Martin Van Kranendonk on the Earliest Life on Earth
Transcript
Note, transcripts are not fully edited for grammar or spelling.
Oliver Strimpel
This is Geology Bites with Oliver Strimpel. Stromatolites are thought to represent evidence for life on Earth as far back as about 3 1/2 billion years ago, which would be far and away the oldest life we know about. They are the layered structures left behind in rocks by the interplay between communities of microbes and their environment. But the interpretation of these structures as biological in origin is controversial, and several non-biological mechanisms have been proposed that can generate stromatolite shapes. Martin van Kranendonk has devoted his long and prolific research career to the study of the early Earth. One major theme of his work has been to use detailed mapping and lab research to develop geological models for the environments of Earth's oldest fossils. This has helped establish the biological origin of many ancient fossils. His recent work on a newly discovered find of exceptionally well-preserved 3 1/2-billion-year-old sedimentary rocks in the Pilbara Craton of Western Australia has provided the strongest evidence to date that structures of this great age were indeed produced by the earliest forms of life.
Martin van Kranendonk is a Professor in the School of Biological, Earth and Environmental Sciences at the University of New South Wales in Sydney.
Martin Van Kranendonk, welcome to Geology Bites.
Martin Van Kranendonk
Hello, Oliver. It's a pleasure to be here and nice to have a chat with you.
Oliver Strimpel
What do stromatolites look like?
Martin Van Kranendonk
Stromatolites look like layered, wrinkly structures in rocks that are different from their surrounding geology. Their structure’s preserved as fossils, just like a fossil elephant bone or a trilobite that are preserved in rocks and then surrounded by the geological matrix: the sandstone, the silt, the shale, the chert, which is made by processes that don't involve life. And they're entombed within that matrix for, in some cases, many billions of years.
Oliver Strimpel
So, roughly how big are the structures that you're talking about?
Martin Van Kranendonk
Stromatolites can be very large indeed. They could be up to 5 meters high, and they look like rocket cones in some exposures. They're packed side to side, but those are in slightly younger rocks than what we're talking about. Those are already 2 billion years old. But the very ancient ones, they're quite a bit smaller. They're about a centimeter scale to sometimes up to 30 centimeters. But even 2.7 billion years ago, there’re stromatolites that are the same height as me, almost 2 meters tall.
Oliver Strimpel
Before we talk about how stromatolite-like structures can be formed biologically… how do proponents of a non-biological origin explain them?
Martin Van Kranendonk
So, you have to be clear that stromatolites are regarded by the entire community as evidence for life through much of the fossil record. So, there are stromatolites that are living today, there're stromatolites that are couple of million years old. There’re stromatolites that are one and two billion years old. And nobody has any doubt that those are made by biology. It's really only the very most ancient ones, these ones that are 3.5 or beyond (because there is one claim in Greenland at 3.7 billion years old). Once they get to that great antiquity, people are a little bit more cautious, just because the rocks are so old that in many cases they've had quite a long history of alteration - movement, you know, by tilting and folding and cracking. And, because the structures are a little bit more cryptic, people are rightfully cautious about a direct interpretation of biology. And, extraordinary claims need extraordinary proof. And so that bar is raised for the most ancient type of life to be able to really document very carefully that it was made by biology and not by something else. And, it's funny when we talk about something else, because some researchers (and I don't think it's very scientific) say “Oh, it could be something else.” Well, what exactly? “We don’t know...” (laughs) But there have been proposals, and the main one is that they could just be the crusts of minerals that have accumulated over time. And we know that happens in a variety of different settings. We can see beautiful, sometimes shrub-like-shaped mineral growths, some beautifully layered smooth, some a bit more regularly textured. And those are structures that can sometimes look like stromatolites. And so people are rightfully cautious to say, well, how can you absolutely determine that that structure is not a mineral crust without the influence of biology? That's a perfectly valid question. If you think about it, we as humans are living. But we're also a chemical system. And we also produce minerals. So, if you tap your teeth, you're tapping a mineral. We make minerals, we make bones, we make teeth, we make fingernails. And so, that divide between life and “just chemistry” or “just geology” is, in some cases, really fine and really hard to define.
