The moment of birth is the moment we transform from an individual into an entire world. We leave the sterility of the womb, pass through a mother’s vagina, and become lathered in her microbes, taking them into our skin, our mouths, our guts. We begin our life as we will always live it: as a community of trillions, enclosed within a single body.
Microbes help their hosts to build their bodies, digest food, and defend against disease, so animals have evolved a multitude of ways for bestowing these tiny partners onto their offspring. Many insects do so at the earliest possible opportunity, adding bacteria directly to egg cells, so that their young are accompanied by microbes from conception. There is literally no part of their life cycle where they are sterile. Others do so in the womb. The tsetse fly, which spreads sleeping sickness, nourishes its grub inside a bizarrely mammalian uterus and feeds it with a milklike fluid—one that’s laden with microbes.
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Yet other species have ways of provisioning their young as they greet the world. Humans do so automatically. The Japanese stinkbug coats her jelly-bean-like eggs in a bacteria-rich icing so the hatchlings become colonized when they emerge. Koala moms package the bacteria that allow them to digest tough and toxic eucalyptus leaves into a special kind of poop called pap for their joeys to ingest.
And perhaps the strangest technique is used by the beewolf—a powerful, bee-killing wasp that daubs the walls of her infant’s nursery with a living plaster that she squeezes out of her own head. You can learn more about its bizarre life, and the scientist who studies it, in the video below—the third in a series of online films produced by HHMI Tangled Bank Studios, which adapt the stories in my book, I Contain Multitudes.