Programmed cell death (PCD) or apoptosis, an active process of cell death, plays a central role in normal tissue development and organogenesis, as well as in the pathogenesis of different diseases. Although it occurs in diverse cells and tissues under the influence of a remarkable variety of inducing agents, the resultant ultrastructural and biochemical changes are extremely monotonous, indicating the existence of a common biological mechanism underlying its occurrence. It is generally accepted that a developmental program leading to cell death cannot be advantageous to unicellular organisms and that PCD appeared in evolution to fulfill the organizational needs of multicellular life. However, the recent description of apoptotic death occurring in three different species of pathogenic kinetoplastids suggests that the evolutionary origin of PCD precedes the appearence of multicellular organisms. The present study proposes that a population of pathogenic Trypanosomatids is socially organized and that PCD is a prerequisite for this organization and for the fulfillment of the demands of a heteroxenic lifestyle. This proposal includes possible roles for PCD in the development of the parasite in the insect vector and/or in its mammalian host and suggests experimental strategies to localize the evolutionary origin of PCD within the kinetoplastids.