Looking for the origin of life
Robert Hazen says a key question is, “What is life?”
Robert M. Hazen, Ph.D., is a research scientist at the Carnegie Institution of Washington’s Geophysical Laboratory and Clarence Robinson Professor of Earth Science at George Mason University, Virginia. Author of more than 230 articles and 17 books on science, history, and music, including Genesis, The Scientific Quest for Life?s Origins. Hazen spoke with Earth & Sky’s Jorge Salazar about scientists efforts to discover the origin of life.
Salazar: Thanks for speaking with me today, Dr. Hazen. I’m hoping to talk a bit about some of the ideas in your latest book, Genesis – The Scientific Quest for Life’s Origin. Other than an occasional reference, it doesn’t have much scripture.
Hazen: There’s only a couple of pieces of scripture, but Genesis is a general word, but it’s also a scientists story as well – it’s the story of the coming into being of something.
Everyone, scientists and non–scientists alike, are fascinated by the origin of life. Where did life come from, how did it arise, how did we go from a primitive Earth with rocks, water, atmosphere and transfer that somehow into biochemistry, the great complexity that we see today. Even the simplest cell is so much more complex than anything that’s non–living. So one asks, how does one make that transition. And of course there’s tremendous debate today. Some people say that life is so complex that it could not possibly have arisen by a natural process. It must have been designed by an intelligent designer, and of course, what they mean by that is by God. Because if you invoke some weird space alien, then someone had to design the designer. Well, there’a a scientific alternative, and that had to do with scientific processes that occurred inexorably throughout the universe – simple processes that lead inevitably to life. Life is a cosmic imperative because of geochemistry. The simple interaction of rock, water, and gasses, leading at first to simple biomolecules, and then those biomolecules self–organize, or arrange themselves on mineral surfaces to give you much larger, more complex molecular systems. Some of those molecular systems will spontaneously begin to self–replicate. And once molecules begin making copies of themselves, then natural selection kicks in. Different groups of molecules compete for the same energy and atoms. And inevitably, that drives the system to greater and greater degrees of complexity.
Salazar: To a non–scientist like myself, I’d be hard pressed to define “life,” other than I know it when I see it. How do scientists define life?
Hazen: It’s surprising that one of the real key, center questions in the origin of life studies is just what is life? How do you know when something is alive and when it’s not alive. Of course today, we have biological assays – we can look for the key molecules, like DNA and proteins. But, in the earliest Earth, there must have been a transitional point, before modern biochemistry, but much more complex than simple geochemsitry, and those highly organized molecular systems must have had the properties of self–reproduction – they’re able to make copies of themselves, and most of scientists agree that the other key ingredient to life is self–replication with some variation, slight differences from one cell or group of cells to the next, and then those molecules would inherit the improvements, the variations that allow them to complete more successfully for resources. This is just the process of natural selection taking place at the molecular scale.
Salazar: Is there scientific consensus about how life emerged?
Hazen: Scientists have as their objective finding some truth about the natural world. And I think that all scientists would agree that there is some truth, probably a knowable truth about an exact way, for example, for the way that life came to be on the primitive Earth, and other sorts of information like this. But while you’re trying to figure it out, it’s a real free–for all. The people from one point of view and opposing points of view, they debate, they argue, they test hypothesis, but ultimately, all scientific hypothesis have to be supported by data, independently verifiable observations and experiments, iron–clad logic, so that when you go from simplicity to the complexity of life, you have to have a process that anyone with the right equipment and background can go into the laboratory and duplicate, or reproduce. And once you do that, then I think consensus will arise. But until that happens, it’s really going to be a lot of fun. One of the exciting things about origin–of–life research is that there’s so much lively debate, there’s so many intriguing ideas, a lot of really smart people coming up with original, creative thoughts, and then throwing them out there and letting people test and find out which ones work and which ones have to be cast aside.
Salazar: One of the themes of Genesis is that life originated by emergent complexity. Could you describe that for our listeners?
Hazen: I think the most compelling scenario for life’s origins is based on the concept of emergent complexity. We observe, over and over in nature, when lots of objects, lots of particles, like sand grains or stars or molecules, or cells, when they interact, they tend to yield structures that are far more complex, that have behaviors that are far beyond anything that the individual sand grain, or star, or cell, could do itself. This kind of emergent complexity is the key to understanding the origin of life. The origin of life was a sequence of emergent steps. First, lots of small molecules came together to form larger molecular structures, structures that were able to condense and form structures like cell membranes, and so forth. And then, some of those molecules actually began to self–replicate – groups of molecules making copies of themselves, and therefore growing at the expense of all of the surrounding atoms and molecules and using energy from their environment. And ultimately, that self–replicating system, the emergence of that system led to the kind of competition and natural selection that drove the evolution into the first cells and beyond.
