Interview with John Baross

November 22, 2005

 

CI:      I usually start with a bit of personal history. How did you become an astrobiologist?

JB:     I started out as a chemistry major wanting to go to medical school. I was part of the molecular biology revolution, I was really fascinated with biochemistry and molecular genetics. I took my first course in microbiology in my junior year and loved it, and opened up a double major. In that same time frame there was money coming into the microbiology department as part of the pre-Viking experiments—in addition to people going to Antarctic dry valleys and looking for microbes, there was a program to look at what kinds of microbes contaminate the nose cones of space rockets and whether or not these organisms could survive that transport. At twenty years old I went to Lompoc with a professor and swabbed the nose cone of a Delta rocket just before it took off, and that was my first astrobiology research project.

That made me want to learn more about environmental microbiology. In particular, I was interested in the role of viruses in the environment, because viruses at that point were studied primarily as genetic tools. I felt we needed to determine their real role in the environment, besides being a genetic tool or killing off organisms. I went to the University of Washington on a special fellowship because they had a marine biology program and they would let me do what I wanted. So I pursued that end of it and became more involved in marine microbiology—but always thinking that, somewhere along the line, this astrobiology thing was going to be important. As an undergraduate I had a sign made, “cosmo-geo-microbiology,” and I put it in my office even as a student. I still have it in one of my labs. I’ve always have this cosmo-perspective.

CI:      When did it become clear that all of the microbial life forms under study were the tip of a much larger iceberg of organisms that were difficult to culture, organisms we knew very little about?

JB:     I don’t think the community was persuaded until the late seventies. I tell my microbiology class that when I was a first year graduate student, I was told by an eminent professor in biological oceanography: “We will not teach anything about microbiology because it’s insignificant; those organisms are small and their numbers are small so they don’t do anything in the oceans, therefore we’re not going to talk about it.” And there I was as a first-year graduate student, wanting to do marine microbiology! It wasn’t until new methods were developed that we realized they play a dominant role and are probably the most important organisms in the ocean. In the nineties, we could finally understand that these organisms are dominant, and that there is a high diversity of organisms we know nothing about—maybe 99%. They’re involved in virtually every kind of geochemical cycling, including primary production. They’re some of the dominant carbon dioxide-fixing organisms in the marine environment. In this same time frame, when I was starting my post-doc in the late 1970s, submarine hydrothermal vents were discovered on the bottom of the ocean, deep-sea volcanoes.

CI:      What’s the story on that—why were people looking, and what were they looking for?

JB:     Years before the expedition was mounted to send a submersible to the bottom of the ocean, temperature anomalies were measured in the water columns near areas where we believed the plates were separating, in particular off the Galapagos Islands. We’re talking about hundredths of a degree, but to measure hundredths of a degree increase in a two- or three-thousand-meter water column indicates that there’s a major heat source. A group of geophysicists and geochemists and geologists mounted an expedition based on those temperature data to search for deep sea volcanoes, using the submersible Alvin, in 1977.

No one thought that biology would be important if these environments indeed existed, because at two or four thousand meters down, we find muddy bottoms with sparse populations of animals that are dependent on whatever organic material floats down from the surface, from photosynthesis. At that time I was a starting post-doc in the microbiology department, working on Antarctic stuff, and the call came back from the ship that on one of their dives they found this incredible oasis of marine animals—huge clams, tubeworms close to a meter high.

CI:      You must have thought, “What were they smoking?”

JB:     Well, they were smoking, it was that era. [Laughs] So what do we do, how do we preserve these animals? There was a tremendous amount of excitement. The whole crews became centered around these amazing animal communities and how they were living, how they were being supported. When the expedition ended I got hold of some of the water samples, which were preserved in various forms of alcohol—including tequila and other things. I made some of the first counts of those organisms and got hooked into the field.

This was 1977. In 1978, another expedition mounted by some geophysicists discovered black smokers off Peru, the East Pacific Rise. That excited me because they were measuring 350 degree Celsius water coming out of these big smoker vents. Back in the Galapagos, there was warm water venting out of the crust—usually a few degrees above the ambient seawater, which is about two to twenty degrees Celsius. That was warm, but not really hot. What got me excited was the idea that here, because of depth, you could actually maintain liquid water at very high temperatures—in this case, because of the seawater, you maintain liquid water up to about 450 degrees Celsius. Here’s the chance to test the hypothesis that liquid water, and not temperature, might be a limit of life. At the same time, it got me extraordinarily interested in what kind of life might exist in crustal material, the sub-sea floor, which nobody knows anything about—it’s a brand-new environment.

