To study the history of life on Earth and look for it beyond our planet, scientists in the field of astrobiology look for signs called “biosignatures.” NASA Goddard researcher Heather Graham discusses some of the oldest evidence of life on Earth and what scientists are searching for when they look for biosignatures in ancient rocks. By looking at what life on Earth was like millions and even billions of years ago, astrobiologists can make predictions about what signs of life could be hiding in the rest of the solar system and beyond.
Jim Green: Life starts with chemicals. The chemistry of life is so important to understand.
Jim Green: So how do we know it’s life, if it’s life like we don’t know it?
Heather Graham:There’s a lot of ways that life could be different, but not related to life on Earth.
Jim Green:Hi, I’m Jim Green, NASA’s Chief Scientist, and this is Gravity Assist. On this season of Gravity Assist we’re looking for life beyond Earth.
Jim Green: I’m here with Dr. Heather Graham. She is an organic geochemist at NASA Goddard Space Flight Center. Heather has a profound curiosity about the natural world, the history of life, the vast connections between biotic and abiotic systems, and what evolution can tell us about our future. Welcome to Gravity Assist, Heather.
Heather Graham:Thank you so much Jim.
Jim Green:Well first off, what the heck is a biosignature?
Heather Graham: So a biosignature is just a physical indication of past or present life. That can be something as simple as a bone that a paleontologist might find, a footprint, an imprint, on the environment. Or a chemical compound that’s highly specialized for life.
Jim Green: So how do we know if a biosignature actually came from life? Can we be fooled?
Heather Graham: Oh yeah. That’s a probably the most important part of biosignature science is making sure that you are looking at something that came from life, and not being fooled by something that can be made by a physics, by nature, by geology, by abiotic chemistry.
Heather Graham: There are a lot of things that we associate with life that are actually very common, being made out in stars. A lot of the chemicals that we associate with life are really an inheritance from the universe. We also have to be very careful to keep our work clean, so that we’re not contaminating it with contemporary life when we’re trying to understand past life.
Jim Green: Biosignatures require certain chemicals. Is there some underlying composition that you look for in a biosignature?
Heather Graham: Yeah. So it really depends on where you’re looking, and what you’re looking for. For example, a really common biosignature that we hear about a lot is DNA. And that’s something that’s universal. Every organism has it, so it’s easy to say, “Oh yes, this is definitely from life. This is a molecule that’s too hard to make without biology being involved.”
Heather Graham: But DNA is very ephemeral. It goes away quickly in nature. It’s broken down by other organisms, and reused very quickly. But there are other chemical compounds that last for a really long time, especially in the geologic record, in the rock record. And those are the things that we can look at as being a more robust signal of life on long time scales.
Jim Green: So getting down to the basics, the really important chemicals, carbon seems to be one. If we look at what we have inside us, it’s carbon, hydrogen, oxygen, nitrogen, phosphorous and sulfur. So are those the fundamental compounds we ought to be looking for?
Heather Graham:So those are the fundamental compounds that we associate with life on Earth.
Jim Green:Life as we know it.
Heather Graham: Life as we know it, correct.. The kind of work that we’re doing here at Goddard, and the research group that I’m involved in, doesn’t always presuppose that though. We’re interested in biosignatures that might not just be about those particular elements. Or more importantly, other elements that might be biosignatures as well.
Jim Green: How do we figure that out? How do we figure out what are the more important elements?
Heather Graham: That’s really from just observing nature in its completeness, in its totality. Not just by looking at life, but by looking at nature in context of its environment to understand what it’s taking from a chemical system to build itself, what it’s eating, what it’s respiring, breathing out, what the by-products of its metabolism are. And then you can track down what are the important things that are moving through a system. And that movement is the essential signature of biology. If something’s static, it’s unlikely to be involved.
Jim Green: So the concept of you are what you eat is true?
Heather Graham: Absolutely.
Jim Green: So we actually pull things out of what we eat, and plug them into our body.
Heather Graham: Yep. So we take carbon from all these other sources. And interestingly though, carbon dioxide is our major by-product. We all breathe it out. But there are lots of organisms that don’t do that with carbon. They use metals and other strange things. And we find all these metals in certain places in the geologic record. And even though we don’t see that organism, we know that this is something it left behind.
