With swine flu circulating the globe, it’s appropriate that May’s Origins essay is “On the Origin of the Immune System.” Immunology is the study of how we and other animals defend ourselves against pathogenic microorganisms—bacteria, viruses, and parasites, for example—and this battle goes back to the beginning of evolution. The first multicellular creatures must have had to learn how to be nice to their own cells yet attack any invading cell trying to exploit their resources. Indeed, when biologists look at sponges and other “simple” creatures at the base of the evolutionary tree, they see many of the same microbial defenses that we and other complex animals use, which suggests that at least some form of immunity arose very quickly in the evolution of animals. But those ancient defenses only constitute what scientists call the innate immune response, an all-out molecular and cellular assault on infected tissue. Most vertebrates have a second level of defense, the adaptive immune response, that targets, and remembers, specific microbes. It’s this adaptive immune response, dependent on white blood cells called B and T cells, that physicians elicit when they vaccinate a person against a virus, for example. In a scientific detective story that has played out over the past few decades, researchers have shown how this adaptive immune response arose after innate immunity, and they have teased out the details of the fortuitous event, a random DNA insertion in an opportune spot, that was the key to its birth. The research on this “big bang of immunology” even played a key role in a 2005 trial pitting scientists and educators against those doubting evolution and seeking to diminish its teaching in school systems in the United States.