When most people hear the word 'explosion', they often think of destroyed buildings and injured people. But geologists have long recognized a different type of 'explosion' in earth's sedimentary rocks, known as the Cambrian explosion.
Cambrian rocks are a group of rocks—found around the world—in which we find fossils of many of the major animal groups. This is significant because Precambrian rocks, found below the Cambrian, don't generally contain much fossil material—but then in the Cambrian—there are fossils in abundance. In fact, this transition is so dramatic that some call it the Cambrian Big Bang.
But probably the most devastating impact of the Cambrian 'explosion' is the damage it does to evolutionary ideas, even with evolutionary dating notions. Since most major animal groups appeared 'suddenly' in the fossil record, without ancestors, the idea that all life evolved slowly over time becomes very hard to swallow.