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Did the immune system evolve to keep out harmful organisms, or is it like a bouncer at a nightclub, trained to allow the right microbes in and kick the less desirable ones out? In the fifth essay in Science 's series in honor of the Year of Darwin , John Travis explores the evolution of the immune system.

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I t was conceived after a yogurt company in 2007 identified an unexpected defense mechanism that its bacteria use to fight off viruses. A birth announcement came in 2012, followed by crucial first steps in 2013 and a massive growth spurt last year. Now, it has matured into a molecular marvel, and much of the world-not just biologists-is taking notice of the genome-editing method CRISPR, Science's 2015 Breakthrough of the Year. CRISPR has appeared in Breakthrough sections twice before, in 2012 and 2013, each time as a runner-up in combination with other genome-editing techniques. But this is the year it broke away from the pack, revealing its true power in a series of spectacular achievements. Two striking examples-the creation of a long-sought "gene drive" that could eliminate pests or the diseases they carry, and the first deliberate editing of the DNA of human embryos-debuted to headlines and concern. Each an