Jim Green: So where do we find biosignatures?
Heather Graham: Usually, when we’re thinking about biosignatures, we think about the rock record, we think about geology. And really, that’s just because that’s a really stable repository. To find a biosignature, you have to find a place where a molecule or a part of an organism is protected from degradation. So, we think of places where there’s not a lot of oxygen, since most other critters that would want to eat that organism are going to be using oxygen.
Heather Graham: We look for rock, areas of sediments, that have been protected from oxygen in the atmosphere. And the rock record’s interesting, not just for people who are interested in land, but a lot of the rocks are actually old ocean. It’s all the ocean sediments. So we can peer into past ancient oceans just by going to certain rocks on Earth.
Jim Green:Well you brought a really nice friend with you. It’s a fossil. What is that?
Heather Graham: So this is a really special fossil. This is from the Ediacaran period. That’s about 600 million years ago. And this was a time in Earth’s history when biology got really experimental. And during this time period, we find all sorts of crazy organisms that only exist during that time period. Biology was trying just all sorts of body plans, and ways of living on the surface, or just below the surface, in really neat ways.
Heather Graham: And these kinds of fossils are only found in a couple of really special places on earth, Nevada and Namibia.
Heather Graham: So this fossil, as you can see, there’s these long tubes, and there’re ridges along them, and they crisscross. And what’s sad is this piece that I brought to you is only about the size of my palm
Heather Graham:And what you’ll see is there’s all these tubes weaving back and forth over each other. But not in a random way, in a very special pattern. And that’s part of what makes us realize this is biology, and not just rocks that happened to fall in a really interesting pattern. This has intentionality and there was energy put into a system to leave this texture.
Heather Graham: This strange structure, there’s a couple of different ways you might be able to try and explain it. Maybe it’s a texture on a rock from a biofilm. Maybe it was a tube worm that was living below the surface. But what’s really important about this is we don’t have any physical way, without biology, of making a pattern like this. And that’s what makes it a biosignature. You can do all sorts of experiments with different sizes of rocks, and different chemistries of solution was of water, but you’ll never make something that looks like this.
Jim Green: Yeah, it looks creepy.
Jim Green: So I heard there’s like 4,700 minerals on earth, and about 300 of them could only be made by life.
Heather Graham: Yes, absolutely. So I know I am a chemist, so I’m biased towards organic chemistry and I’m giving a lot of examples of organic chemistry that are biosignatures. But really, there are lots of minerals that are biosignatures as well. There’s a lot of great work that’s been done to show what minerals you absolutely need.
Heather Graham: For example, oxygenic life, the kind of life that made oxygen, all the algae in the ocean that made oxygen. There are minerals that would only be associated with those really high levels of oxygen, that those organisms created. So if those organisms never developed on another planet, we would be unlikely to see many of those minerals on those planets.
Jim Green: How long do these biosignatures last? Once they’re in the rock, is that it?
Heather Graham:No. Actually, one of the real arts of biosignature science is knowing what things look like when they break apart. Unfortunately, a lot of the chemical compounds that we use as biosignatures don’t last in that state that your body or another organism’s body made them. They break down in very particular patterns. And so, we find those degradation products in the rock record.
Jim Green: So once life is created, and it decomposes, you end up with the parts. And so figuring out, do you have all the right parts, or even some of those parts, may or may not be preserved over time.
Heather Graham: Yeah.
Jim Green: That ends up being a real problem. So what kind of tools do you use to really identify these parts and pieces in biosignatures?
Heather Graham:Yeah, this is a real puzzle. So this is the kind of area we would really think of as interdisciplinary science. When I’m looking at a particular sediment for example, I’m looking for particular compounds, I’m taking a lot of information that I’ve learned from other geologists that have described that rock. So I will know something about, was this rock once a lake? Was it the ocean? Was it buried and heated? So I’ll have a lot of information from geology to help me direct those tests about what sort of molecules I’m likely to find.
Heather Graham: And the other important thing about these molecules that we think of is biosignatures that are being preserved in the rock record, is there an example of a community. They’re usually not just one particular organism, you’re looking at a habitat, you’re looking at a whole bunch of different metabolisms. So you’re really not just saying, was there life, was there not life, but you’re getting a sense of what the ecosystem was.
