First Life and Next Life
Synthetic biology is a new field, but it’s targeting an old question: How did life begin?
The driver turned off the engine of his rumbling Russian- army troop carrier at the edge of a deep canyon carved by a stream of glacial meltwater. Our little research group–which included Stanford graduate students Jamie and Meaghan, postdocs Jan and Jake from the Carnegie Institution of Washington, and our guide, Vladimir–clambered down from the truck for a welcome stretch after a jarring five-hour drive from Petropavlovsk. Then we shouldered our packs and began to climb, crunching over packed snow and ice between house-size boulders. When we stopped for breath and looked back downhill, we could see the ash and lava flows from past eruptions eroded into hills and valleys, with scattered patches of low shrubs in sheltered areas far below. The jagged volcanic landscape of Kamchatka defined the horizon. Above us loomed our goal: the blasted peak of Mount Mutnowski, a volcano that had erupted just a few years before.
Two hours later and 2,000 feet higher, we peered over the edge of the crater. It was hard to grasp the chaos beneath us. There was nothing alive in this landscape of black and gray rock except our team of six. A small glacier on the other side was melting into the crater, and distant roaring sounds emanated from deep inside as steam rose into the blue sky. Earth, air, fire, and water, I thought–the ancient elements, brought together here in far eastern Russia, stirred by heat energy left over from the beginning of our planet’s history. Except for the glacier, this place seemed like a remnant from that time–a model of what Earth was like four billion years ago, before life began. We made our way down into the crater, at times wearing gas masks to protect our lungs against caustic gases.
My fieldwork in Kamchatka was supported by a NASA grant, and our main goal was to better understand geochemical conditions related to the origin of life on Earth and perhaps on Mars. Earlier publications in Russian-language journals had reported that organic compounds, including amino acids, were present in the boiling springs and vapors of volcanoes in Kamchatka. Everyone agrees that the origin of life required a source of organic compounds, but no one really knows what the primary source might have been. One possibility is that most of the compounds were produced by geochemical synthesis in volcanic regions early in Earth’s history, and it would be a real breakthrough if we could detect similar reactions in volcanoes today.
The second goal was basically to hedge my bet. What if we got all the way to Kamchatka and found no organic compounds? That would be embarrassing. For this reason I brought along a mixture of compounds similar to those we thought might have been available four billion years ago to kick-start life: four amino acids, a fatty acid, phosphate, glycerol, and the four bases of nucleic acid. We knew that under laboratory conditions, these components can react to produce more-complex compounds related to the molecular structures and functions characteristic of life. I proposed to add these to a volcanic pool to see what would happen. Most of my colleagues believe that this kind of experiment is a bit silly because the conditions are so uncontrolled, but I think of it as a reality check. We can get interesting reactions to work in a laboratory, but what if we are overlooking something that becomes apparent only when we try to simulate those reactions in a natural environment?
Symbiosis and Synthetic Biology
When I first began to hear the term astrobiology a few years ago, it sounded strangely discordant. And then another new discipline appeared that was even more of a stretch: synthetic biology. But this is how science progresses–by a kind of symbiosis between seemingly unrelated disciplines, in which traditional biology and chemistry become biochemistry, and biology and physics become biophysics. I began my career doing traditional biophysical studies on membranes, but now some of my research is funded by NASA’s astrobiology program, and many of our experiments could be described as synthetic biology: the application of engineering techniques to design or redesign biological functions and systems.
The field of synthetic biology is hot just now, because its methods are potentially very powerful. Synthetic biologists know enough about living systems to alter genetic programs in useful ways, the way expert computer programmers alter software. But what does such high-tech science have to do with volcanoes and the origin of life? Louis Pasteur once commented that chance favors the prepared mind; very often, even the most basic research produces an undreamed-of application. For example, one of the most powerful tools of molecular biology is the polymerase chain reaction (PCR), which is used to amplify DNA–that is, to make multiple copies of a given sequence. In PCR, cycles of heating and cooling combine with DNA synthesis by a polymerase, an enzyme that catalyzes the building of large molecules (polymers) from small molecules (monomers). Kary Mullis came up with the idea in 1983, first using a polymerase from ordinary E. coli bacteria, but a polymerase was needed that could survive near-boiling temperatures. In 1965–in completely unrelated research–Thomas Brock discovered a primitive bacterium, which he named Thermus aquaticus, living in the volcanic hot springs of Yellowstone National Park. This organism is the original source of the heat-resistant Taq polymerase now used in all commercial PCR devices.