           I want to backtrack slightly. Carl Sagan had given some talks at the University of Washington when I was a graduate student, a year or two before Viking. He was saying that we may not find life on the surface of Mars, but we’re likely to find life on the subsurface of Mars if we can get down into the regolith. He was talking about organisms resembling earthworms, he wasn’t talking about bacteria. But I got this idea of the sub-surface—not only understanding it on Earth, but that the sub-surface was where life might exist on other planets and moons.

CI:      While Viking was returning ambiguous results—but showing a pretty dry, arid, and dead-looking planet—people were simultaneously becoming aware of these extraordinary ecosystems on Earth. That must have been when astrobiology shifted; the nature and extremes of life on Earth have become one of the central pillars of astrobiology. Is that what started it?

JB:     There were a couple of things. There was definitely a sense of depression during the Viking period. It was an expensive experiment that failed; the normal conditions on the surface of Mars probably aren’t going to support life. That set astrobiology back considerably. Meanwhile, hydrothermal vents were discovered, along with a variety of new research methods—particularly in microbiology and ocean sampling.

The interest in extreme environments came about as a result of the discovery of hydrothermal vents. In the eighties we discovered all these incredible, interesting, bizarre, novel microorganisms. In that same time frame, molecular methods were developed to show that many of the organisms found in these extreme environments represent a separate domain of life, distinct from normal bacteria, and these organisms may represent the most ancient groups on Earth. We started making the connection between analog extreme environments, origins of life, and life on other planets.

CI:      You’re talking about the Archaean branch of life. That must have been controversial when it was first put out by Carl Woese. How good were the dating techniques?

JB:     Time was not put on the genetic trees. It was the distance between certain very highly conserved gene sequences, the distance one organism has from another. There are some conserved sequences in genes shared by humans and archaea that grow at 110 degrees, and bacteria that grow at pH 2, and tomatoes, fungi, et cetera. We have an evolutionary molecule to play with. By looking at that molecule we’re able—for the first time ever—to compare unicellular organisms like bacteria to humans or tomatoes or fungi.

That was exciting, a lot of people jumped in. It wasn’t accepted initially, and in some cases still isn’t accepted by people who work with metazoans and higher organisms. They feel that, in some cases, structure may be more important than these molecular clocks. Many papers in the last four or five years have tried to put time onto these genetic trees—not only using the gene that Carl Woese pushed, but a variety of other genes. We’ve matched these trees with paleontological data and geological data, and placed different organisms in the different time frames. We’ve been able to do that; there’s some controversy there, but also some strongly accepted ideas. Where we have a problem is before about 2.7 or so billion years ago, when it gets more difficult.

           A lot of things came together that really helped the development of astrobiology. The National Science Foundation started a program called Life in Extreme Environments, which took off in the mid-nineties. They wanted an interdisciplinary approach to study extreme environments. Out of that came the teams of people who eventually started the astrobiology program. That NSF program was instrumental in getting teams of people together to put together some of the first NASA astrobiology proposals.

CI:      Earlier in your career, when you were a junior professor and not yet established, did your oceanographic colleagues look at you strangely because of the “astro“ part of what you did? How strongly were the disciplinary tramlines drawn back in the eighties?

JB:     I didn’t call myself an astrobiologist back in the eighties, and I wasn’t working directly on that topic. It was clearly something we were thinking about. As soon as there was any kind of culture in which you could actually call yourself an astrobiologist, the community became extremely divided, as it is today. Many of my close colleagues think astrobiology is nothing, that it has nothing to give. I call it the science of optimism, because we’re going after something we know may not exist, and even if it does exist it may take more than our lifetime just to find it. That’s where astrobiology tends to bring philosophy and even theology into its field. A lot of people who enter astrobiology are attracted to the marriage between philosophical issues and astronomical issues.

CI:      Even in advance of the hoped-for dramatic discovery of biology elsewhere, there’s this wonderful pincer movement going on. Astronomers are trawling extrasolar planets at the rate of twenty or thirty a year, and they’re implying all sorts of potentially habitable environments; meanwhile on Earth the envelope of extremophiles opens up year by year. The framing of the subject is exquisite. I wouldn’t be surprised at this point if there was microbial life in a lot of other habitable places beyond the Solar System.