Salazar: Could you describe your origin of life research for our listeners?
Hazen: Our research proceeds on two fronts. First of all, we do chemical experiments, particularly at the higher temperatures and pressures that might exist at the shallow crust or at the bottom of the ocean, where you might have a volcanic activity, or just the Earth’s inner heat driving chemical reactions. We see the formation of organic molecules, biomolecules, if you will, with great facility. And then we study how those molecules might have been selected and organized, particularly on mineral surfaces, because minerals have the ability to select various molecules, and concentrate them and order them onto the surface. But that’s not all. We’re also interested in the theoretical side of emergent complexity – what sorts of natural conditions foster this emergent complexity, where you go from simplicity to much more complex systems in a natural environment. And we find that you need a flow of energy, sometimes cycling that energy back and forth, from hot to cool, or wet to dry, or other sorts of environmental conditions that cycle are very important. We also find that we need a certain minimum number of interacting molecules. If the concentration of molecules isn’t high enough, you can’t form life. So these are all hints and clues to the sorts of environments, to the sorts of conditions that led to life on the primitive Earth.
Salazar: What’s the goal – to create life in a test tube?
Hazen: The objectives of our research, the ultimate goal, the holy grail if you will, would be actually to create a group of molecules, in a test tube or on a mineral surface, that started to make copies of itself – a self–replicating system of molecules. Now this wouldn’t be anything like cellular life that we see today. It’s much, much simpler, and it’s certainly isn’t the type of life form that would take over and cause some sort of plague on Earth. This is a very simple chemical system. Many people wouldn’t even say it’s quite alive yet. But once you have a self–replicating set of molecules, then that set of molecules could gradually, over time, evolve. There’d be slight variations in which molecules were used. It would compete with itself. That’s the kind of step that we know must have preceded the life that we see on Earth today. So that’s really our goal. It’s rather modest, and it certainly doesn’t raise any serious ethical dilemma at this point, but if we could do that, if we could get that far, I think that we’d be well on our way to understanding in quite some detail, how life arose on the primitive Earth.
Salazar: What are some of the difficulties scientists face in attempting to create life?
Hazen: One of the main impediments of succeeding, of finding that self–replicating molecular cycle is that there are just so darn many organic molecules, carbon–based molecules. There are millions upon millions upon millions of different ways to arrange a few carbon atoms with oxygen, hydrogen, nitrogen thrown in, and those are the critical building blocks. You can make thousands upon thousands of different kinds of molecules in very simple geochemical experiments. The problem is selecting just the right ones, finding the subset that will start making copies of itself and win out over all of those other useless, unnecessary molecules. That’s the key – it’s finding the right subset of molecules out of all of the millions that are possible, and we’re gradually winnowing them down. We’re gradually getting insights into what works and what doesn’t work.
You know, there are two approaches to the origin of life, this bottom–up approach starts with simple geochemistry and tries to develop biochemistry out of that, going from simplicity to complexity. But we have another tack – we can use the top–down approach, where we look at very primitive modern cells, cells that were alive deep in the oceans or in extreme environments on Earth, understand the chemistry or the biochemistry of those cells, and try to work backward in time to say, what is the simplest sort of chemistry that we can imagine in a cell. What is the biochemistry, that’s common to every living cell, because that would point us to what might be considered biochemical fossils. If we understand the simplest life on Earth today, it pushes us back, back, back into time so. And so we’re trying to bridge that gap – between the bottom–up geochemistry and the top–down biochemistry.
Salazar: Could you describe when life might have originated here on Earth?
Hazen: Current thinking is that life only had a very narrow window in which to emerge. Earth formed about 4 1/2 billion years ago, and most experts think that for the next 500 million years, Earth was under a heavy bombardment of meteors, and comets, asteroids hitting the Earth, pulverizing the surface, killing every living thing, wiping out the ocean – that would have made it very difficult for life to have arisen and survived before about 4 billion years ago. And yet the oldest preserved sedimentary rocks on the Earth’s surface, those that are about 3.85 billion years old in Greenland, they show tantalizing signs which have been interpreted as life. Well if life was around 3.85 billion years, and it could not have existed 4 billion years ago. That leaves a window of only 150 million years to go from non–life to life. That is a short window, in geological time, and if life arose that fast, many people are suggesting that life is a cosmic imperative, that life arises relatively quickly any place that you have carbon, that you have energy, that you have water, that you have the raw materials that you need for life. That’s a very exciting prospect, because that means that life could have arisen on Mars, it might have arisen on Europa, or Titan, or any of a number of other places in our own solar system, not to mention countless billions and billions of worlds in other galaxies other than our own.
