Lynn Margulis died on Tuesday, the 22nd of November.
She is most famous for her theory of endosymbiosis, which says that eukaryotic cells—the complicated cells of protozoa, animals, plants, and fungi—were formed in a series of stages where one organism swallowed another but, instead of digesting it, took it on as a symbiotic partner!
I didn't know she died this week... so sad, she was really a great woman and a great scientist.
Edit: more on her.