First Signs Of Animal Life On Earth May Be From Microbes - Astrobiology Magazine
http://www.astrobio.net/news-exclusive/first-signs-animal-life-earth-may-microbes/
Evidence suggests that microbes existed on Earth as far back as 3.7 billion years ago, a billion years after the planet formed.
Animal remains, however, don’t appear in the fossil record until 600 million years ago during the Ediacaran period, though there
are indirect signs that animal life may have gotten started much earlier.