Oliver Strimpel
What is the mechanism whereby these intricate layer structures as much as five meters tall can be formed by, well, by biology and what kind of biology?
Martin Van Kranendonk
Yeah. So, the great thing is that we have living remnants of these microbial communities that are still making stromatolites today. So, we can study those and understand very clearly how these rock structures are formed because they are forming in front of our eyes today. So, the most famous ones are in Shark Bay, Western Australia, and they're large structures up to a meter tall. And, they're made of rock for 99% of the structure. They are made of rock. But, the very topmost layer is green and squidgy. And so if you press on it, it's a little bit slimy. It's goopy. It moves. So, that's where we can see where the life is. And, of course, when we take a microscopic view, we can see the microorganisms. But what these communities do is, they make their structures in three different ways. One is that because they are a bit sticky, they actually trap sediment that's loosely in the water. So, just think about, like a piece of sticky-tape on your dining room table with the sticky side up. Any dust that's floating through the air will stick onto that sticky-tape. And so, stromatolites grow the same way. They trap, and then they grow over and bind the sediment, and make a layer that way. But they can do other things as well. And, because of their life force — their metabolism, they actually change a very small volume of sea water — they change its composition. And sea water has dissolved minerals in it. When you evaporate the water, it leaves behind those minerals, and so limestone reefs are one example of that. Limestone is dissolved in water. And we know that salt is dissolved in seawater. And so, what microbes can do, is they change that composition of the seawater, and that will induce the precipitation of minerals behind. And the microorganisms always stay one step above. They deposit the minerals below, and they keep growing upwards. And so, you build up these layered, kind of apartment buildings, if you like, from the ground floor up over time. And then the third way is that you can get a biological mineral precipitation. So, the presence of those microbes just induces the minerals to precipitate on them because they have a charged surface. So, there's been interplay of three different mechanisms that allow for stromatolites to grow. Some only grow by trapping a bonding sediment. Some grow really primarily by precipitating materials as a result of metabolism. And then there's any combination in between. And that's caused some of the confusion in the geological literature. How can we prove that these are made by life if you can also have a biogenic mineral crust precipitation? So it is a tricky game.
Oliver Strimpel
Are the layers analogous to growth rings in trees — each successive wrinkle, if you like, in the stromatolite?
Martin Van Kranendonk
It's very, very tempting to interpret the layers as annual growth layers, but the reality is that a colleague of mine has sat on a beach in the Bahamas and watched stromatolites grow for years on end. And it turns out it's not very regular at all. It depends a lot more on changing environmental conditions, and storm events turn out to be really important markers, and then they grow very slowly and not much happens for a long time in between. So, it's not as easy as saying they’re annual layers, but they do create over time if the conditions are right.
Oliver Strimpel
OK, so let's talk about the recent find of these exceptionally well-preserved 3 ½-billion-year-old stromatolites. Where exactly were they found and what did they look like?
Martin Van Kranendonk
So, these ancient structures were found in the Pilbara region of Western Australia, which is right up in the northwest corner of the continent in Western Australia. But they weren't recently discovered. They were actually found in the very late 1970s already. And they've been looked at for a long time, using mostly just quite simple techniques: their shape and the outcrops; some thin slices of rocks. But it's only recently that we've been able to really penetrate deep inside them into fresh materials. And that's the recent advances that have been made by our group. So, all the previous studies have been work from surface deposits, and the problem is, is that the surface of the earth is exposed to weathering. It's exposed to sun and rain and drying and oxygen. And in very old rocks, that causes the minerals to get altered by those surface processes. And so, even though the textures are really well preserved at the surface, what made the minerals and what the fine kind of textures were preserved, we couldn't determine — until we were able to retrieve samples of fresh, unaltered material from deep below the surface through diamond drilling. Just like the way that exploration companies will take a drill rig to explore from mineral deposits deep beneath the ground, my group in two different studies were able to take a drill into the Pilbara and penetrate below the level of the surface alteration and extract fresh materials. And, it's by studying those fresh materials that we've learned so much more about these ancient structures.
Oliver Strimpel
How do you know where to dig?