Jim Green:Another thought I had, relative to biosignatures in rocks, is a structure. A structure that, how geologically could that have possibly have happened? Like a stromatolite. So when we study biosignatures, how does that help us think about the origin of life here on Earth?
Heather Graham:Yeah, that’s really interesting. So a lot of these chemical compounds that we’re thinking of don’t really last on really, really long timescales. And when we’re thinking about something like the origin of life, a lot of what we’re looking at is physical structures in the rock. Really old evidence of cells, really old evidence of biofilms, which are just big accumulations of microbiota together.
Heather Graham: So we’re looking for those physical things in the oldest rocks. And that’s where textures and rocks become really important. And also something that you want to be able to make sure that those textures aren’t something that nature makes all by itself without biology being involved.
Jim Green: So the key to finding life way in the past. Is to start looking at the old rocks. Well, how old are the rocks here on Earth?
Heather Graham: So that’s a bummer about Earth, is that most of our rocks have been recycled-
Jim Green: Oh no.
Heather Graham: And heated, and cooked, and moved, and washed with lots and lots of water. So it’s really hard, actually. We think of Earth as being this life saturated planet. And so, it must be so easy to find life. But when we look in the deep past, it’s actually hard, because much of that evidence of life has been destroyed by tectonics, by the planet moving around. We’re a very dynamic planet.
Heather Graham: So we have to look for what we call quiescent parts of the planet. These are old cratons, the oldest rocks on earth that are up in Canada and Greenland, and places like that, where we find really interesting textures in the rock. And we like to think that those are probably the earliest forms of life. And then that gives us a sense of, what was the chemistry in that early ocean? What was the planet like? And that gives us a sense of the kind of chemistry that was possible.
Jim Green: So just recently, a team from the Mars 2020 Rover group, this is the group that is going to be looking at the rock record from Mars, trekked out into Australia. And I was wondering, why are they going to Australia for a meeting? Well, they’re out in the middle of nowhere in Western Australia, and they went to the Australian chert.
Heather Graham: Cool.
Jim Green: Yeah, so-
Heather Graham: Jealous.
Jim Green: And it’s a trek, let me tell you. But that’s old rock.
Jim Green:Yeah. So what do we know about that Australian chert?
Heather Graham: Yeah, so those are some of the really old rocks where we find the oldest evidence of life. What’s really special about some of those rocks in Australia is, you can find really old evidence of microbial biofilms, these mats, microbes that form these layers and they grow up, up, up upon each other in these layers. And that’s a stromatolite, like you mentioned.
Heather Graham: And what’s really interesting is, you can see living examples of those right off the coast in Australia. So it’s such a special place, to be able to have that window into the deep past, and then see something that’s contemporary, that still has the same lifestyle. And is conducting its biology in pretty much the same way for billions of years. It’s a really special place.
Jim Green: Yeah. In fact, when they measure the date at these rocks, they find they’re 3.6 or 3.8 billion years old.
Heather Graham: Yeah.
Jim Green: But yet, we know the earth is 4.6 billion years old. So there’s nearly a billion years record that we’ve lost.
Heather Graham: Yeah. And the really sad thing is… Because that’s our earliest evidence, we know that somewhere in that time period between formation and that 3.8-ish number that you give, that’s the exciting part. That’s when life evolved, and we don’t have really anything to go on there. And that’s where experiments in the laboratory can become really illustrative for giving us a window, and giving us something to imagine for that past.
Jim Green: Well, this is where Mars comes in a really neat way, because it doesn’t appear to have had some plate tectonics, but not like Earth. And it appears to have been a blue planet like earth early on, but then went very arid. And so maybe the record of life starting, perhaps, on Mars before 3.8 billion years, is attainable in the rock record there that we can bring back.
Heather Graham: Yeah, that’s what’s really exciting, is: It hasn’t had that heating and cooling and moving and crushing that Earth has. And that really can tell us a little bit about why we choose to go the places we go on Mars. That we’re looking in these sediments, that we know were buried once upon a time. And if there is something organic there, we’re looking in the best places to find something that wasn’t exposed to the surface where it could be altered.
