Lynn Margulis | Evolution biologist, 73

Lynn Margulis, 73, a biologist whose work on the origin of cells helped transform the study of evolution, died Tuesday at her home in Amherst, Mass.

She died five days after suffering a hemorrhagic stroke, said Dorion Sagan, a son she had with her first husband, the cosmologist Carl Sagan.

Dr. Margulis, who had the title of distinguished university professor of geosciences at the University of Massachusetts Amherst since 1988, drew upon earlier, ridiculed ideas when she first promulgated her theory, in the late 1960s, that cells with nuclei, which are known as eukaryotes and include all the cells in the human body, evolved as a result of symbiotic relationships among bacteria.

The hypothesis was a direct challenge to the prevailing neo-Darwinist belief that the primary evolutionary mechanism was random mutation.

Rather, Dr. Margulis argued that a more important mechanism was symbiosis; that is, evolution is a function of organisms that are mutually beneficial growing together to become one and reproducing. The theory undermined significant precepts of the study of evolution, underscoring the idea that evolution began at the level of microorganisms long before it would be visible at the level of species.

"She talked a lot about the importance of microorganisms," said her daughter, Jennifer Margulis. "She called herself a spokesperson for the microcosm."

The manuscript in which Margulis first presented her findings was rejected by 15 journals before being published in 1967 by the Journal of Theoretical Biology. An expanded version, with additional evidence to support the theory - which was known as the serial endosymbiotic theory - became her first book, Origin of Eukaryotic Cells.

A revised version, Symbiosis in Cell Evolution, followed in 1981, and though it challenged the presumptions of many key scientists, it has since become accepted evolutionary doctrine. - N.Y. Times News Service