywhere. Margulis and Sagan describe this as a form of global metabolism, a single distributed experiment running continuously for billions of years. Set against it, the evolution of animals looks almost conservative.

Every cell in your body, and in every plant, animal and fungus, is what biologists call eukaryotic: it keeps its DNA packed in a nucleus, and it contains small working parts with jobs to do. Two of those parts are peculiar. Mitochondria, which burn oxygen to power the cell, and — in plants — chloroplasts, which run photosynthesis, both carry their own separate DNA and divide on their own schedule, almost as if they were guests rather than organs.

Margulis's great argument, which she pressed for years against heavy scepticism, is that they were guests. The complex cell did not evolve its machinery from scratch. It assembled it by swallowing other bacteria and failing to digest them. An ancient host cell took in an oxygen-breathing bacterium, and instead of a meal it gained a power plant; the descendants of that captive are the mitochondria in your cells right now. A later host took in a photosynthetic bacterium, and that captive became the chloroplast. Separate living things merged into one, permanently.

Step back from the chemistry and the deep time, and Microcosmos is making one large claim about how evolution actually works. The popular picture — the one that filters down into business books and gym slogans — is of nature as relentless competition, every organism clawing against every other, the strong eating the weak. Margulis and Sagan do not deny that struggle exists. But they argue that the biggest jumps in the history of life came from the opposite impulse: from organisms joining forces rather than defeating one another.

The complex cell is the headline example, but the pattern repeats at every scale. Lichen is a marriage of fungus and alga. Reef-building corals carry photosynthetic partners inside their tissues. Cows and termites digest their food only because of the microbes in their guts; so, to a large degree, do we. A human body contains roughly as many bacterial cells as human ones. On this view, no large organism is really an individual — each is an ecosystem, a negotiated settlement among lives that were once distinct.

Return to those first cells in the scalding early ocean. Nothing about them looks like a beginning in the usual sense — no ambition toward complexity, no ladder to climb. Yet almost everything that followed was built from their inventions and, in the case of our own cells, from their merged bodies. The two billion years in which bacteria ran the planet alone were not a waiting room for the real show. They were the show, and in the deepest sense they still are: the chemistry that keeps the biosphere turning remains theirs.