SCIENCE AT THE SHINE DOME canberra 6 - 8 may 2009
Symposium: Evolution of the universe, the planets, life and thought
Friday, 8 May 2009
Professor Malcolm Walter FAA
Australian Centre for Astrobiology, University of New South Wales, Sydney
Malcolm Walter is professor of astrobiology at the University of New South Wales and director of the Australian Centre for Astrobiology based there. He has worked for 45 years on the geological evidence of early life on Earth, and more recently on the search for life on Mars. He has also worked as an oil exploration consultant and a consultant to museums. In 2004 he was elected Fellow of the Australian Academy of Science. He has been awarded a Eureka Prize for his interdisciplinary research.
The early record of life on Earth guides the search for life on Mars
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We have taken a journey from before the origin of the universe, before the big bang, up until the formation of planetary systems this morning, which is all just physics, and now we get to the extremely complex stuff – life. I am going to add a universal qualification at the beginning: I do not have the time to qualify everything that I am saying. I am going to talk firstly about the rock record of early life. There are severe constraints on understanding the rock record. I’m going to give you what I believe is the best current interpretation, but every point can be argued. So we will leave it at that, and you will see how I like to interpret it.
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Firstly, to put it in a broad time context, we have a very vague hint of life in Greenland, in carbon isotopes, here at about 3.8 [on the biogeologic clock]. It is not at all convincing. We have convincing evidence of life from about 3.5 billion years ago, as I will show you, which is all microscopic. There is no evidence of any macroscopic life until around two billion years ago, and then it is very rare and is still contentious, and then there is the more familiar later fossil record. So life for most of time has been microscopic.
The rock record is severely limited because, as we go further back in time, we have fewer and fewer well-preserved rocks in which to search for fossil evidence of life, simply because of tectonic mountain-building processes. So, by the time we get back to 3.5 billion years ago, we only have two areas of Earth to study, as best we know: the Pilbara in Western Australia and the Barberton Mountainland in South Africa. I’ll show you the evidence from the Pilbara because that is by far the richest record of early life. I am going to focus on early life for the sake of comparison with Mars, which I will come to later.
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Because the rock record is so limited, I find it useful to augment my thinking about what might have been around by looking at this: the universal tree of life, which has come out of molecular biology since the 1970s. This diagram, I believe, is one of the greatest achievements of science over the last century; I find it quite staggering. Of course, genomics and proteomics are accelerating the observations these days and so wonderful things are happening. From last week, we now have the cow genome and heaven knows where that will lead.
But, if we look at the universal tree of life, we see three super-kingdoms or domains of life and we see that only here [pointing to Eucarya] are there macroscopic organisms. All the rest of the universal tree of life is represented by microscopic organisms. It is telling us the same thing as the geological record; that is, if we are looking for life elsewhere in the universe, what we are looking for really, mostly, is microscopic life of one sort or another – if this sample of one, Earth, is any indication of what might be out there elsewhere. The fact that we have only one sample of life is something that is important to remember, and that is why it is worth looking.
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The Pilbara in north western Australia is up in here [pointing to map] and Port Hedland is here. These rocks are three billion years old, or older. There is a 100-kilometre scale. Marble Bar is over here, and I was there last week. If I had shown this map 20 years ago or maybe only 10 years ago, there would be almost no stars. These stars are where there is some evidence of life at 3.4 to 3.5 billion years ago. Our knowledge of this area has proliferated enormously in the last 20 years or so. What was very contentious once in terms of the fossil record when it was first published in about 1980 has become, while still contentious to some extent, far more cogent and convincing.
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This is what the terrain looks like. The layered rocks that form the hilltops are 3.5 billion years old. They are volcanic rocks and so-called cherts – siliceous sediments. It is in the siliceous sediments that most of the evidence of early life is found.
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Africa and the Pilbara. We know how to find them because many of them are gold deposits or copper, lead and zinc deposits, so there is an imperative to look for them.
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I want to concentrate on a slightly younger time – ‘slightly younger’ in geological terms – of 3.42 billion years ago. At that time in the Pilbara, there was a small but still widespread continental platform. These are a series of times as that platform developed: from times when there were rocky shorelines with bolder conglomerates on the wave-cut platforms [bottom panel], and so on, through to a time when the sea covered the whole of that platform [top panel]. On that platform these structures called stromatolites formed, and they can be seen in the rocks in the Pilbara. This is what they looked like when they were forming.
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It is contended – and I believe and consider it to be so – that they were built by benthic microbial mats, such as the ones that we can still see forming in Shark Bay in Western Australia, which look like this.