If we follow Pasteur’s advice, we can increase the chances for more such serendipitous discoveries. In particular, we can prepare our minds by broadening the scope of synthetic biology to encompass studies of the origin of life. I will begin by describing nature’s version of synthetic biology; then I will show how our growing understanding of life’s molecular mechanisms suggests a way to reproduce the origin of life in the laboratory.
First Life: Synthetic Biology in the Wild
To take on the question of life’s origin, we need to have some idea of what Earth was like four billion years ago. There is good evidence that oceans were already present, predating life by several hundred million years. The oceans were salty, probably somewhat acidic, with volcanic land masses rising above sea level. Precipitation onto those islands produced freshwater ponds, so a marine environment is not the only one in which life could have begun. The atmosphere was a mixture of carbon dioxide and nitrogen, with little or no oxygen, and the average global temperature was 60 to 70 °C, much higher than today’s 15 °C. Thus the first forms of life probably resembled the thermophilic bacteria that inhabit hot springs today.
How could life begin in such an unpromising environment? Charles Darwin occasionally wondered about that, though he was too conservative to speculate in public about the origin of life. In a private letter to his friend Joseph Hooker, he wrote: “But if (and Oh! what a big if!) we could conceive in some warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.” And his great book On the Origin of Species touches on the question in a single sentence: “Looking to the first dawn of life, when all organic beings, as we may believe, presented the simplest structure, how, it has been asked, could the first steps in the advancement or differentiation of parts have arisen?”
Less eloquently, what would be required for the evolution of life to begin? First of all, evolution works on populations, not single organisms, so we need to find a way to generate large numbers of molecular systems in the prebiotic environment. Furthermore, there must be great variation in their properties. The requirement of variation within a population means that the first life forms capable of evolution could not be random mixtures of replicating molecules unable to assemble into discrete entities; instead, they would be systems of interacting molecules encapsulated in something like a cell.
The systems would have to exhibit the two primary functions of life: growth and reproduction. Cells grow by taking in nutrients–simple molecules from the environment. They use energy to link those molecules into the polymers that we call proteins and nucleic acids. Reproduction requires a mechanism by which genetic information can be stored and then replicated, so that the information, in the form of genes, can be passed on. But the transfer of information is necessarily imperfect. A certain number of errors–mutations–must occur to produce variations in the population such as those that enabled primitive life to explore different niches and begin evolving toward the magnificent biosphere of today’s Earth.
We’re talking about forms of life much simpler than even the most primitive bacteria that exist now. Still, how could cells of any kind spontaneously appear out of random mixtures of simple organic molecules? The prospect is so mind-boggling that a few scientists state flatly that we will never understand how it came about. I’m more optimistic. But attempting to discover how life began is hard work, with no certainty that we will ever find answers. We need to formulate and test hypotheses, and be willing to venture into vast unexplored territory. I will briefly describe some of the milestones on this journey. As we acknowledge them, we can begin to assemble a logical cage that constrains speculation and guides us toward answers.
First milestone: A Source of Organic Monomers
The four basic varieties of biomolecules are amino acids and proteins, carbohydrates, nucleic acids, and lipids. There is little doubt that similar–even identical–organic compounds were present in the prebiotic environment. That was the conclusion of Stanley Miller’s famous experiment in the early 1950s in which he exposed mixtures of ordinary gases to electrical discharges and observed the formation of amino acids. Since that time, virtually all the primary carbon compounds of life have been synthesized in prebiotic simulations.
