Why? Why have animals effectively outsourced parts of their development to other species? Why not just do everything in-house? “I think it’s unavoidable,” says John Rawls, who has worked with germ-free mice and squid. “Microbes are a necessary part of animal life. There’s no getting rid of them.” Remember that animals emerged in a world that had already been teeming with microbes for billions of years. They were the rulers of the planet long before we arrived. And when we did arrive, of course we evolved ways of interacting with the microbes around us. It would be absurd not to, like moving
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