George Simpson. Portrait of the American palaeontologist, George Gaylord Simpson (1902-84), with a bushbaby. Simpson got his PhD from Yale University, USA, in 1926. After working as a curator at the American Museum of Natural History he became Agassiz Professor of Vertebrate Paleontology at Harvard University, USA. His main work was in clarifying the history of the early mammals in the late Mesozoic era, and in the Palaeocene and Eocene epochs through extensive fieldwork. Simpson is credited with devising the modern classification of mammals. He also found evidence for neo-Darwinian evolution in population genetics, palaeontology and chromosomal studies.