Carl Zimmer has written another good post on his blog about the evolution of the immune system: The Whale and the Antibody.
You can find this same remarkable system in humans, albatrosses, rattlesnakes, bullfrogs, and all other land vertebrates. You can also find it in most fish, from salmon to hammerhead sharks to sea horses. There are some variations from species to species, but they’ve all got B cells, T cells, antibodies, thymuses, and the other essential components. But you won’t find it in beetles, earthworms, dragonflies, or any other invertebrate on land. Nor will you find it in starfish, squid, lobsters, or lampreys in the water. All these other animals rely instead on rudimentary immune systems that cannot learn.
For those who reject evolution, this sort of pattern tells them nothing. Like everything else in nature, they can only wave their hands and declare it the inscrutable work of a designer (lower case d or upper case D as they are so inclined on a given day). But immunologists and other scientists who actually want to learn something about the immune system find this view useless. Instead, they look at how animals with an antibody-based immune system are related to one another. And what they find is both straightforward and astonishing. All of the living animals with an antibody-based immune system descend from a common ancestor, and none of the descendants of that common ancestor lack it. That means that the antibody-based immune system evolved once, about 470 million years ago.