So if we look at the history of Earth, here the history of Earth is given as 5 billion years, this is in thousands obviously. The Earth is probably not that old. It’s probably 4.5 or 4 or 3 billion years but, anyhow, that’s when the planet first aggregated, as far as we know. One believes that no life existed for perhaps the first half billion years, but after half a billion years, which is a lot of time to be sure, there already begins to be traces of life forms on the surface of this planet.