Hi, I'm Sean Tvelia, and we're here at Suffolk County Community College to discuss when life first appeared on the planet. Geologists measure time in two different ways. The first, relative time, basically allows us to determine when events occurred based on their placement in the rock record, things closer to the surface happened relatively recent and the deeper we go down, the further back in time we go. The other way we measure time is through absolute dating methods. Absolute dating methods allow us to determine the years measured from the present when something occurred based on the radioactive decay of unstable isotopes. When we look through the rock record we can also see fossils are layered within the sedimentary deposits and since they are within the Earth or within those rocks they also obey relative dating principles meaning the further down they are the older they are. As we look through the fossil record, we can find that life changes through time. The oldest life that we find, well the oldest sure or certain life that we find actually occurs about 3.5 billion years ago. Now that life is photosynthetic life which is a relatively complex process and if we find that 3.5 billion years ago, geologists assume that life probably originated shortly before 3.5 billion years ago but as of now we have no fossil evidence suggesting an earlier time frame. So when we look at the fossil record, we've determined that life began roughly 3.5 billion years ago.