I was sad to read in the New York Times that the biologist Lynn Margulis has died; she was 73. She is known for having been married to Carl Sagan (they divorced); their son Dorion Sagan was co-author of several of her books. But she is famous for her work on evolution. She concentrated her efforts on the evolution of eukaryotic cells (cells with nuclei), which happened long before multicellular organisms appeared. Her work strengthened the theory that such cells originated as symbiotic relationships between different species of bacteria. In particular, chloroplasts (the organelles in plant cells responsible for photosynthesis) closely resemble cyanobacteria (which perform photosynthesis). Mitochondria, which are found in animal as well as plant cells, resemble proteobacteria (a family of bacteria that includes Rickettsia, which is a pathogen responsible for certain human diseases). A major theme of her work is that evolution is as much about symbiosis or cooperation as it is about competition. I've read her fascinating book Microcosmos (written with Dorion Sagan), which tells the story of the origin of life from a bacterial perspective. I have another book of hers, Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth (written with Karlene Schwartz). This book is amazing. It is a catalog of all the major divisions of life, showing exactly how they differ (the five kingdoms of the title include plants, animals, fungi, bacteria and archaea, which resemble bacteria). The book devotes only several pages to each phylum---including Craniata (phylum A-37 in the classification scheme advanced in the book), which includes all familiar vertebrates (fish, amphibia, reptiles, mammals).