The results of simulation studies were strongly supported when a remarkable meteorite fell to Earth near Murchison, Australia, in September 1969. It was clear that the meteorite contained organic material, because a strange smell emerged from the hot surfaces. Forty years later, when I grind Murchison samples in the lab, the same odor wafts up from the mortar–simultaneously dusty, oily, and sour. This is an ancient odor, older than Earth itself, preserved for five billion years in a comet or asteroid from which the original meteorite was derived.
There are thousands of organic compounds present in the Murchison meteorite and other carbonaceous meteorites that have been analyzed–confirming Miller’s experiment by showing that biologically relevant organic compounds are in fact produced by nonbiological processes. This makes it plausible that organic compounds were present on prebiotic Earth, either synthesized by geochemical processes or delivered as meteoritic and cometary infall more than four billion years ago.
Second Milestone: Self-Assembly of Compartments and Protocells
The unit of all life today is the cell. My research background is in membrane biophysics, and I began my career by studying the lipid membranes that are the essential boundaries defining living cells. Twenty years ago, when I obtained a golf-ball-size stone from the Murchison meteorite, I wanted to see whether anything resembling lipids was present in the mix of meteoritic organics, perhaps shedding light on how life became cellular.
In earlier research I had often used a mixture of chloroform and methanol to extract lipids from a variety of biological materials, such as red blood cells, chloroplasts, mitochondria, and even egg yolks–the last being a rich source of a phospholipid called lecithin. And in 1975 I had spent a sabbatical leave in the lab of Alec Bangham, who discovered in the 1960s, working at a research institute near Cambridge, England, that lecithin could spontaneously self-assemble into membranous sacs, or vesicles, that have come to be called liposomes. I now employed the chloroform-methanol mixture to isolate compounds from samples of the Murchison stone, then used a chromatographic procedure to purify those that might be capable of self-assembly into membranes. The left panel in the figure on page 71 shows what happened when a few micrograms of the extract were dried on a microscope slide and exposed to water to simulate the cycles of wetting and drying that would have been frequent on the early Earth. The results were very exciting. Not only were lipidlike molecules present in the mix, but they readily self-assembled into cell-size vesicles.
When we analyzed the mixture of meteoritic organics, we found that some of the compounds were short-chain fatty acids, soaplike molecules that feature a tail of 9 to 13 carbon atoms. This meant that we no longer needed material from precious meteorites to carry out experiments; we could investigate the properties of the pure compounds purchased from chemical-supply companies. We began with decanoic acid, a 10-carbon fatty acid, and found that it readily produced vesicles similar to those produced by the meteorite extracts. The next question was whether such compartments could encapsulate larger molecules to produce protocells, which are defined as encapsulated systems of molecules–like RNA–that have the potential to act as catalysts and carriers of genetic information. This turns out to be so easy that it could be done for a high-school science fair. If the microscopic vesicles are mixed with large molecules like proteins or nucleic acids, then put through a dry-wet cycle, about half of the large molecules end up inside the vesicles. The glowing lipid vesicles shown in the right-hand panel of the micrograph are composed of decanoic acid surrounding DNA molecules.
The bottom line is that protocells are very easy to produce by simple self-assembly processes. It follows that such structures would also be expected to occur in a prebiotic setting.
Third Milestone: Polymer Synthesis
All life today uses enzymes to catalyze the synthesis of polymers. And nearly all polymeric molecules of life, including proteins and nucleic acids, are synthesized from monomers that are chemically activated–that is, they gain the energy to undergo polymerization–through complex metabolic processes that extract the equivalent of a water molecule from each one. Ribosomes link activated amino acids through peptide bonds to produce proteins, and enzymes called polymerases catalyze the formation of ester bonds between activated nucleotides to produce nucleic acids.
