550 million years ago, during the Cambrian period, animals appear suddenly in the fossil record. This spectacular phenomenon has been given the name of Cambrian explosion. Why did it happen then, and not before, has been, for over a century, one of the great mysteries of paleontology.
The evolution of life on Earth seems to have been rather discontinuous. Life is likely to have appeared 3,500 to 4,000 million years ago. Not much later, around 3,000 million years ago, photosynthesis appeared. Eukaryotes (cells with nuclei) emerged 2,000 million years ago. From then until the Cambrian explosion, nothing much seems to have happened for 1,500 million years. Then all the types of organization of the animals existing today appear suddenly. Why?