
Fossil traces were found in an oil field in the Arabian peninsula are considered to be the oldest evidence of animal existence. The fossils found can confirm that life existed more than 635 years ago. The fossils found don't look like traditional fossils, but they look more like fossil echoes which are chemical traces of a compound. The chemical traces were similar to the demosponge, which is considered to be the last common ancestor for all animals. "It is, definitively, the earliest evidence for animals," said geochemist Gordon Love of the University of California, Riverside.
Source: Wired / Nature
http://blog.wired.com/wiredscience/2009/02/earliestanimal.html