Studies on the Evolution of Some Disease-Producing Organisms

Abstract

Since little is known about the evolutionary development of most of the organisms producing disease, an attempt is made here to approach the subject from a study of the relation of contemporary parasites to their hosts. The organisms chosen are all arthropod-borne and belong to the rickettsiae, the spirochaetes, and the protozoa; they all represent parasitism of a high degree in which there are no longer any free-living stages. The evidence seems to indicate that parasitism among these groups began in the invertebrates and was secondarily transferred to vertebrates when the invertebrate hosts became blood-sucking. Indications that a given parasitism is older in one host (the invertebrate) than in the other (the vertebrate) are: its lack of pathogenicity in the older host, its relative efficiency of transmission in the older host (e.g., from parent tick to offspring through the eggs of the parent), degree of parallelism between the evolution of the parasite and the host, and degree of parallelism between the life cycles of parasite and host. The author stresses the usefulness of this method of approach to an otherwise very difficult problem.