Nothing nearly this complicated could have happened before life began, but a variety of simpler reactions can also produce interesting polymers. For instance, James Ferris, at Rensselaer Polytechnic Institute in New York, showed that a clay mineral called montmorillonite promotes the synthesis of polymeric RNA from activated nucleotides. The mineral surfaces adsorb and organize the nucleotides, which then zip up into polymers. Furthermore, once RNA molecules are formed, they can undergo a kind of limited replication process that does not require enzymes. Leslie Orgel and his associates at the Salk Institute demonstrated in the 1980s that chemically activated nucleotide monomers line up on synthetic RNA templates by Watson-Crick base pairing, as they do in the double helix of DNA, and then polymerize into a second strand of RNA.
The seminal observations of Orgel, Ferris, and others clearly suggested that something like RNA might have been the first polymer to be associated with life processes. Additional evidence was provided when Thomas Cech at the University of Colorado and Sidney Altman at Yale found that certain types of RNA had catalytic properties, a discovery for which they shared a Nobel Prize. Such RNA molecules, now referred to as ribozymes, can make and break specific chemical bonds within their own structure rather than depending on protein enzymes. The discovery of catalytic RNA led Nobel-winning chemist Walter Gilbert at Harvard to propose an “RNA World,” positing that life did not begin with the complex systems of DNA, RNA, and proteins that characterize all life today. Instead, RNA molecules could have served as catalysts as well as storing and transmitting genetic information. The RNA World concept dominates current thinking about the origin of life. Research groups led by Gerald Joyce at the Scripps Research Institute, David Bartel at the Whitehead Institute, and Peter Unrau at Simon Fraser University are attempting to incorporate RNA into a self-replicating system of molecules. Significantly, they often employ a technique in which evolutionary principles are used to select specific catalytic activities from mixtures containing trillions of different RNA molecules.
That brings us to the next milestone.
Fourth Milestone: Evolution of Catalysts
Can genetic information somehow emerge in random mixtures, essentially by chance? If the answer is no, then we’re in trouble, because those of us who work on the origin of life claim that this is exactly what happened four billion years ago, when the first forms of life emerged from a sterile mixture of minerals, atmospheric gases, and dilute solutions of organic compounds. To address that question, I will revisit a classic experiment that David Bartel and Jack Szostak published in 1993, while Bartel was a graduate student in Szostak’s lab. Their experiment is moderately complicated, but the result is so important that it is worth explaining here. The goal was to see if a completely random system of molecules could undergo evolutionary selection in such a way that molecules with catalytic properties could evolve. The first step was to synthesize trillions of different RNA molecules consisting of approximately 300 nucleotides, arranged in random sequences. Bartel and Szostak reasoned that buried in those trillions were a few ribozymes that happened to catalyze a ligation reaction, in which one strand of RNA is linked to a second strand. They developed a procedure that captured those rare molecules even if they only weakly catalyzed the reaction. Then they used enzymes to amplify them. The amplified sequences were put through another round of selection and amplification, and the process was repeated for 10 cycles.
The results were stunning. Increased catalytic activity began to appear after four cycles, and after 10 rounds the rate of catalysis was seven million times the uncatalyzed rate! It was even possible to watch the RNA evolve. Nucleic acids can be labeled with radioactive phosphate, then separated and visualized through a technique called gel electrophoresis. A mixture of RNA molecules is placed at the top of a gel and a voltage of several hundred volts is applied, which causes the molecules to migrate downward through the gel. Larger molecules don’t move very far, so they appear as bands near the top of the gel; smaller, faster-moving molecules form bands near the middle and bottom. At the start of the experiment, nothing could be seen in the gels, because the RNA molecules were all different. But after three cycles, distinct bands appeared, meaning that certain catalytic species were already being selected. With further cycling, other species appeared for a few cycles and then went extinct. After 10 cycles, two distinct RNA species survived, representing those RNA molecules that were most efficient in catalyzing the ligation reaction.