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We know more or less how these form. We can use these as a model for interpreting the ancient examples.
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The modern ones are constructed by cyanobateria – which used to be called bluegreen algae – particularly filamentous and also coccoid forms. The story in Shark Bay used to be quite simple. There were different sorts of cyanobacteria that made different sorts of stromatolites. We did not really understand the processes by which they made them, but they did make them. We can see the different mat communities and we can see the different types of stromatolites that resulted from those mat communities. Molecular biology has greatly complicated that simple picture.
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There is a great range of microbes in the mats. The study of those microbial mats and the actual biogeochemical processes that lead to the formation of the stromatolites have really started a whole new phase of trying to figure out what was really going on, in terms of the chemistry that leads to calcium carbonate precipitation, for instance, and the micromorphology of the mats and so on. So we are at the beginning of a new era of understanding how they form and that is sobering, when we try to interpret what we see in the rock record.
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There are lots of different sorts of cyanobacteria in the Shark Bay stromatolites.
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Perhaps most surprising, lots of archaea, which are obligately anaerobic in what superficially seems to be a highly aerobic, oxygen-rich system; nonetheless, they are there.
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If we go back to the rock record, this is what we deal with. We have the stromatolites. We have some poorly preserved filamentous microfossils. Again, the point is that, when these were first found and described in 1980, they were thought to be extremely rare and, just because of their rarity, they were particularly contentious. But now it turns out that they are abundant. It’s a bit like the story of discovering extrasolar planets.
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One of the things done now to try to reduce the uncertainties of interpreting these poorly preserved things is to use such techniques as three dimensional and two dimensional Raman spectroscopy to demonstrate that they are, in fact, carbonaceous objects. Here they are seen in optical photomicrographs in a few thin sections of rock.
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Here they are seen with a three dimensional Raman microscope, showing their overall shape and the fact that they are made of carbonaceous material.
You can see that the cellular structure is still preserved in this two dimensional Raman spectroscopy. An intriguing recent discovery in rocks a little younger than those 3.5 billion-year-old ones is these objects in three billion-year-old rocks in the Pilbara. In terms of microfossils, these are very large. The scale is 50 microns.
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There are spheres within spheres and intriguing spindle-shaped objects, also very large.
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I have been doing some recent work on these with colleagues in NASA and CNRS using NanoSIMS, secondary ion mass spectrometry, to make elemental and isotopic maps of the composition of these objects. We see carbon, sulfur and nitrogen in an isotopic composition consistent with the palaeobiology of these objects. Intriguingly, they look as though they might be eukaryotes, but we do not have any firm evidence for that yet.
One of the powerful techniques that has been used in recent years to uncover the biological record of early life is the use of so-called biomarkers; that is finding preserved hydrocarbon molecules in rocks that can be related to lipids in the organisms that died and were buried in the sediments that became rocks.
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For example, the hopanepolyols that are in the cell membranes of bacteria can be preserved: through the loss of the functional groups; the carbon skeleton can still be preserved and found in rocks and, through careful work, can be shown to be the same age as the rock. This is highly contentious and has been disputed in recent times. However, I consider the way that I am describing it as valid; it is the equivalent of finding skeletons of animals in rocks. Those are original lipids, so they are very specific in terms of the types of organisms they came from; therefore, the hydrocarbons that are found in rocks provide quite specific information.
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For example, in 2.7- to 2.8-billion-year-old rocks in the Pilbara region, a range of steranes derived from sterols can be found and a range of hopanes derived from hopanepolyols can be found, and they give quite specific information about the sorts of organisms that were present at that time. It is a very powerful technique, I believe, which has a long way to run yet in providing us with information.
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So we have those sorts of approaches to reconstructing the biology of the early Earth. This is a classic picture of what the Earth would have looked like, say, 3.5 billion years ago; it could just as well have been Mars.
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What do we know about the biology of four billion years ago? Nothing.
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What do we know about 3.4 to 3.5 billion years? This is what I think I know. These sorts of organisms made stromatolites. The carbon isotope record tells us that there were autotrophs fixing CO2, or bicarbonate, to make organic matter. The rock record tells us that some of them like to lived in salty places [halophiles]. The rock record tells us that some were living around hot springs on the sea floor [benthic thermophiles] and some were living subterraneally in hot places [hyperthermophiles]. But there was no proof of oxygen releasing photosynthesis by cyanobacteria at that time.
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By the time we get to this time, we have added a few other things. We know that there was an active biological methane cycle. I am quite confident that there were cyanobacteria and so on. So that is a rough picture of early life on Earth.
