700-660 million years ago



worm trail worm trail

In this fictional treatment of the history of life on earth, I am going to err on the side of caution and suggest that the oldest worms date to about 620 million years ago (rather than the studies which have suggested ages of 1.1-1.2 billion years for the first worms). Since cnidarians and sponges are more primitive than worms, I will postulate that these animals evolved prior to that, starting around 700 million years ago.

Fossil strata which have been dated at more than 1.2 billion years old contain animals--not only contain fossils of radial-soft bodied organisms, they contain burrows which seem to have been made by mucus-producing worms (simulated in the images above; Rasmussen, 2002). There is a site in India whose worm burrows have been dated at 1.1 billion years ago, although some dispute this date. Recently, another site with apparent signs of primitive animals (jellyfish relatives) and worm burrows has been dated at 1.2 billion years old at the youngest (Seilacher, 1998; Kerr, 1998b; Morris, 1993). Other Ediacaran fossils are known by 610-600 million years ago and more diverse assemblages of organisms are known by 555.3 million years ago (McMenamin, 1996, Minnerop, 2000). They were largely replaced by the animals of the "Cambrian explosion" which began about 545 million years ago.
Molecular comparisons of modern organisms also support the conclusion that animals had produced a variety of lineages prior to the Cambrian. For example, one study of 129 proteins suggest that eukaryotes arose between 950 and 1,260 million years ago, animals arose between 761 and 957 million years ago, and coelomate lineages separated between 642 and 761 million years ago (Douzery, 2004). Other analyses have produced similar results (Peterson, 2004)
How significant are these dates? Until recently, it was thought that a diversity of complex animals appeared suddenly in the "Cambrian explosion" starting about 545 million years ago. We now know that animals existed for at least 60 million years prior to this and perhaps as many as 600 million years prior to the Cambrian. Sixty million years may not seem like such a long time given the vast expanses of time in the Precambrian, but a great deal of animal evolution can occur in such a period. It took about sixty million years for rhipidistian fish to evolve into the first amphibians and about 60 million years for the subsequent diversification of these amphibians into a variety of lineages including the first reptiles. A sixty million year period starting at the end of the Permian would include two mass extinctions. It would begin with a world dominated by synapsid reptiles and primitive archosaurs end in the Jurassic with its diverse lineages of dinosaurs, pterosaurs, and marine reptiles. In about sixty million years, the primitive mammals which survived the extinction of the dinosaurs radiated into the diverse fossil and living groups of the Cenozoic.