Transcript: Fossils have existed only since creatures had skeletons or hard bodies or shells, about a half a billion years, but life existed on Earth at least three billion years before that. Almost all of these life forms were microscopic. Scientists can learn about the history of microscopic life over the whole span of Earth’s history from DNA itself, which acts as a kind of living fossil telling the story of life. Imagine the DNA of an early living organism that was the ancestor of all life today. Over time, genetic changes would accumulate and the accumulated changes cause species to diverge in an evolutionary sense. So now looking backwards, we can estimate the time to a last common ancestor by the cumulative amount of changes to the DNA. Two species with more similar DNA must have diverged more recently in the evolutionary tree. Not all the changes are due to mutations. Sometimes entire genes can be incorporated, a process called lateral gene transfer.