A couple of days ago it was announced that fossils have pushed back the date of earliest known life on Earth to 3,430,000,000 years ago. The New Scientist article does an excellent job of summarising the science and the relevance of the find, so I won’t go into that here.
What I would like to highlight is the implications for life more generally. People occaisionally see these findings as somewhat abstract and meaningless – what does it matter that life appeared a bit earlier than we thought?
The Earth is somewhere in the region of 4.54 billion years old. We now have direct evidence that life was around at least 3.43 billion years ago, and indirect evidence that it was firmly established at least 3.8 billion years ago.
Now, bear in mind that dating the Earth is a somewhat tricky job, so let me explain how radiometric dating works (this is somewhat simplified but the details are correct). When you have an amount of a radioactive isotope, it will gradually break down (decay) into other isotopes. For example, K40 gradually decays into Ca40 and Ar40. The half life of that decay rate is 1.248 billion years (in other words, every 1.248 billion years 50% of the remaining potassium will decay). 89.1% of those decays will result in the Calcium isotope, the remaining 10.9% will result in the Argon isotope. By measuring the amount of Ar40 and K40 in a sample, you can therefore calculate how long that sample has been sat around for.
In order for radiometric dating to work you need to have a closed and stable system; if the rock is re-melted, or heavily weathered, or subject to hydrothermal alteration, you will find that elements become mobile again so the radioactive clock gets reset (or just broken). You therefore need good samples, and as you might expect – the further back in the geological record you go, the higher the chances of a rock having been buggered about with in one of the aforementioned ways. As such, the dates for earth formation are taken from moon and meteorite dating. The assumption is that the rocky bodies in the solar system formed at about the same time, and these tectonically inactive bodies will have much better dates than we can get from earth crustal rocks. This is a reasonable assumption.
What the dates mean, however, is that the dates given are the dates at which solid material was present on much smaller bodies than the Earth – and hence bodies that will have cooled more quickly. It is likely that Earth took longer to cool and form a solid crust. In fact the oldest piece of crustal rock we have is 3.8 billion years old. The best method we have for identifying the earliest crust is this:
If you have solid crust, you must also have erosion beginning to occur. If you have erosion beginning to occur, then you must also get deposition. therefore, by looking at the mineral grains in the oldest sedimentary rocks we might get an idea of the dates of formation of the rocks that were originally eroded. Hence we look at detrital zircons very long lived and stable little buggers, and indicate that we have solid crustal material around about 4.35 billion years ago, and the oxygen isotope ranges suggest we probably had liquid water in some quantities by about that time too.
What that tells us is that life on Earth formed within 500 million years of there being a cool and stable enough environment for it to have a chance – and that ignores the fact that the atmosphere for the first part of that period was probably so thin everything was under very hostile radiation bombardment.
Furthermore, we only know that life has been consistently present since 3.8 billion years ago – we don’t know how many times life initiated but was wiped out again before getting a solid foothold – it would be naive to assume that the first time life occurred it found itself in a hospitable and stable enough environment to flourish on its first attempt.
Ultimately, what I’m driving at is that the chance of life generating on other planets has got to be pretty good.
You may or may not have come across the Drake Equation, but basically it’s a way of estimating the number of advanced civilisations in our galaxy at any given time, and one of the most important controls is the potential for life to occur in environments which might support it. If life occurred as early as it seems to have done on Earth, the implication is that that value tends to 1, which would support the values Drake originally used to suggest that at the present time there are 10 advanced civilisations in the Milky Way.
Who said dating old fossils was boring?