The Tangled Tree: A Radical New History of Life

by David Quammen

A recent estimate suggests that each human body contains about thirty-seven trillion human cells. It also contains about a hundred trillion bacterial cells, for almost a three-to-one ratio of bacterial to human. (Another study proposes a lower ratio, roughly one-to-one, but that still means thirty-seven trillion bacterial cells in your body.) And this doesn’t even count all the nonbacterial microbes—the virus particles, fungal cells, archaea, and other teeny passengers—that routinely reside in our guts, our mouths, our nostrils, our follicles, on our skin, and elsewhere around our bodies. These stowaways may represent more than ten thousand species (or “species,” given the fuzziness of the category as applied to prokaryotes). About a tenth of that diversity, maybe a thousand species, are bacteria living just within the human gut. Because each of our trillions of microbiomic cells is generally much smaller than a human cell, the complete microbiome constitutes only about 1 percent to 3 percent of our body’s mass. In a two-hundred-pound adult, that’s roughly two to six pounds. In volume, maybe three to nine pints. Still, they’re busy and they’re consequential, those few pints of cells.