So it appears life was underway at least within 700 million years of the formation of the Earth (4.5 billions years ago). To-date these are actually the oldest sedimentary rocks yet discovered and the most ancient surface rocks on the planet. Maybe life had an even earlier foothold on the planet but the traces have long since been wiped out.

We do know that the Earth was subjected to intense bombardment from meteorites in the first few 100 million years, perhaps up to 700 million years after its formation. It is unlikely that life could have got a foothold during this bombardment of the young Earth as the devastation caused would have vapourised and sterilized the Earth's surface thereby destroying any chances of the first life forms - prokaryotes - emerging. It is thought that life would have to wait a few hundred million years until the intense bombardment had ceased or diminished, and the surface conditions on the Earth were more stable, to stand any chance of evolving.

However the size of the impacts throughout this bombardment is also important. Large impacts would probably prevent early life forms from gaining a foothold anywhere on the planet, but smaller impacts would be more localised and maybe allow life in certain regions of the planet to gain a foothold. More research needs to be done to predict the impact sizes as well as the impact frequency in these first few hundred million years. But would it take long for life to gain a foothold? It is thought that the chemical reactions required to go from non-life to life would be considered rapid on geological timescales. So self-replicating systems could emerge and flourish perhaps faster than their destruction rate by massive impacts.

The oldest fossils known (microfossils) date to 3.5 billion years ago. Found in Western Australia, perfectly preserved in rocks called cherts which were formed when silica was precipitated from hot vents in a region of intense volcanic activity deep within the seas. These rocks give us a picture of life on the early Earth where complex chemicals engaged in ever-changing reactions resulting eventually in the first living organisms. The energy required for these reactions was derived from the heat of these underwater volcanoes. It must also be noted that there is currently a 'heated' debate within the scientific community as to whether these microstructures are indeed fossil microbes. But if they are, then what was the kickstart for life? And did the molecules of life form on planet Earth initially or form in the molecular clouds of our galaxy before being transported by comets to seed our planet?