Carl Woese
The tree of life shows the diversity of microbes. Single-celled organisms represent the vast majority of the biosphere's diversity. This is surprising to some, given our familiarity with the larger metazoan world. Microbes are responsible for many biochemical cycles and are crucial to the whole biosphere. Woese's efforts to clarify the evolution and diversity of microbes was an invaluable service to ecology, conservation and evolutionary biology.