Evidence for life on earth more than 3.8b years ago
Science Digest:
"Ten years ago, an international team of scientists reported evidence, in a controversial cover story in the journal Nature, that life on Earth began more than 3.8 billion years ago--400 million years earlier than previously thought. A UCLA professor who was not part of that team and two of the original authors will report in late July that the evidence is stronger than ever."
http://www.sciencedaily.com/releases/2006/07/060721090947.htm
I think Nick's figure is for undisputed evidence of life, there's other evidence of varying levels of acceptance for older dates.