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.
Keywords: Paleontology--Cambrian, Life (Biology), Life--Origin, Fossils
Geochemical evidence, in the form of traces of organic carbon in rocks, suggests that life existed nearly 3.9 billion years ago.
This 3.5-billion-year-old black chert from the Warrawoona Group of Western Australia contains microscopic forms believed by some scientists to be fossil bacteria.