The idea that complex biological systems can evolve through a series of simple, random events is not universally accepted. The structure of a vital immune protein shows how such evolution can occur at a molecular level. Before antibodies evolved, primitive multi-cellular organisms devised a general defence system against bacterial and viral invaders called 'innate immunity'. The system has survived in vertebrates with its core components little changed during the intervening 700 million years. A central element of this defence strategy is an activated thioester — a molecular warhead — that is today used only in this setting, perhaps because it is potentially so destructive. The protein C3, a member of a small family of related proteins carrying this warhead, is a large molecule of the 'complement' system, which identifies foreign agents and targets them for destruction. On page 505 of this issue, Gros and colleagues present the atomic-resolution crystal structure of C3, as well as that of an inactivated fragment, C3c.
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