Salazar: What do you mean by “cosmic imperative”?
Hazen: You can think of life in a number of different ways. You can think of it as an incredibly rare and precious event. Perhaps Earth is the only living planet in the entire universe. And that’s a universe of perhaps 100 billion galaxies, each with hundreds of billions of stars. On the other hand, maybe life is common, in which case we consider life a cosmic imperative, life will form anywhere that there’s water, that there’s energy, carbon, the essential building blocks of life, that life will arise very quickly, and that there will be literally billions of billions of living worlds throughout our universe. That’s a very exciting prospect to a scientist, and it’s one that drives us to try to understand that origin of life process.
Salazar: So, where else besides Earth do scientists think is the most likely place to have life?
Hazen: Right now, scientists are focusing their attention on Mars. Mars had lakes, or even oceans, very early in its history. And, indeed, if life is a cosmic imperative, if it arises quickly, then it’s possible that Mars was habitably before Earth. Mars may have even generated life before Earth. And what’s amazing, is that it’s very possible that asteroid impacts on Mars could have transferred living cells from the surface of Mars to Earth. It is very possible, according to some scientists, that all life on Earth is indeed Martian life. It arose on Mars first, because Mars is habitable before Earth, and then was transferred to Earth. And then Earth became a living planet as a result of this seed of life from another Planet, our planetary neighbor Mars. Now we don’t know if Mars is alive now, but even if it isn’t, we might be able to find fossil evidence for that early life if we go to Mars. That’s an exciting prospect.
Salazar: How long ago could Mars have housed life?
Hazen: According to current models, Mars was habitable shortly after its formation, 4 1/2 billion years ago. It was cooler than Earth. It was not as blasted by comets and meteorites. It had liquid water on the surface, and there’s abundant evidence for that from the recent rovers on the surface of Mars. We also have evidence that the early atmosphere was much denser, and perhaps more conducive to life. So Mars, we suspect, at least for a billion years of its early history, was a rather benign world, a nice temperature, liquid water, all the sorts of things that you want to form life. But gradually, over time, Mars became colder. Because it’s a smaller planet, it lost its atmosphere to gravitational escape. It became dried out. And if there’s any life remaining on Mars, and that’s life deep underground where it’s warm and wet, and much more protected than the surface of Mars.
Salazar: What did those Martians look like?
Hazen: We don’t have a real good idea of what Mars life would look like, but the chances are that it was microscopic, very much like microbes on Earth today – tiny single cells that you’d have to use a microscope to see. They might have made the early oceans of mars a little bit cloudy, there’s certainly no evidence at this point that Mars would have gone any farther than that. But we don’t know. That’s one of the exciting things about going to Mars, collecting rocks, of actually returning samples to Earth and examining them with every tool at our disposal to see if there’s signs of life in Mar’s rocks.
Salazar: What are some of the recent discoveries regarding the origin of life?
Hazen: There’s so many exciting discoveries made all of the time. Just in the last couple of months, scientists have discovered that very small organic molecules, molecules of only a few carbon atoms can act as catalyst to drive biological reactions. Previously, everyone thought that you had to have huge molecules, things like the enzymes in our bodies, very complicated, very unlikely molecules. But now we find that some of the key biological chemical reactions can be enhanced, can be accelerated, just with simple molecules that would have been available in the environment. So that warrants a lot more study – small molecules acting as catalysts. And that’s just one of many, many discoveries being made that points to the richness of this field in the study of the origin of life.
Salazar: Thank you again for your time in talking about a surprisingly controversial topic today, with the recent trial involving the Dover school board and intelligent design.
Hazen: That’s the other thing. A lot of people are very happy to discuss the more theological aspects. Intelligent design, to me, seems like such a shame, because it kind of says, “oh, this is so complicated, we can’t possibly figure it out. So, we’re going to have to resort to some intelligent designer.” And to me, that demeans my concept of God, a god that can set the universe in motion, with all of the natural laws, and with all of the majesty of creation. And, inexorably, it leads all over the universe in countless billions of spots to life, and life becomes to know itself and to ask the questions that we’re asking here on this show. That to me is a really profound universe, a universe that that I enjoy studying. I don’t want to throw up my hands and say that we can’t possibly know because it’s too complex.
