Thanks, guys. I don’t remember a whole lot from my Geology classes, but I think the scientists reckon that the earliest forms of life on the planet go back to the Pre-Cambrian Era, around four billion years ago. I’m thinking that’s a WAG.
Not really a WAG. Methods such as uranium-lead dating of zircon allow for what theory suggests is an extremely accurate date, well beyond the age of the earth if necessary. I'm sure the Internet has a description, but the bottom line is that the method should be reliable. [Then you draw inferences from strata, and that may be risky, but at least the uranium-lead dates have a reliable basis.]