Jim Green: So, what’s an agnostic signature of life?
Heather Graham: So that is a biosignature where we’re not presupposing a connection to Earth biology.
Jim Green: A carbon-based life, like we know it?
Heather Graham: That could be one or it could be carbon-based life that has different informational polymers besides DNA, something like that. There’s a lot of ways that life could be different, but not related to life on Earth. And these kinds of studies get really interesting as we’re starting to move farther and farther out into the solar system, where there’s less and less likelihood that we’re related to creatures out there, if they exist.
Jim Green: So do you think we’re alone in the galaxy? Do you think life is confined to just Earth?
Heather Graham: I tend to think not. And I base a lot of that on probability. This is another area of expertise that there’s lots of people working in biosignatures, and that’s the kind of people who think about statistics in likelihood and probability. And it’s rare you see something as a one-off in nature. If something works, it usually happens again and again. And so I just can’t imagine that we’re the only instance of that kind of unique chemistry happening.
Jim Green:So if you think we can find life beyond Earth, where do you think we’ll find it?
Heather Graham: Where? Oh, that’s interesting. Boy–
Jim Green: In other words, will we find it first in the solar system? Or will we find it in exoplanets first? Planets around other stars.
Heather Graham: I would tend to say that we’re probably going to find something unambiguous in the solar system first. And I would say that’s really just because for this kind of evidence, you really do need a bunch of different ways of viewing the problem to be convincing. We can’t just find one molecule and say, “Oh look, that’s life,” because you need to be able to see it in context.
Jim Green: So I noticed you have some really unique tattoos. Just long streaks that are emanating from particular points. What are they?
Heather Graham: Yeah, these are high-speed photographs taken from inside a bubble chamber where they were crashing particles into each other in a very particular way to see what kind of other particles are made. And this is imitating processes that happen in certain kinds of stars that make light that has mass. So light from our star is massless, but that’s not always true. And I got these tattoos because light was something that was a big part of my PhD work. I actually looked at biosignatures to tell us about light harvesting molecules and photosynthetic organisms.
Jim Green: Well, Heather, I always ask my guests to tell me what was the event, or person place or thing that got them so excited about being the scientists they are today? I call that a gravity assist. So Heather, what was your gravity assist?
Heather Graham: Oh, I think if I think of any person, place or thing, the place that immediately pops to mind is the community college that I went to, Santa Monica College in LA. It’s a really great place, and I think what made it so great is I got to take so many different classes. There’s all these different people who work on biosignatures, and you need to have a really firm grasp of physics and geology and chemistry and math.
Heather Graham:And that’s something I was able to do at this community college because I had the grace to just explore every topic I wanted to. I took every, every science class they had. I took field ornithology. And I had a wonderful organic chemistry professor there, Jamie Anderson, who just pushed me into every research opportunity he could find. And I think if I hadn’t been given that allowance to have all that curiosity, I wouldn’t have the background I really benefit from in biosignature science.
Jim Green:You had mentioned to me earlier that you were the first one in your family to go to college. How hard was that?
Heather Graham: Yeah, I am the first person. And I think it’s hard because I don’t think it was something I ever expected I would do. I didn’t actually go to college until I was an adult. And I’m not saying that people that are 18 coming out of high school aren’t adults, but I was a real adult. I was 30 when I went to college. And I think it really changes the intention, and how you approach your studies. I wasn’t doing it because my mom and dad told me to. I was doing it really because these were things I wanted to know.
Heather Graham: This was interest that I had. And I took it really seriously because I didn’t have any examples in my family of how to do this. And I was really helped by a lot of wonderful professors. And when I think about the way I approached it, and that curiosity that I brought to studies, there’s a saying by an old author, Charles Baudelaire, “That I sought the why of it, and turned pleasure into knowledge.”
Jim Green:Well, thanks so very much. This has just been a delight, and you’re a wonderful biosignature.
Heather Graham: Oh, why, thank you.
Jim Green:Well, join me next time as we continue our journey to look for life beyond Earth. I’m Jim Green, and this is your “Gravity Assist.”
Lead Producer: Elizabeth Landau
Audio Engineer: Emanuel Cooper