JB:     Absolutely. Each year that goes by gives me more and more confidence that there is at least microbial life out there; to me it’s not even an issue. The issues are: how do we detect it, how do we get it, how do we discover the specific habitats on these planets where it may exist? Are there separate origins of life, or different evolutionary pathways? Even within the realm of carbon-based life, what are the other options besides the Earth life option? Those are pressing issues. I’m chairing a task group at the National Academy on what we call weird life, or the limits of organic life in the universe. The motivation is that we don’t want to miss out on finding life by being too Earth-centric with our detection methods.

CI:      I wanted to ask about origin-of-life issues. What’s the evidence that the Archaean organisms, or life’s earliest organisms, were extremophiles? How similar are existing extremophiles to their ancestral versions?

JB:     We don’t know for sure that the most deeply rooted organisms on our trees of life are really the earliest organisms. These deeply branched organisms give us some clues—in terms of their metabolism, the way they derive energy, what they feed on—and those clues can tell us something about those early processes among organisms. But it’s hard to extrapolate to what the early life forms were like.

We know that we have some common ancestor. All extant organisms have the same genetic code, the same way of using a genetic code to translate gene messages into proteins, a limited number of ways in which you can derive energy—from either light or chemistry. There’s biochemical unity. This means that before the separation of the three major domains of life, there was a genetic pool, probably of high diversity, with lots of experiments going on within the evolutionary context. That eventually selected out mechanisms—genes—that were the best, and created an ancestral pool of organisms. That’s what we’re most interested in, how we got this common ancestral pool of genes, and how it developed into our unity of biochemistry. Why that one version and not other alternatives?

In looking at extant organisms, many of them deeply rooted on the tree, we’re trying to figure out what some of those ancestral genes were like, and perhaps how the genetic code was formed. How did genetic material get from one organism into another to homogenize this diversity into a common pool? Understanding that means going back to a better understanding of the origins of viruses, a better understanding of how organisms exchange genetic material, of how to make a large genome or a large chromosome in a relatively short time, of how cells fuse together, of symbiotic associations—all this becomes extraordinarily important in developing that ancestral pool of genes that led both to the unity and the diversity that we have today.

It’s more of a top-down study with regards to extremophiles and what they can tell us about that scenario, rather than the idea that they were really ancient organisms. Instead, they may represent ancient metabolisms, or they may have some genes carrying out functions that are very ancient. But these are still important aspects of four-billion-year-old organisms, organisms that are sophisticated, high-tech.

CI:      What is the range of metabolic diversity of life’s earliest organisms? Many of them did not rely on photosynthesis. Were there a lot of chemical energy sources?

JB:     It’s hard to know exactly what was going on, but I would claim that a hydrogen-based ecosystem was the early driving energy source. That means organisms, for example, that can make methane as hydrogen reduces carbon dioxide. There are other groups that use hydrogen and carbon dioxide coupled with sulfur. There appears to be a diversity of pathways for reducing that carbon dioxide to other organic compounds. There may have been a wide range of ways to reduce carbon dioxide with hydrogen at some point, but the ones we’ve been studying are primarily the pathways that make methane, which is also considered an ancient pathway. I think hydrogen was a crucial early energy source.

A second one would be what we call the anoxygenic photosynthetic microorganisms. They photosynthesize but in the absence of oxygen, and they don’t make oxygen; so rather than split water, they split hydrogen sulfide, which was very plentiful in the early stages of ocean chemistry. What’s interesting about these organisms is that in the process of reducing carbon dioxide, they oxygenize hydrogen sulfide, so you end up with oxidized forms of sulfur including sulfates. We do see sulfates at greater than three billion years ago, so we know there was some process making and reducing sulfates microbiologically in a very ancient system. I’ve given you two metabolisms; they can also absorb at wavelengths closer to the infrared, so they can be out of the UV penetration range in the ocean. I feel that where there’s hydrogen on any other planet, along with carbon, there’s a key energy source.

CI:      Hydrogen’s so abundant. It shows how primitive humans are, because we’re still trying to get to a hydrogen economy. Microbes figured it out way before us.

