What was Earth like when life began?

Back when the first life got going, what was earth actually like? Georgia Mills spoke to David Rothery from the Open University..

大卫- We’re pretty sure that the Earth and the other rocky planets in the solar system formed about just over 4½ billion years ago through a series of collisions between progressively larger and larger clumps of debris called planetary embryos.

The early Earth was very different from the Earth we see today. After the last large collision, which is probably the collision, which formed our Moon as debris round the Earth, you were left with a body which was, essentially, molten on the outside and iron core in the middle. The outer part would have been molten and it slowly froze over from the top down giving you an early crust. That would be unstable, it would be rifted apart many times, and you’d have eruptions onto the surface. So the surface would be continually reforming itself and the atmosphere above this surface would not be breathable anyway. There was no free oxygen; there was a lot of carbon dioxide. There was nitrogen which we still have today but, apart from that, carbon dioxide and sulphur dioxide - it would have been unbreathable. Hot, unstable and no breathable atmosphere.

We can trace it back to nearly 3.8 billion years old. Beyond that the record is obscured because the Earth was being hit still by the kind of debris which formed the large basins on the Moon. There was still debris around in the solar system so the Earth would have been hit periodically by impacts which would sterilize a large part of the surface. It’s possible that life started more than once but was then obliterated and had to start over again. So we could have had life older than 4 billion years ago but no traces of that survive.

Georgia - We were back in time when we know the oldest life was starting to appear - 3.8 billion years ago. What would the Earth have been like and how do we know?

大卫- The geological record is pretty patchy but it’s fairly clear that the continents had knot grown to anything like their present size by then. There would only be small continental areas rather like present day islands arcs. If you think of the island arcs in the Pacific, or think of Japan or bits of Indonesia, if you will, but no large continents. That continental crust hadn’t grown and above this an atmosphere which was as dense, possibly denser than today, but no free oxygen only carbon dioxide, nitrogen, and sulphur dioxide. It’s not until 2 billion years ago that the oxygen levels started to creep up because that’s when photosynthetic organisms appeared in the sea that would start breaking carbon dioxide apart and liberating oxygen.

Georgia - So when we look at the geological record, how can we tell there wasn’t oxygen around or there was this sulphur and this nitrogen?

大卫- You can tell the lack of oxygen from the minerals that form. Today many metals, for example, in the surface environment will oxidise - iron will turn to rust. You get none of this going on 2 or 3 billion years ago. The oxygen levels really were miniscule at that point. So it’s a very difficult jigsaw puzzle but fascinating to think what the Earth would be like. You wouldn’t recognise it if you flew by in a spacecraft because it would be a very watery looking planet. There’d be shallow oceans everywhere and small clusters of islands peeking up above the oceans.

Georgia - Do we know anything about what the temperature was like?

大卫- Surface temperatures in the past are probably quite variable. The young Sun would have been a little bit fainter than it is today so giving slightly less warmth. But if we had more carbon dioxide in the atmosphere during the past, as we think we did, that’s a good greenhouse gas so it would have trapped the solar heat more efficiently. So fainter Sun but stronger greenhouse gas effect, you might think they would balance out.

But there are traces in the geological record that more than once in the past the Earth was trapped in what’s called an icehouse condition where the surface water everywhere would have been frozen over. It would have been a ball of ice and these ice house conditions lasted for tens of millions of years at at time. So, we’ve got icehouse conditions - a snowball Earth some of the time but mostly exposed water in between.

Georgia - Everything you’ve said to me so far about early Earth is the fact that there’s no oxygen, it’s been very cold, it’s had lots of stinking sulphur everywhere, there’s only rocks to eat, it doesn't sound to me like a good place for life to be?

大卫- The early Earth would not be a good place for our kind of life, but our environment that we do so well in has been manufactured by life. It was microbes that liberated the oxygen. It was microbial plants and then the higher plants spread to land. Until that happened, our kind of life could not exist at all. The early Earth was a great place for microbial life that doesn’t like oxygen and would find the early Earth a very pleasant place to live, but not for us. If you say was it a good place for life, you have to specify what kind of life are you thinking of?

Georgia - Understanding that life way back then was very different today has implications in our quest to find E.T…

大卫——如果我们在其它地方寻找生命的商店uldn’t expect planets to be in the same state that the Earth is. If there is a exoplanet similar to the Earth, we’d hope to be able to identify it in it’s own atmosphere, but the atmosphere’s been put out of balance by living processes, and that’s probably the easiest kind of extraterrestrial life to detect at remote distances. But I would expect that there are many exoplanets which are Earth-like which have not developed yet into the state that the present day Earth is, which still have these primitive atmospheres hardly touched by life. So there could be lots of worlds with microbes living on the deep ocean floors which leave no clear trace in the atmospheres yet so they’ll be hidden from our view, even with our best instruments. So there could be lots of life-bearing planets which are going to be exceedingly hard to recognise as such.