For more than three billion years, the Earth harbored only single-celled organisms. At some point, multi-cellular life appeared, in the form of jellyfish, worms, and sponges. But these early animals, being soft-bodied, left few fossil traces. About 560 million years ago, animals with shells formed, and their skeletal remains left markers in the sediments. Within tens of millions of years, most groups of organisms that we recognize today had appeared, in what is known as the Cambrian explosion.