These results demonstrate a fundamental principle of evolution at the molecular level. At the start of the experiment, every molecule of RNA was different from all the rest, but then a selective hurdle was imposed in the form of a ligation reaction that allowed only certain molecules to survive and reproduce. The result was that specific catalytic molecules emerged by a process closely reflecting Darwinian natural selection. The conclusion: genetic information can in fact appear in random mixtures, as long as the mixtures begin with large numbers of polymers defined by a variety of nucleotide sequences from which specific sequences having a catalytic property can be selected and amplified. It seems reasonable to propose that similar selective processes could have occurred on the prebiotic Earth when the first forms of life self-assembled in a mixture of organic compounds and then began to evolve.
Fifth Milestone: Combinatorial Chemistry & Garbage Bags
Most chemists learn to do their experiments in series, one per day. But experiments can also be done in parallel with a technique called combinatorial chemistry. This approach is particularly useful in the pharmaceutical industry, in which it is often necessary to experiment with large numbers of compounds in order to optimize a reaction or test a new drug. A robotic device loads hundreds or even thousands of small reaction chambers with the desired mixtures, each chamber containing a droplet that is slightly different from the rest. After the reaction is completed, the chambers are individually tested for activity.
In my lab, we perform a version of combinatorial chemistry when we prepare liposomes by adding water to a few milligrams of dry lipid in a flask. A milky suspension is produced that contains, not thousands, but trillions of individual microscopic vesicles in the size range of small bacteria–half a micrometer in diameter. If the vesicles are prepared in a solution containing small peptides and short nucleic acids such as RNA, each of the vesicles will contain a different set of components, so each represents a microscopic experiment. Now let’s think about the early Earth. Instead of milligrams of lipid in a flask, it would have had billions of tons of organic material assembling into enormous numbers of microscopic structures, and half a billion years to do the experiment.
The origin of life can be understood metaphorically as combinatorial chemistry on a global scale. A few of the microscopic experiments must have been successful, resulting in primitive cells capable of capturing energy and nutrients in order to grow by means of polymerization reactions. Evolution began when the cells filled a limited niche and competed for resources. At that point, natural selection took over, placing a premium on how efficiently a given cell could capture nutrients in order to grow. I imagine that once robust cellular life got under way, it expanded exponentially. Earth, seen from space, may even have blushed red or turned green for a while when photosynthetic bacteria filled the oceans.
Will we ever discover the combination of ingredients that gave rise to life? Again, I am optimistic. We need to apply what we know about the chemistry and physics of living systems to narrow down the possibilities, then be brave enough to actually do some experiments. But what experiments should we try? This is where theory can guide us. Freeman Dyson, one of the great theoretical physicists of our time, has also taken an interest in the origin of life. In his book Origins of Life, Dyson succinctly summarizes what I have told you:
Life began with little bags, the precursors of cells, enclosing small volumes of dirty water containing miscellaneous garbage. A random collection of molecules in a bag may occasionally contain catalysts that cause synthesis of other molecules that act as catalysts to synthesize other molecules, and so on. Very rarely a collection of molecules may arise that contains enough catalysts to reproduce the whole population as time goes on. The reproduction does not need to be precise. It is enough if the catalysts are maintained in a rough statistical fashion. The population of molecules in the bag is reproducing itself without any exact replication. While this is happening, the bag may be growing by accretion of fresh garbage from the outside, and the bag may occasionally be broken into two bags when it is thrown around by turbulent motions. The critical question is then, what is the probability that a daughter bag produced from the splitting of a bag with a self-reproducing population of molecules will itself contain a self-reproducing population? When this probability is greater than one half, a parent produces on the average more than one functional daughter, a divergent chain reaction can occur, the bags containing self-reproducing populations will multiply, and life of a sort has begun.
The life that begins in this way is the garbage-bag world. It is a world of little protocells that only metabolize and reproduce themselves statistically. The molecules that they contain do not replicate themselves exactly. Statistical reproduction is a good enough basis for natural selection. As soon as the garbage-bag world begins with crudely reproducing protocells, natural selection will operate to improve the quality of the catalysts and the accuracy of the reproduction. It would not be surprising if a million years of selection would produce protocells with many of the chemical refinements that we see in modern cells.