Martin Van Kranendonk
(Laughs) Yeah, well, that takes a long time. So, I've been tromping around that country for about 30 years now, and I'm finally starting to get a bit of understanding about it — because you're right, there are places where the structures might be faulted away. And so, one of the best places where we take visitors to come and see, we didn't drill there because there's a little fault structure and we weren't sure how deep it might have offset the units. So, we went somewhere else, where the conditions were much better preserved and we were able to drill there first in 2007, and we retrieved really good materials. And then more recently in 2019, we drilled in that similar locality, but in a couple of different places, and we retrieved really good material. And listen, sometimes it doesn't always work out. One of our drill holes in 2019 completely missed, and another one got stuck in a fault that was full of clay and we lost the drill stem. And, you know, it's a risky business, but the success is worth the effort. And we got 3 drill holes that were really perfect — right through the sequence. And we actually had one piece of drill core that we pulled out of the ground, and it had stromatolites exposed on the edge of the core. We could see it come out of the ground. It was just incredible!
Oliver Strimpel
How far down was it?
Martin Van Kranendonk
80, 90 meters down. The depth of alteration can extend down to about 70 meters, and so we purposely drilled down till about 80 or 90 meters to be below the effects of surface oxygen.
Oliver Strimpel
So, what exactly did you look at in these pristine rocks from this formation then, that you were able to recover from the core to try and clinch the biological origin of these very, very ancient rocks?
Martin Van Kranendonk
So, on the surface, these stromatolites appear rusty red and black, and they're wrinkly textured, and the wrinkly texture is the signature of biology, because that doesn't happen so easy with just geology. But then at depth, these structures that looked exactly the same, were bright yellow, made out of what's commonly known as fool's gold or pyrite - iron sulfide. And, when that's exposed at the surface, the sulfur combines with oxygen, and it weathers and it makes rust, basically. So, we were looking at the rusted equivalents, and we couldn't see the fine-scale texture. But down, under the ground, we found that perfectly preserved pyrite. And when we pulled those structures out, we could see these lovely little dome-shaped columns, in some cases only one centimeter high, but very clearly growing off of the ancient sea floor 3.5 billion years ago. And below that we could see ripples in the sandstone. You know, we could see where the beach environment was. We could really reconstruct the environment from these fresh materials. But most importantly, to be able to look inside the layers… So, remember when I talked about living stromatolites at Shark Bay, how there's that sort of green squidgy layer at the top? That's the growth material. Well, as the minerals precipitate, they sometimes grow around the organic matter that those communities produce, and they trap it inside. And a colleague of mine, Raphael Baumgartner, devised a way of etching away some of the iron sulfide and revealing what was buried entombed within the stromatolites, and he found remnants of organic matter. And so that was a real “aha moment,” because if they were just non-biological mineral crusts, you wouldn't expect to see organic matter. It shouldn't be there! And so we found this material preserved inside the iron sulfide, this wrinkly lamination that made the structure of the stromatolites, and I was “Oh my goodness, there's something unexpected here in terms of abiology.” But, there's a feature that's consistent with these structures being made by ancient life.
Oliver Strimpel
When you say organic matter, did you actually find biomolecules like lipids or something?
Martin Van Kranendonk
No. So these very ancient rocks have been heated up by metamorphic processes by temperatures and really just heat coming through the crust for so long. So, they've been up to about 300 degrees Celsius. They've been cooked. And that's too hot for biomolecules to be preserved. What we did find were two features that were of great interest. One is that the type of carbon preserved in the rocks was consistent with that heating. And so one of the options that the skeptics would say is “Oh, that organic matter could have been brought in later and has nothing to do with the material and the texture that you're looking at.” But there are two features we could argue against that. One is that it had been heated to the same temperature as the rocks around. So, we knew it was very early. It had to be there at the same time as the layers were formed. And the second was that the isotope ratio of carbon… So, carbon forms in a couple of different types, 12 and 13 carbon, the isotopes of carbon, and in just normal organic matter, that's fractionated at a certain degree. But then life changes that dramatically, because it prefers the lighter carbon, the 12 carbon. And this organic matter, it turned out, was fractionated in the same way that life fractionates carbon, down to about -27, -30 per mil. It's a very strong fractionation. And so again, that was consistent with life. The other really important textual observation was that there were no veins — so, later cracks that could have allowed the carbon in. It actually sat within these very, very small (and I'm talking nanoscale here, so extremely tiny much, much, much, much, much thinner than the width of a human hair), these kind of pores within the sulfur. And that showed again that the material was present as that pyrite was being precipitated, which we think was actually induced by the microbial activity.