JB:     Yes. One of the things that came out of my involvement with the Vatican Summer School in 2005 was the cosmo-microbiology that affected how I think about the formation of planets and the fusion of heavy metals. I can see parallels in the way proteins are made and were made early on—primarily iron or other metal clusters with sulfur. I see all the evidence for metabolic pathways and other catalytic systems without protein on metal-mineral surfaces. We see these catalytic reactions occurring once metallic compounds start forming, not dissimilar to how planets are formed. Little organisms—essentially little planets—are formed in very much the same way early on, with a lot of spontaneous creation of structure and of energy-yielding pathways in the absence of any sophisticated catalytic protein, perhaps in the absence of any kind of information macromolecule. I think there’s something inevitable about creating some kind of carbon-based life, based on the physics of how elements come together and how reactions occur on various metallic minerals.

CI:      All these metabolisms you’ve talked about are speculated to be primitive metabolisms. Are there modern analogs, or living relics if you like, of these modalities that can be studied?

JB:     Absolutely. One that’s being studied now is a pathway called the reductive TCA cycle, the cycle that we and other respiring organisms use to derive energy from oxidation reactions that occur in our mitochondria, for example. You feed organic material into the organism and it carries out these various oxidation-reduction reactions—it produces carbon dioxide and it produces energy as ATP. If you take that cycle in reverse, then you’re pulling down carbon dioxide and reducing it into organic material and you’re using energy. As it turns out, the reductive TCA cycle exists in most organisms, and it’s thought by many to be the most ancient of metabolic cycles. There are groups now finding that we can almost replicate the whole cycle, without enzymes, on pyrite and other minerals. We can form these intermediate organic compounds for life and also derive energy for those reactions directly from minerals. There’s a lot of interest in mineral catalysis.

CI:      When we’re thinking about how early life might have started, and we’re asking the question empirically, at some point we run out of unaltered rocks; even evidence of the molecular tracers gets tricky to interpret. From what you know about the likely metabolisms and Earth conditions, how early do you imagine life started, and where? If not Darwin’s warm little pond, was it a deep sea vent?

JB:     There’s pretty good evidence that there was life somewhere around 3.8 billion years ago or even slightly older. There’s also evidence that there was an ocean at somewhere between 4.2 and 4.3 billion years. In that time frame, between 3.8 and 4.2 billion years ago, we were being blasted by large bolides—planetessimal bodies—and models indicate that some of those would have evaporated a worldwide ocean. So that would not have been a good place to try to start life. There might have been some small land masses that could protrude through that early ocean, but continent formation was not robust before 3.5 billion years ago, and less than five percent of our continental mass was around prior to that.

I think the best place for life to have originated would have been in the sub-sea floor associated with hydrothermal activity. There are a lot of people who think along those lines today, given the geological history of the early Earth. You can generate energy—catalytic surfaces with minerals in the Earth’s sub-surface that may have produced the organic compounds and condensed them into larger molecules. It’s certainly going to be a site to look for various intermediates in the origins of life, and we may have to think a little outside the box. Some people are looking at metabolic pathways in the absence of protein enzymes using hydrothermal vents. Others are looking at a variety of other catalytic functions that may reduce nitrogen gas into ammonia for life, and they’re looking at hydrothermal models.

The big problem is how to make nucleic acid. The gradients in deep-sea hydrothermals, in bottom ocean water, probably offer our best shot as a setting for understanding many of the components of the origins of life. That environment, the sub-surface associated with hydrothermalism, would have been a plausible habitat for the earliest microbial ecosystems because of the abundance of energy and the abundance of carbon. Everything is there, and at the same time they haven’t yet evolved mechanisms to protect themselves from ultraviolet radiation, particularly in the absence of an ozone layer, which wasn’t around when early life evolved.

CI:      I have trouble understanding this: if there was an origin in that kind of environment, how would it propagate and take a grip? These sub-oceanic environments are little ecosystems or worlds on their own, but aren’t they transient at some level? It’s not a long-term stable environment. So if you had a set of these isolated ecosystems originating life, wouldn’t each be somewhat different from another, and how would biology become global?

JB:     First of all, there’s no comparison between the early Earth and what it looks like today. Tectonics would be so much stronger and hydrothermalism would be robust. We’re not looking at major plates moving around, we’re looking at a jigsaw puzzle, with the whole crust being hydrothermally active and lots of subducting crust. You still have a lot of heat and you still haven’t formed large plates. Hydrothermalism was universal on the bottom of the ocean. Those environments were not transient.