Next Life: Synthetic Cells
Theoretical concepts like the RNA World and Dyson’s garbage-bag world have inspired experimental approaches in which systems of molecules enclosed by membranes are sufficiently complex to have some of the properties of life. The ultimate goal is to assemble a cellular system that can use energy to grow through a process of catalyzed polymerization, replication of genetic information, and evolution. Several laboratories have initiated such studies, and there is reason to believe that the goal of artificial life may be achieved in the next decade. I will now recount a brief history of research on fabricating artificial cells.
Perhaps the first thing to understand is that assembling a system of molecules capable of reproducing is old news. More than 50 years ago, Heinz Fraenkel-Conrat and Robley Williams at Berkeley discovered that the tobacco mosaic virus could be separated into its coat protein and RNA. If the two components were mixed together, they reassembled into the infectious agent. More recently, in a remarkable display of modern molecular-biology methods, Jeronimo Cello, Aniko Paul, and Eckard Wimmer at the State University of New York at Stony Brook fabricated a functional polio-virus genome by stitching together hundreds of smaller fragments that were synthesized using chemical techniques. And two years ago Hamilton Smith and his colleagues at the J. Craig Venter Institute in Rockville, MD, managed to synthesize a complete genome of a small bacterial species called Mycoplasma genitalium. The uproar this caused is an indication of what will face the first claims that a living cell has been reassembled from its parts.
The synthesis of viral and bacterial genomes suggests that even more-challenging fabrications may be possible. We have known for years that spontaneous self-assembly processes can produce surprisingly complex systems of functional molecules. Efraim Racker, working at Cornell University, pioneered the effort to dissect and reconstitute mitochondrial membranes in the 1970s. Mitochondria are subcellular organelles that are present in most cells, and embedded in their membranes are enzymes that remove electrons from metabolic products derived from nutrients such as glucose. The process is called electron transport, because the electrons then pass through a chain of enzymes in the mitochondrial membrane and are delivered to oxygen. The electron transport is tightly coupled to a second transport process, in which positively charged protons derived from water are pumped outward, producing an electrical potential of approximately 0.2 volts across the membrane. This voltage provides the energy source for the synthesis of adenosine triphosphate (ATP), which transports chemical energy within cells and therefore drives most life processes. The universal mechanism by which ATP is synthesized, now referred to as chemiosmosis, was proposed in 1961 by Peter Mitchell, a remarkable British scientist who later carried out research in his home in Bodmin, Cornwall.
Racker and his students dissolved mitochondrial membranes with a detergent called deoxycholic acid. One of his first discoveries was that the membranes contained an enzyme that coupled ATP synthesis to electron transport. He referred to this as a coupling factor, but it is now called an ATP synthase. Racker also found that the detergent could be removed by dialysis–simply by placing the clear solution in a bag composed of a material resembling cellophane and letting it sit overnight in a dilute salt solution. The small detergent molecules leaked out of the bag, but larger molecules could not get through the porous material. The next day the solution was turbid, because membranous vesicles containing the original protein components had reassembled. The vesicles were fully capable of electron transport reactions and ATP synthesis. It was the first reconstitution of a very complex biological function.
At about the same time, Walther Stoeckenius at the University of California, San Francisco, became curious about the pigmented membranes of a bacterial species called Halobacterium halobium, which lives in extremely salty water. Stoeckenius and Dieter Oesterhelt were able to isolate the purple pigment–bacteriorhodopsin–and found that its function was to absorb light energy and use the energy to transport protons across the bacterial membrane. The energy of the proton gradient was then used to synthesize ATP. Racker and Stoeckenius, both members of the National Academy of Sciences, then initiated a rare collaboration between two senior scientists. They used Racker’s dialysis method to reconstitute a system of membranous vesicles containing only the proton pump of purple membranes and the ATP synthase of mitochondria. In 1974, they reported that the hybrid vesicles could use light as an energy source to synthesize ATP. Their paper added to the weight of evidence that finally confirmed chemiosmotic synthesis of ATP, for which Peter Mitchell was awarded the Nobel Prize in 1978.
