The origin of the Toll-like family of receptors pre-dates the evolutionary split between the plant and animal kingdoms. These receptors are remarkably conserved across the taxonomic kingdoms and have fundamental roles in triggering immune responses. How they trigger such responses, and how these mechanisms arose in evolution, is a topic of extensive debate. We postulate a surveillance model: these receptors "keep watch" for both endogenous and exogenous molecules that indicate tissue injury, infection and remodeling. Furthermore, we suggest that the first Toll-like family receptors that arose in evolution might have acted in both development and immunity by recognizing the degradation of endogenous macromolecules.