Secondly, when you form new crust, called ultramasic rock, it’s very high in iron and magnesium, usually the silicates. That in turn reacts with water and produces hydrogen, and in the process also produces heat; it’s an exothermic reaction. It’s one we’ve just recently discovered in the mid-Atlantic ridge, and we realize that also would have been rampant. So we have these magma-driven systems, and then we have new rock, these ultramasic rocks, interacting with sea water that’s also producing heat and lots of hydrogen. That was universally found throughout the bottom of the ocean; as soon as we’ve got liquid water accumulating on Earth, we’ve started hydrothermalism. If one particular vent site clogged up, there were certainly others popping up all around it. That would perhaps enhance chemical reactions by creating even more gradients, in terms of temperature and different quantities of minerals.

The sub-surface back in the Archaean, and even to some extent as it’s localized today, is an open system that behaves like a chemical reaction. Sea water comes down, it interacts with hot rock, extracts nutrients and minerals; it’s basically the whole periodic table and all these rocks and volatiles, and then all that remixes and creates another set of minerals and volatiles. All in an open system. There’s nothing like it that we could have constructed on Earth, and it gives the most options for creating the most diverse kinds of chemistry, and also the most options for habitats.

CI:      So when we’re thinking of other planets and other habitable zones, the places most likely to be living worlds are very dynamic environments with lots of sources of free energy. They sound very different from current Mars. Perhaps instability in the environment isn’t necessarily a negative thing as far as the early stages of life go? People talk about the Goldilocks or the “rare Earth” concepts, where the habitable nature has to be in a fairly well-defined bound of physical conditions without too many extremes—but maybe that’s not the case.

JB:     In terms of supporting life, you definitely need a continuous source of energy; light does that, and hydrothermal vents do it with chemistry. Then you need all the other basic elements that are part of life—not just an energy source, but carbon, nitrogen, phosphorous, and at least twenty different trace metals that are essential for living systems. In a sense you have two kinds of environments that can support life: one is photosynthesis, provided all the other nutrients are there; the other is more of a chemosynthesis, which, under hydrothermal conditions, can support life in the absence of photosynthesis. It’s the only alternative I know of to a light environment that can actually support life with a primary producer and all the secondary producers, totally in the absence of photosynthesis.

If we go to Mars, where surface life is impossible, there’s plenty of water; most of it is ice in the regolith. There is evidence of past volcanism that has spilled out water, and if there’s still any kind of a heat source, then it’s possible that there are still small pockets of heat generating the kinds of chemistry that can sustain life, and perhaps enough heat to melt through part of the buried permafrost. That’s where we may have life.

           Then there’s the discovery of methane on Mars—where is that methane coming from? We can probably rule out a biological source. If methane is being generated geophysically, then the most likely processes involve hydrothermalism. If there’s water buried somewhere down in the sub-surface along with ultramasic rock, that’s an exothermic reaction that generates hydrogen. That hydrogen continues to react with metals—iron and nickel and others—and minerals to produce a variety of organic compounds, including abundant methane. We see methane abundances that can reach extremely high levels in natural systems where ultramasic rock is reacting with water. It’s also an exothermic reaction, so that could produce heat up to 150 degrees Celsius or higher. And that’s just a chemical reaction. A lot of geochemical people involved in astrobiology are excited by the systems that might be generating the methane. We could have the energy sources, we could have the warmer temperatures to release liquid water, and we could sustain a group of living organisms.

CI:      If we hop now to sub-ice Europa, with the limited amount that’s known about the physical conditions there, what does your knowledge of the colder oceanic environments on Earth tell you might be going on there?

JB:     That’s the foot of a big cipher right now, and one I’m very interested in, because models have been coming out that are trying to invoke hydrothermalism on Europa. There’s one that claims the tidal heating and the flexing of the bottom core would create some kind of hydrothermalism, and that means a number of nutrients that I’ve talked about, and warm water at the same time. They and others have tried to imply that you could have enough heat concentration out of a robust venting system to get up to the ice and cause some melt. Those are just models. The key to Europa is not so much the cold temperature, and not so much that there’s no oxygen—it’s whether or not nutrients are being generated. The only thing we know chemically about Europa is that there appears to be sulfate and there appears to be plenty of carbonate, and that’s based on spectral data. But what about the energy sources? We don’t know anything about those. What about some kind of hydrothermalism or maybe some early history of a completely aquatic Europan planet? What about an ice cap, so that it had some kind of an atmosphere? Then it could have generated enough energy sources to start on a life form that may still be a little bit active down there now.