The point of this brief history is that a surprisingly complex biological function can be reconstituted through self-assembly of dispersed components. Why not try to reconstitute a whole cell? If this turns out to be possible, perhaps it will help us untangle what we mean by “life” and even elucidate the major steps that led to the origin of cellular life.
Pier Luigi Luisi and his research associates in Zurich made the first attempt by encapsulating ribosomes in lipid vesicles in 1999, together with a synthetic form of RNA that told the ribosomes to incorporate the amino acid phenylalanine into a protein. A few short peptides were produced, but lipid bilayers are impermeable to amino acids, so synthesis was limited to those phenylalanines that happened to be inside the vesicles. Vincent Noireaux and Albert Libchaber at the Rockefeller University had a clever solution for the permeability problem: why not add a channel to the lipid bilayer of the vesicles? They reported in 2004 that they had succeeded in encapsulating a complete translation system isolated from E. coli, along with messenger RNA that directs ribosomal synthesis of green fluorescent protein (GFP) and of hemolysin, a protein that serves as a channel allowing externally added amino acids and ATP to enter the vesicles. The system worked for as long as four days, and at the end of the incubation period the vesicles glowed green from the accumulated GFP. Tetsuya Yomo and his research group at Osaka University have gone a step further with a similar encapsulated translation system in which the GFP gene is present in a strand of DNA. They refer to their system as a genetic cascade, because the GFP gene is transcribed into messenger RNA, which then directs synthesis of the protein.
These encapsulated translation systems exhibit a fundamental property of life: they use genetic information to synthesize a protein, but only a few specific proteins are produced, and everything else is left behind. To be truly alive, the protocells would need a DNA strand with genes for more than 200 different proteins and RNA species, including genes for a polymerase enzyme so that the DNA can be replicated. Enzymes that catalyze lipid synthesis must also be present, because the membrane boundary needs to grow. Transport proteins must be incorporated into the lipid bilayer; otherwise the vesicles have no access to external sources of nutrients and energy. A whole set of regulatory processes should also be in place, so that all this growth is coördinated. Finally, when the vesicles grow to approximately twice their original size, they need to divide into daughter cells that share the original genetic information.
It follows that even the simplest life today is astonishingly complex and could not have sprung into existence on the early Earth with a full complement of hundreds of genes. There must have been something simpler–a kind of scaffold life that was left behind in the evolutionary debris several billion years ago. Given all this, how likely is it that the ultimate promise of synthetic biology will be fulfilled–that an artificial version of a primitive living cell can be assembled? The best bet is probably a ribozyme that catalyzes its own complete synthesis from ATP, UTP, GTP, and CTP–the four nucleotide monomers of RNA–using genetic information encoded in its structure. If someone succeeds, we will have in hand the essential property that is lacking so far in artificial cell models: reproduction of the catalyst itself. Given such a ribozyme, we already know how to incorporate it into a system of lipid vesicles that can grow along with the ribozyme and allow nutrient nucleotides to enter the cell to support growth. The encapsulated ribozymes will have the capacity to evolve, as Bartel and Szostak demonstrated 15 years ago. In short, the system will be alive.
And then what happens? There will be headlines, of course; textbooks will be rewritten; and early in the morning someone will probably be awakened by a phone call from Stockholm. But after all the hullabaloo dies down; someone else will ask, “Well, so what?” That same question could have been asked when the double-helix structure of DNA was published in 1953. The magnitude of the discovery was not apparent until years later. I think that the first system of molecules capable of reproducing itself will also seem to be an academic exercise at first. But to put it in proper perspective, recall that food, antibiotics, oil, wood, methane, and hydrogen are produced by living cells resulting from more than three billion years of evolution. I think the next revolution in technology will begin when the synthetic functions of life can be performed by simplified versions of cells that are designed from blueprints rather than through evolution.
David Deamer is a research professor of biomolecular engineering at the University of California, Santa Cruz. He is currently writing a book on the origin of life, to be published by the University of California Press.
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