ABSTRACT
Evolution of life on Planet Earth has not been constant; rather, there have been pulses of new organisms and losses of old ones. This was recognized by early nineteenth century geologists, including Adam Sedgewick, a Cambridge geology professor. He found deposits of fossils in strata representing an early period that he named the Cambrian (i.e., the so-called Cambrian explosion), and he also named the Paleozoic Era in 1838 (Benton 2003). Phillips and Gould (1860) also noted that fossil species were very different during a certain period of the Phanerozoic (abundant + life; the last 540 million years (my) when larger fossils were present) and divided it into three periods accordingly: Paleozoic (old life), Mesozoic (middle life), and Cenozoic (originally Caenozoic = recent life). The presence of these different fossil layers was later attributed to episodes of mass extinction and replacement by new forms. The extinctions were presumably due to global catastrophic events that resulted in ecosystem-level extinctions.
