[BLANK_AUDIO] We've seen that the conditions on the early Earth became clement for life rather early on. But what is the evidencee for early life on Earth? The only direct evidence we have is from fossils and chemical signatures preserved in the geological record, and this evidence may be proof that life was established on the Earth by 3.5 billion years ago, possibly as early as 3.8 billion years ago. It's thought that the evidence does suggest life by 3.8 billion years, then it's likely to have arisen some time before that, because that early evidence is probably not preserved in the fossil record. This would suggest that life was present soon after the formation of the crust and oceans. And at the end of that period of late heavy bombardment, that period of intense asteroid and comet impacts on the surface of early Earth. Now, before we look at some of that evidence, it's worth reminding ourselves that the evidence is subject to intense scientific controversy. And to base, in fact, the evidence for early life on Earth is one of the most hotly debated areas of astrobiology. There are several lines of evidence that have been developed to suggest the presence of life on early Earth. One line of evidence of early life on Earth, is features called stromatolites. Stromatolites is laminated mounds which formed today in shallow marine water. You can find them in Shark Bay in Australia, for example. They're built up by successive accumulation of sediments and microbes, and also rocks such as calcium carbonate. So as the microbes form, sediments collect on top of them and new layers of microbes form, and eventually you end up with these macroscopically visible mounds of microbial activity. The oldest stramphrolites have been found in 3.46 billion year old apex church silica rocks in Western Australia's Warraweena group. One of the reasons why they're so subject to controversy is because these mounds, as you can understand, look like features that can form by non-biological processes. For example, if sediments are laid down in shallow marine environments or rivers for example, they can also form these wavy textures that look at least, on the large scale, a little bit like stromatolites. So, further lines of evidence have been the search for microfossils. Fossils of individual microorganisms in these stromatolites or other types of rocks that can be evidence for life. This 3.46 billion year old stromatholites in Western Australia contain these filamentous structures which are thought to resemble modern cynobacteria. That composed of kerogen, which is the alteration product of heated and pressurized organic matter. And you can see some examples here of proported filaments in these apex charts. These micro fossils have also been subject to intense debate. Particularly because non-biological processes can also form filamentous structures. We don't really know the context of these ancient rocks, so this is another line of evidence, though, that astrobiologists seek in order to demonstrate early life on Earth, fossils of individual microorganisms. There are also indirect ways to show the presence of life. And one way is through chemical fossils. For example, carbon that we met earlier in this course and is the backbone of most molecules used by life, comes in different forms or isotopes. For example, there's Carbon-12, which is the most common type of carbon in the environment, and Carbon-13. Carbon-13 has seven neutrons, Carbon-12 has six neutrons. In other words, they have a slightly different atomic mass. Life will preferentially use this lighter isotope, or Carbon-12. So wherever there has been life, the carbon in those molecules within that life will tend to be made up of the Carbon-12 rather than the much rarer Carbon-13. And this preferential uptake of this isotope Carbon-12 can be used as evidence for the presence of life. And you can see some calcium carbonate shells here, in which these early signatures can be preserved. These are much more recent fossils. But similar sorts of organisms taking out carbon through photosynthesis, depositing that carbon either in minerals or in organic matter, can also be used to find chemical signatures on early Earth. This so called isotope fractionation, this preferential uptake of light, lighter isotopes compared to heavier isotopes by life, is seen in carbonate rocks from as far back as 3.5 billion years ago and slightly more controversially earlier as well. So what is the problem with all of this evidence? These three lines of evidence, macroscopic features such as stromatolites, microfossils of individual microorganisms, and indirect evidence such as chemical signatures. Well, first of all, these are rocks very heavily metamorphosed. In other words, they've been altered by heat and pressure since their formation those many billions of years ago. This makes it much more difficult to interpret the evidence. The geological settings are usually uncertain while heavily disputed. We don't know exactly what the conditions were like in the regions where those early rocks were formed, and so were they really conducive for life? That's very important to determine to find out whether the fossils or chemical signatures that we see are really plausible evidence for life. And many of the features that we observed can be produced by nonbiological processes. The wavy textures in stromatolites can be produced in sediments without biology. Microfossils can be formed with similar types of structures by non biological processes. Filamenters, non biological structures that just look like biology. Chemical signatures, isotopic fractionation can also be caused by non biological processes. But scientist think that when this evidence is taken together, it does provide some compelling evidence for life on the early earth. So what have we learned? We've learned that it's thought that life is established on the Earth by at least 3.5 billion years ago. The main evidence of this life comes from macroscopic features such as stromatolites, microfossils of creatures, and independent chemical alterations, such as the fractionation of carbon isotopes. Many people dispute this evidence on the grounds that similar features could be a result of non-biological processes. But it does show that the search for life on early Earth and the evidence for life on early Earth is an ongoing challenge of astrobiology. And this work will reveal whether the early life on Earth was established 3.5 billion years ago or earlier, and what the nature of this life might have been. [BLANK_AUDIO]