Oliver Strimpel
So, that organic matter then is carbon? A certain kind of actual carbon that came out of the skeletons of these microorganisms?
Martin Van Kranendonk
It's a material that's called kerogen, and it's a complex mix of carbon-bearing molecules, but it's attached to oxygen and hydrogen. And because it's been cooked-up, it's quite a structured material. The most structured type of carbon is graphite that we use in our pencils, and that comes from much higher temperatures. That's got to be up at about 500 degrees. But keratin is an intermediate step. So, there's no indications of the primary organisms — the microbes, because it's all been heated up so much. But it's the decayed remains of microbes and the material that microbes make called extra-polymeric substance EPS. And what that it is, is it's kind of a sugary goo that microbes make, so that they can move around their environment. Microbes need to be able to move towards sunlight, towards food sources, and they do that by gliding through the sugary goo. This EPS. And in fact, it turns out that in stromatolites, the EPS goo is actually much more in volume than the microbes themselves. They make quite a soup that they can move around in. And it's this soup that actually gets preserved most commonly. And when we dissolve that pyrite away with a little bit of nitric acid, where we found ropy remnants, sometimes curling back on itself and knotted and then breaking into strands all entombed within that pyrite. And that looks exactly like the EPS of modern microbial communities. And it was that kind of feature that really sealed the deal. So, in addition to the carbon, the kerogen being preserved in the pores, having the right carbon isotope value and then this ropey remnant EPS, that was a triad that showed that, yes, these were formed by biology.
Oliver Strimpel
You also looked at the concentration of certain other trace elements in these formations, didn't you?
Martin Van Kranendonk
That's correct. So, we were able to take a very thin slice of one of these centimeter-tall stromatolites down onto the synchrotron in Melbourne, which is a source that generates high- energy particles. And we use the X-ray analytical facility. And we were able to map changes in element concentrations through those layered pyrite stromatolites. And it turns out that pyrite can absorb different elements as it grows, and so it can be rich in arsenic. It can be rich in copper, zinc, lead, etcetera. And what we found was this very gorgeous microscale lamination of changes between arsenic-rich pyrite, zinc-rich pyrite, and nickel-rich pyrite. And the exciting thing there was that it demonstrated there were textures of the layering that showed it wasn't just regular abiological growth, but highly irregular with, cutoff structures — growths like you see in trees, some layers that grow regularly and then they get overprinted by another layer, whereas mineral growth is very simple and continuous. So, there was a good textural contrast. But what we also know is that there's a very early type, a primitive type of microbe that prefers using zinc and nickel, and those are called sulfate-reducing bacteria. They take sulfate that's dissolved in the seawater in very early Earth conditions, and changing that to sulfide, so SO4 goes to S2. Most of it binds with iron, but because microbes of that type need nickel and zinc, they leave behind traces of enrichments and nickel and zinc. And that was another smoking gun to say: “Wow! This fits exactly what we predict.” And there it was, looking us straight in the face. It was very cool.
Oliver Strimpel
Wow. So, these metals are byproducts or part of the metabolism of these anaerobic microbes?
Martin Van Kranendonk
That's right. They need these trace metals to just enhance the chemical reactions between these major elements, sulfur and iron and stuff. So that's known by studying modern sulfate-reducing bacteria: they always find enrichments of zinc and nickel. And here we found them 3 1/2 billion years ago.
Oliver Strimpel
So, 3 ½ billion years ago was before the Great Oxidation Event, and when the Earth's atmosphere became rich in oxygen, as it is today, and so, did these early microbial communities then have different anaerobic metabolisms, so those that are around today in Shark Bay?
