From Wikipedia - Lynn Margulis (March 5, 1938 – November 22, 2011) was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted for how certain organelles were formed. She is also associated with the Gaia hypothesis, based on an idea developed by the English environmental scientist James Lovelock.
I first heard about Margulis in my Introduction to Plant Biology class. Like any good scientist, she put her neck out there and stirred up some controversy; her death is a loss to everyone.