The early history of Europa is still something that really needs to be worked out—whether or not it is an active hydrothermal volcanic moon like Io and Ganymede and Callisto. People assume it will be like that, and that there is some hydrothermalism going on. My view would be this: if it is a hydrothermically active moon and life did get established, say in the sub-surface where the nutrients would be, then it’s been pumping out microbes in the sub-surface for more than 3.5 billion years, and they’re being pumped out into the water column. Whether or not they’ve adapted to grow in the low nutrients in that water column or not, there still would be an accumulation of a lot of organic material. It’s possible we could detect that, if we could get samples of some of the brightly colored ice along some of the ridge areas—that color could actually be organic material. Or we could get into an area of shallow ice that we could penetrate somehow, get some of that material, and see what’s there.

CI:      Do we know enough about the conditions to even speculate that someplace like Lake Vostok is a good analog; do we have any good analogs?

JB:     Lake Vostok is used as the analog because it’s been ice-covered for a long time and it doesn’t have any external sources of nutrients, although there’s evidence that there might be some hydrothermal activity still on the bottom. It is an analog, but it’s a freshwater analog, and the concentrations of sulfate would already make Europa a little bit different from Vostok. Things are going to be different if it’s seawater because of the way the ice forms. It forms brine pockets, and can concentrate nutrients in those brine pockets that can remain liquid at very low temperatures, because you are increasing the salt content. We need to know how salty the Europa ocean really is, and that will tell us a lot.

The freshwater Lake Vostok has its limitations in terms of extrapolation to Europa. The biological interests in Vostok are what kinds of organisms could be growing under extraordinarily low nutrient conditions. They’re thinking in terms of organisms that grow on organic nutrients. One of the papers co-authored by Ken Neilson pointed out that you would run out of energy sources on Europa. Chris Chyba was saying in his paper that perhaps even with photolysis reactions, you might be generating low molecular weight organic material like formate, and there are organisms that can use that. Both cases just point out that we need to understand more about the chemistry of Europa, and that we need a better understanding of whether or not it has been and may still be a hydrothermally active moon.

CI:      We’ve been mostly talking about microbes. You must have your favorites among the larger extremophile creatures; I’ve read about these ice worms, and there are some hot versions that are found near the hydrothermal vents. What are some of the more extraordinary larger organisms that can live in extreme environments?

JB:     In my Vatican Summer School lectures I started out each lecture with a weird animal life form to hook the students, because they were not biologists. I discussed a whole range of my favorite organisms—for example, a worm that lives down in hydrothermal vents, and others with lineages dating back to the early part of the Cretaceous. They exist under the most extreme conditions of any animal. They live on these black smokers, they’re bathed in toxic levels of metals—and they’ve been able to adapt to that, in some cases by using microbes that can detoxify metals. There’s still controversy as to what the high temperature range is for that animal. It’s kind of cool.

My big interest right now in the whole metazoan picture, multicellular animals, is this new emerging science called Evo-Devo, evolution-development. The fascination for me is that there’s a group of toolbox genes found throughout the animal kingdom that instigated the formation of structure in animals, including limbs, wings, and eyes. The idea of a convergent evolution—that is, that the eye was invented separately forty different times—is not completely true. Most of the animals with either some kind of light-sensitive paths or complex lens-type eyes have a set of these toolkit genes, which are capable of turning on and turning off other genes that instigate the formation of a structure, like an eye. The convergent evolution occurs afterwards, but they all start out with this set of toolkit genes. To me that’s just earth-shaking, it’s completely changed the way I think about evolution: that it’s not just mutation, natural selection, slow steps to creation of structure and organs, but a set of designated genes that can control this process and so allow it to happen at a much more rapid rate and in a greater diversity of organisms.

CI:      The difficult thing is determining whether these same mechanisms would exist in other Earth-like environments with biology, even if they have the same fundamental genetic basis as us. In vertebrates it’s the Hox gene, isn’t it?

JB:     It’s the Hox gene, and then the Pansix gene for eyes.

CI:      Is the mechanism itself compelling enough to speculate that it’s a convergent mechanism, that it would manifest beyond a certain level of complexity given the same start to biology elsewhere?

