"The ancestors of major groups of animal species began
populating Earth more than 600 million years earlier than
indicated by their fossil remains, according to the largest
study on the subject using gene sequences, recently
completed at Penn State.
"The research suggests that animals have been evolving
steadily into different species for at least 1,200 million
years (1.2 billion years), which challenges a popular theory
known as the Cambrian Explosion that proposes the sudden
appearance of most major animal groups, known as phyla,
530 million years ago."
http://www.science.psu.edu/alert/Hedges1-1999.htm