Editor's note: This essay, by renowned evolutionary biologist Lynn Margulis, was published in the August 1994 issue of Scientific American with the title, "Sex, Death and Kefir." Margulis died on Tuesday in her home, according to a statement released by the University of Massachusetts at Amherst, where she was a Distinguished University Professor of Geosciences. She is best known for her work on how symbiosis led to the evolution of organelles, which were once independent organisms (she describes her theory in her August 1971 Scientific American article "Symbiosis and Evolution" (pdf), which you can read if your library has an institutional subscription). She was also a major contributor to the Gaia theory, which posits that Earth is a self-regulating complex system, and was once married to astronomer Carl Sagan.
Sex, Death and Kefir
The certainty of death was absent at the origin of life. Unlike humans and other mammals, many organisms do not age and die. The process of programmed, inevitable death evolved only after our symbiotic microbial ancestors, some two billion years ago, became sexual individuals.
Any organism can die because of circumstances beyond its control: the ambience grows too hot, a predator attacks, poison gas permeates. But programmed death happens independently of environmental action: cornstalks topple at the end of the season, or a healthy elephant succumbs at the end of a century. Monthly, in menstruating women, the dead cells of the uterine lining flow through the vagina. Each autumn in the deciduous trees and shrubs of the North Temperate Zone, rows of cells at the base of the leaf stem die.
Unlike animals and plants that grow from embryos and die on schedule, all bacteria and most other microorganisms remain eternally young. These other organisms are protoctists and fungi. Protoctists constitute a diverse group that includes our animal ancestors, as well as seaweeds, ciliates, slime molds, foraminifera, diatoms and many others. Like the fungi (yeasts, molds and mushrooms), protoctists are symbiotic aggregates of nucleated cells that reproduce by cell division. Protoctist and fungal individuals can grow and reproduce without any sexual partners.
But in some protoctists—those that became the ancestors of the fungi, plants and animals—our kind of sex, which involves mating and cell fusion by fertilization, first appeared. I propose that it did so as an accidental consequence of a desperate strategy for survival. Sex began when unfavorable seasonal changes in the environment caused our protoctist predecessors to engage in attempts at cannibalism that were only partially successful. The result was a monster bearing the cells and genes of at least two individuals (as does the fertilized egg today).
The return of more favorable environmental conditions selected for survival those monsters able to regain their simpler, normal identity. To do so, each had to slough off half or more of the "extra" cell remains. Death and the genes that caused death evolved. "Death genes" have now been isolated and their operation studied. Lawrence M. Schwartz, here at the University of Massachusetts, for example, can predict the demise of cells in a laboratory culture to within a few hours when he introduces DNA containing death genes.