 In 1972 Niles Eldredge and Stephen Jay Gould used fossil evidence to advocate the theory of punctuated equilibrium, which maintains that evolution is characterized by long periods of relative stasis and much shorter periods of relatively rapid change.One area of paleontology that has seen a lot of activity during the 1980s, 1990s, and beyond is the study of the Cambrian explosion during which many of the various phyla of animals with their distinctive body plans first appear.The well-known Burgess Shale Cambrian fossil site was found in 1909 by Charles Doolittle Walcott, and another important site in Chengjiang China was found in 1912.