When Did the Cambrian Explosion Happen?

For most of Earth history (the first 4 billion years), life consisted mainly of single-celled organisms and simple multicellular forms. Then, over a span of perhaps 15-25 million years during the early Cambrian period, the fossil record suddenly bursts with an astonishing diversity of complex animal life: arthropods, molluscs, chordates (our ancestors), echinoderms, brachiopods, and many phyla that have no modern descendants. The Burgess Shale of British Columbia (discovered 1909) and the Chengjiang fossils of China are the most famous Cambrian fossil sites, preserving soft-bodied organisms in extraordinary detail. What triggered the explosion is debated: increased atmospheric oxygen (following the Ediacaran period), the evolution of eyes (sparking an "arms race"), the evolution of predation, changes in ocean chemistry, and the emergence of Hox genes enabling complex body plans have all been proposed. The Cambrian Explosion remains one of the most dramatic events in the history of life.