Martin Van Kranendonk
So that's something we're still trying to tease out of these ancient structures. It's very difficult to be able to prove a type of metabolism. Now, we think that the zinc and the nickel combined with fractionated sulfur isotopes that previous groups have documented strongly suggests that there was sulfate-reducing bacteria alive at that time. But there may have been other microbes present in those communities as well. And we think that because in modern environments, sulfate-reducing bacteria are quite a small volume of the overall community within a stromatolite. And when I say a community… at Shark Bay, there are more than 10,000 different types of microbes in a stromatolite. But there's usually one dominant type of bacteria. And, in modern stromatolites, that's cyanobacteria, what used to be called blue-green algae, it's actually bacteria. And that harnesses light energy, and it gives off oxygen as a byproduct. And they're the ones that changed our planet about 2 1/2 billion years ago to this beautiful blue marble that we live in today. But prior to that, there were almost certainly oxygenic photosynthesizers, as we call them, the cyanobacteria, present. But we don't know how far back in time they went. And certainly, in the very early earth there's no evidence for free oxygen in the atmosphere. And so the question is, what was the primary producer? What was the big volume of microbes that could build these structures, that, even though they're small at a microbial scale, are still very big, right, like a microbe is maybe 10 microns in diameter, 100 microns in diameter. And these stromatolites we're looking at are centimeters, to some case, 30 centimeters. So that's a lot of microbial activity. And we think, but we haven't been able to prove, that there were probably anoxygenic photosynthesizers — so, bacteria that had been able to harness the energy from sunlight but gave off a different byproduct from oxygen. It's actually a much more complicated metabolism to give off oxygen, and to break those molecular bonds than it is by using photosynthesis to metabolize without producing oxygen. And that's known to be a much more primitive lineage. And so it's likely that there were anoxygenic photosynthesizers. But we haven't been able to show the definitive proof for that. And that might be beyond our science at the moment, something we're still working on.
Oliver Strimpel
But it's interesting that even though the conditions could have been radically different back then, from what they are today, and therefore that the community, the population, if you like, of the microbes, would be quite different from those that are around today in Shark Bay, nonetheless, you have structures that are very reminiscent, and morphologies that are very reminiscent of what we see today. So, it's kind of interesting that the same microfilm and columnar branching and all the things that you described would be so closely analogous across the billions of years.
Martin Van Kranendonk
I guess in some ways it's not so surprising because what stromatolites do is… they actually, of course, they want to survive and outcompete the other aspects of their environment. Now, in the very early earth, there were no competitors like crabs or snails or seashells that would eat them today, but they were competing against the environment. And so their environment was introducing sediment into the area where they were trying to grow, and sediment kills them off. If there's too much sediment, they can't grow. And so stromatolites compete against this by growing upward to stay above the sea floor. And so that kind of aspect is common throughout time. If you only cover a flat surface, you can only grow so much. And so, just like humans build apartment buildings to fit more people into a small area, so stromatolites grow upwards because they're increasing their surface area; can fit more microbes into that volume of space. It's just a common survival technique, and life is about surviving. And so it's maybe not surprising that even the oldest remnants of life show that battle to survive.
Oliver Strimpel
You've been heavily involved in the search for life on Mars with the Curiosity and Perseverance Rovers. Is that because if there was once life on Mars, it might have resembled terrestrial stromatolites? Or because we think that Mars may once have had an environment similar to that of the Archean on Earth?
Martin Van Kranendonk
It's for both of those reasons, Oliver. We have an example of “1” here on Earth. Everything that we do in our exploration for the search of life elsewhere is based on our understanding of early life on Earth. Particularly around Mars, because Mars is a smaller planet, it became geologically dead much earlier in its history. Most of the rocks exposed on the surface of Mars are between 4 and about 3 billion years, and so really the Pilbara in Western Australia, one of the few places on the globe that has rocks 3 1/2 billion years with evidence of life, is the place that everybody goes to, to not only see the textures that life preserved, but also to understand the habitats where life lived. If you have a whole planet to explore, like Mars, where would you go to maximize your chance of success to look for life? And so, a large part of the study that my group has done is to really be able to understand very carefully the environments where life flourished on early Earth. And one of the exciting things is that we look at the Dresser formation, these ancient 3.5 ones, but they're also stromatolites at 3.4 at 3.35. And through the geological record we can build up a picture of where life lived. What's extraordinary is that, if you talk about older than 3 billion, there are many different niche spaces or environments where we find evidence for ancient life, and that's incredible. It just documents that life got started on Earth very early and diversified into these different environments, also, very early. And that gives us hope about searching for life on Mars, because Mars had that short history. But, if life got started on Earth and diversified very early, well, maybe it got started on Mars and was able to gain a foothold in that very early record.