JB:     What it says is that evolution is capable of taking something that works well and disseminating it throughout a very wide range of organisms. If we root it down to the ancestor of the first organism to make a Hox gene, the fact of that Hox gene allows that lineage to form very high diversity. Getting the high diversity that we see in the metazoan community, for example, may actually require those sorts of genes. I’ve written a little article for the astrobiology book that Woody Sullivan and I are working on, called “Evolution in Astrobiology,” in which I try to discuss these issues—whether or not evolution is an essential feature of life, why it’s important, and what we would expect to see again if we rewound the clock. I perhaps get myself into some trouble with some of the things I’ve said, but it’s been an interesting exercise.

CI:      You’ve been pretty involved in issues of weird life. Recognizing the huge range of habitable planets that are likely to be out there, what are the most useful ways for us to relax the bounds of how we define life? Is it not just following the water and looking for different solvents, is it looking at different genetic architectures? What should we be considering?

JB:     I try to divide up “weird.” There’s “slightly weird,” which means essentially carbon-based, using a lot of the same biochemical processes but maybe different building blocks. That’s one level of weird: different amino acids, maybe different bases for nucleic acids. The second is a little more weird, still carbon-based but using a completely different molecular architecture for the cell than what we have. The central dogma of DNA-RNA-protein is not there, it’s something different. How do we imagine that? Even if a scenario is carbon-based, we’re still thinking about an informational macromolecule and some kind of translation of that into a product.

Then we start getting the “seriously weird,” and the first seriously weird option is still a structured entity but perhaps not carbon-based; it could be silicate-based or a silicate-carbon-based system; but structurally radically different. Can there be carbon-based life or silicate-based life that can live in solvents other than water? That’s a separate issue. We do know that a lot of enzymes work in the absence of water and organic solvents, and in some cases they behave quite well and differently. But in order to form, the structure of the enzyme itself has bound water to it; no experiment has been done in which water is completely absent. Even though water might be less than one percent, it’s absolutely essential to create the three-dimensional structure that allows catalytic activity. In terms of the solvent issue, we don’t have any information.

And then the most radical are the kinds of life forms that people speculate on, like something living in the atmosphere of Venus that’s more sulfur-based. Or the Titan kind of life that would live in organic solvents like an ammonia ocean—there are tons of these kind of speculations. What we have done is said, “We only know one kind of life and that’s carbon-based, so what are the ranges of life forms or active biochemical forms that can come out of a carbon-based system, with and without water as a solvent?” We can look at carbon-based systems and other solvents; those are going to be the dominant recommendations. From a cosmo-chemistry point of view, if we have any kind of a planet or moon with liquid water, and it’s a rocky planet with any kind of similarity to Earth, Mars, Venus, then carbon-based life is the way to think. I think it’s extreme speculation to say there’s enough energy in Saturn’s rings to generate some form of a living system. You can define life any way you want, and include something living in virtually any environment you want, in fun, if you like. For now, I’m going to stick with what we can do with a carbon-based life system.

CI:      Acknowledging the metabolic diversity on Earth and relaxing to encompass some of those slightly weird forms of life must create an issue as far as what the biomarkers will be, especially when we’re inspecting the atmospheres of distant terrestrial planets where we won’t have much more to go on than a single noisy spectrum.

JB:     It’s really difficult. There are very Earth-centric approaches, looking at organic biomarkers, falling water, and perhaps distributions of minerals or other volatiles that may be indicative of life. And then, which of those can be looked at spectrally as we probe extrasolar planets—it’s difficult. Those are all good biomarkers, it’s just that any one individual probably isn’t. We’re going to have to figure out how to use spectral data in ways that we’ve never thought of, resolutions we don’t have today; perhaps that will happen as we let powerful telescopes look into space. It will be a long time before we can do that. We have a hard enough time just knowing what to look for on Mars, and I think we’re going to have a more difficult time when we set up the next missions to Europa.

CI:      Let me finish with a question about the way the field’s going. Astrobiology has matured to the level where you have no trouble recruiting students into your area. Do you see your profession growing at the grassroots level?

JB:     There are more students applying to work in astrobiology than in oceanography. Astrobiology is attracting some of the best students out there, and they’re attracted to a lot of the same questions that we senior scientists are interested in; not just a search for life elsewhere, but a lot of the philosophical issues. When you start asking them, “Why do you want to be an astrobiologist, why do you want to study this?” it becomes very personal, very existential: “This is my way of finding out more about myself.” Once you free them to talk about that, you realize that they’re thinking a lot more about why they’re entering science than most science students. I think astrobiology students are coming at science from a much more personal perspective. And I really like that, because it’s rekindling my interest in having those discussions again.