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It has taken us 50 years of intensive study of field mapping and field observations, detailed petrology and detailed palaeobiology and so on to get to that point, and now we are trying to do the same on Mars. It is not going to be fast. Phoenix was up there late last year. You might have followed that to some extent. It was a static lander.
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We have known for a long time, since the Pioneer missions of the 1960s, that there are dry river valleys on Mars; something flowed on the surface of Mars early on in the history of the planet at about the same time as we have evidence of life on Earth.
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We know that there was relatively abundant volcanism on Mars. Here is the largest volcano in the solar system: Olympus Mons, 500 kilometres wide and 27 kilometres high. This has been active in relatively recent times.
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There is Phoenix; that is what it looked like.
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We know that there is water ice on Mars. In that first image showing the Phoenix landing site, you saw the North Pole with its carbon dioxide and water ice. Phoenix scraped into the regolith, or soil, underneath the lander and exposed what turned out to be water ice.
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Liquid water is not stable on the surface of Mars at the present time, supposedly. The reason is – here is what the weather was like at the Phoenix landing site. The average maximum was minus 30oC, the average minimum was minus 79oC and the pressure was around only 8.5 to 7.85 millibars, so liquid water is not stable. But there was evidence of water droplets on the legs of Phoenix and there was evidence of a highly saline environment, so there are some questions about whether or not there are some special conditions, such as very high salinity, that would allow liquid water to still exist on the surface of Mars.
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It is quite clear that there was abundant liquid water in the past, which is the prime need for life; so, from what we know about Mars, it is clear that there could well have been life there.
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But do we have any evidence? Just in January this year – although it has been known for about five years – Mike Mumma and his colleagues published this information on the distribution of methane in the atmosphere of Mars.
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Methane is a gas that is quickly dissociated by various processes in the atmosphere. This indicates that the methane is being actively produced currently. There are two ways to do that. One way is by purely geological processes – volcanism, serpentinisation and so on. The other is methanogenic bacteria living on Mars. So maybe we are homing in on the possibility of life on Mars. This is the strongest possible indication that we have at the moment, anyway. We have learned that life on Mars was possible, probably still is possible and perhaps still is there; methane may be an indicator of that. We will have to wait to find out whether that is correct. It would be nice to know the isotopic composition of the methane, for example.
With the future of the exploration of Mars, the big thing that is coming up next is the Mars science lander of NASA. That is a very capable and complex laboratory that will be launched in 2011 and I am not sure exactly when it will land on Mars. The European Space Agency will do the same. That may well reveal significant information in terms of life; I certainly hope so. Then there is the plan for a sample return mission. All the Mars missions so far have been one way. The Mars sample return will be an international mission, if it ever occurs. The earliest it is likely to be launched is about 2020. That gives us the best chance of really finding out whether there is current life or past life on Mars. If that is launched in 2020, it will be 2023 when the samples come back – and we will just have to wait until then to find out. Thank you.
Discussion
Malcolm McCulloch: Thank you, Malcolm.
Question: Can you say a few words about the possibility of life that is not carbon based? I presume that everything you have been talking about is carbon-based life as we typically know it. People talk about the need for, say, broader horizons, when one thinks about extraterrestrial life. What are your thoughts on that?
Malcolm Walter: I think we have to stay alert to all of the possibilities, but this is the most conservative approach. We are getting into enough trouble as it is, thinking and talking about life elsewhere. I am encouraged just to focus on what we know about life on Earth, extrapolate from there and leave it at that. But it would be wise to keep in the back of our minds other possibilities.
Question: You showed some lens-shaped structures from those Pilbara rocks that you thought were eukaryotic. What are the lines of evidence for that?
Malcolm Walter: I wish you had not asked that question. It was either naive or foolish of me to have used that word ‘eukaryotic’. It is really based only on size. Eukaryotes tend to have much larger cells than prokaryotes and bacteria and archaea. That is all there is at the moment. My colleagues and I have been trying to think of a way to be more rigorous and convincing than that. Things like nuclei do not preserve, in general; they do occasionally, but it is extremely rare. At the moment all we have is what I would term a ‘very vague hint’ and I would never put it in writing.
Question: Could you comment on claims of finding bacteria-like structures on electron microscopy in rocks from Mars, including meteorites?
Malcolm Walter: I deliberately did not speak about that. In 1996, there was a flurry of interest in one of the Martian meteorites, ALH84001. They were so-called fossil microbes but not living ones. Most of us who had worked on fossil microbes in the past thought from the beginning that that was naïve – and it was naive. I do not have time to go into it, but there were a number of lines of evidence and they have all gone away.
