For more than 3 billion years - most of earth's history - all organisms were simple, composed of individual cells occasionally organized into colonies. Starting around 580 million years ago, the rate of evolution began to accelerate rapidly, and a remarkable amount of biological diversity appeared, culminating in the Cambrian Explosion. Here, the majority of types of modern animals appeared in the fossil record for the first time, as well as unique lineages that subsequently became extinct.
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, and Charles Darwin saw it as one of the main objections that could be made against his theory of evolution by natural selection.
The long-running puzzlement about the appearance of the Cambrian animals, seemingly abruptly and from nowhere, centers on three key points: whether there really was a mass diversification of complex organisms over a relatively short period of time during the early Cambrian; what might have caused such rapid change; and what it would imply about the origin and evolution of animals. Interpretation is difficult due to a limited supply of evidence, based mainly on an incomplete fossil record and chemical signatures left in Cambrian rocks.
Wikipedia Article: "Cambrian Explosion"
