The Cambrian explosion describes the profound diversification in life on Earth. Prior to around 580 million years ago, organisms were on the whole simple, comprised of individual cells occasionally organised into colonies. Over the subsequent 70-80 million years, evolution would accelerate by an order of magnitude, and the diversity of life would begin to resemble today's.
The fossil record provides us with a few cases of exceptional preservation around the end of this explosion, with famous units such as the Burgess shale offering a glimpse into a bustling yet strikingly different world, in stark contrast to the microbe-dominated seas of just 80 million years before.
The Cambrian explosion has generated extensive scientific debate. The seemingly rapid appearance of fossils in the "Primordial Strata" was noted as early as the mid 19th century; Charles Darwin saw it as one of the principal objections that could be lodged against his theory of evolution by natural selection.
Phanerozoic