I Contain Multitudes

Even when we are alone, we are never alone. We exist in symbiosis

Microbes have opened many doors for animals, allowing them to take up all kinds of peculiar lifestyles that would normally be closed off to them. And when animals share habits, their microbiomes often converge. For example, Knight and his colleagues once showed that ant-eating mammals, including pangolins, armadillos, anteaters, aardvarks, and aardwolves (a type of hyena), all have similar gut microbes, even though they have been evolving independently for around 100 million years

Every organ is also variable in itself. The microbes that live at the start of the small intestine are very different from those in the rectum. Those in dental plaque vary above and below the gum-line. On the skin, microbes in the oily lakes of the face and chest differ from those in the hot and humid jungles of the groin and armpit, or those colonising the dry deserts of the forearms and palms. Speaking of palms, your right hand shares just a sixth of its microbial species with your left hand

Some species are common, but none is everywhere. If there is a core, it exists at the level of functions,not organisms. There are certain jobs, like digesting a certain nutrient or carrying out a specific metabolic trick, that are always filled by somemicrobe – just not always the same one

Islands are where you go if you want to find life at its most outlandish, gaudy, and superlative. Their isolation, restricted boundaries, and constrained size allow evolution to go to town. The patterns of biology resolve into sharper focus more readily than they would do on the extensive, contiguous mainland.

There are fewer than 100 species of bacteria that cause infectious diseases in humans

The variations that exist between body parts dwarf those that exist between people. Put simply, the bacteria on your forearm are more similar to those on my forearm than to those in your mouth.

There are fewer than 100 species of bacteria that cause infectious diseases in humans;8 by contrast, the thousands of species in our guts are mostly harmless. At worst, they are passengers or hitchhikers. At best, they are invaluable parts of our bodies: not takers of life but its guardians.

When animals get sick, we frequently lose our appetite – a sensible tactic that diverts energy from foraging and towards getting better. It also means that our gut microbes experience a temporary famine. Sick mice deal with this problem by releasing emergency rations: a simple sugar called fucose. Gut microbes can snip off this sugar and feed on it, staying alive while they wait for their hosts to resume normal service.40

These interactions matter, because they foster stability. If a single bacterium was too efficient at harvesting glycans, it might eat away the mucus barrier itself, creating openings through which other microbes could enter. But if there are hundreds of competing species, they can all keep each other from gluttonously monopolising the food supply. By offering a wide array of nutrients we feed a wide range of microbes and stabilise our enormous, diverse communities. And those communities, in turn, make it harder for pathogens to invade. By setting the table correctly, we ensure that the right guests turn up to dinner, while gatecrashers are locked out