So people come to look for the geological traces — what should we be looking for on Mars? What's the guide in terms of the textures and the fabrics and the environments, rock types, but then also what kind of habitat? So, would you go to a flowing river, or a quiet lake, or a delta, or a deep-sea hydrothermal vent? Or a shallow caldera? Where would you look? So, you've got all those choices, and our group has brought planetary investigators through the Pilbara, so that they can really see that evidence for themselves. So, we've had a little bit to do with site selection and understanding textures and what the search strategy should be.
Oliver Strimpel
If microbial life does indeed go back to 3 1/2 billion years ago, do you think it was somehow facilitated by some particular change in the environment at that time or do you think there was no particular reason for the emergence of life at that time, and if we just keep on looking, we might find still earlier life, perhaps going back to near the time the Earth first cooled enough to generate a hospitable environment, nearer to 4 billion years ago?
Martin Van Kranendonk
Oh, I would love that to be the case, Oliver, don't get me wrong. But, unfortunately, the history of Earth was that it had a very hot and almost cauldron-like beginning, and it's only until about 3.5 billion years ago that you get very well-preserved ancient rocks. So there are older rocks on Earth going back to about 4.03 billion. But they've been really cooked up. They've been brought down deep into the crust. They've been heated to temperatures above their melting point and then swirled around by tectonic movements. And there's almost no primary information left in the textures of the rocks, which is so critical for understanding early life. So the Pilbara, one of these sort of geological anomalies, just one of these fortunate places that have survived intact, basically, from three and a half billion years ago — most of the rest of the world (and we’ve looked now across a lot of the rest of the world) has rocks that are either younger or… older, but more strongly deformed. And as I mentioned earlier, there is a record — in fact, there's two different records — of ancient life from West Greenland in rocks that are 3.7 billion years old. And there are little isolated pockets within that otherwise kind of soupy mess, where there are preserved some primary textures. Now, the community is not convinced by these older records, because it doesn't have all the different arguments like we've described for the Dresser formation. They have one or two features that are really compelling. But, because they're so heated, and these are up to about 500, 550 degrees, they've got some big metamorphic minerals growing around them. People are less convinced of the fact that these were made by biology. I am personally 100% convinced, because I've been there. I've seen them. I understand that setting and so getting that record of life back to older periods in the geological record is really difficult. And you mentioned about life emerging at 3.5. So that story I just recounted actually suggests that life probably emerged much earlier, but we just don't have a record of it. And that's also part of the exciting reason for going to Mars, because, it doesn't have that young history of cooking things up. We can actually see a bit of our own early history on Mars, because it was frozen in time. And so there's value in going back to Mars to learn about what our early Earth might have looked like and perhaps even get a better record of where life might have got started, if it ever gained a foothold on Mars.
Oliver Strimpel
You're not suggesting a common origin of life between here and Mars, are you?
Martin Van Kranendonk
No, it's just that both planets probably had a similar very early start. And on Earth, that early start has been erased by younger events. But on Mars, that's been preserved. And so we might just be able to go and see what happened in parallel there. So, yeah, if we go and find signs of ancient life on Mars, that might be a separate beginning that would have huge implications for our understanding of the universe. If it happened on two planets next door to each other, oh, my goodness, just imagine how much life might be out there in the universe!
Oliver Strimpel
Martin van Kranendonk, thank you very much.
Martin Van Kranendonk
It's my great pleasure to talk to you, Oliver, and thanks for the opportunity to talk about stromatolites.
Oliver Strimpel
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