Alive! The race to create life from scratch

YOU might think Norman Packard is playing God. Or you might see him as the ultimate entrepreneur. As founder and CEO of Venice-based company ProtoLife, Packard is one of the leaders of an ambitious project that has in its sights the lofty goal of life itself. His team is attempting what no one else has done before: to create a new form of living being from non-living chemicals in the lab.

Breathing the spark of life into inanimate matter was once regarded as a divine prerogative. But now several serious and well-funded research groups are working hard on doing it themselves. If one of them succeeds, the world will have met alien life just as surely as if we had encountered it on Mars or Europa. That first alien meeting will help scientists get a better handle on what life really is, how it began, what it means to be alive and even whether there are degrees of "aliveness". "We want to demonstrate what the heck life is by constructing it," says Packard's business partner and colleague Steen Rasmussen, a physicist at Los Alamos National Laboratory in New Mexico. "If we do that, we're going to have a very big party. The first team that does it is going to get the Nobel prize."

Although the experiments are still in the earliest stages, some people, especially those with strong religious beliefs, feel uneasy at the thought of scientists taking on the role of creators. Others worry about safety - what if a synthetic life form escaped from the lab? How do we control the use of such technology?

Finding a way to address these worries will have benefits beyond helping scientists answer the basic questions of life. The practical pay-offs of creations like Rasmussen's could be enormous. Synthetic life could be used to build living technologies: bespoke creatures that produce clean fuels or help heal injured bodies. The potential of synthetic organisms far outstrips what genetic engineering can accomplish today with conventional organisms such as bacteria. "The potential returns are very, very large - comparable to just about anything since the advent of technology," says Packard. And there is no doubt that there is big money to be made too.

Only a few research groups have explicitly set themselves the goal of making a synthetic life form (see "Race for the ultimate prize" - bottom). Most are adapting bits and pieces from existing organisms. ProtoLife's plans are the most ambitious and radical of all. They focus on Rasmussen's brainchild, which he has nicknamed the Los Alamos Bug. Still but a gleam in its creator's eye, the Bug will be built up from first principles, using chemicals largely foreign to existing creatures. "You somehow have to forget everything you know about life," says Rasmussen. "What we have is the simplest we could dream up."

To achieve this radical simplicity, Rasmussen and his colleagues had to begin with the most basic of questions: what is the least something must do to qualify as being alive? Biologists and philosophers struggled to answer that question for decades (New Scientist, 13 June 1998, p 38). However, most now agree that one key difference - perhaps the only one - between life and non-life is Darwinian evolution. For something to be alive, it has to be capable of leaving behind offspring whose characteristics can be refined by natural selection. That requires some sort of molecule to carry hereditary information, as well as some sort of process - elementary metabolism - for natural selection to act upon. Some kind of container is also needed to bind these two components together long enough for selection to do its work.

Containment, heredity, metabolism; that's it in a nutshell. Put those together in the simplest way possible, and you've got the Los Alamos Bug. But every step is completely different from what we're used to (see graphic - the four stages shown are described further in "The Los Alamos Bug" - below).

Take containment, for example. Terrestrial life is always water-based, essentially a watery gel of molecules enclosed within an oily membrane. Modern cells move nutrients across this membrane with the help of an array of different proteins embedded in the membrane. The Los Alamos Bug, however, is completely different. For a start it is oil-based, little more than a droplet of fatty acids. "Instead of having a bag with all the good stuff inside, think of having a piece of chewing gum," says Rasmussen. "Then you stick the metabolic molecules and genetic molecules into the chewing gum, so they are attached on the surface or sitting inside the chewing gum."

The container is the easy part. The next step - heredity - is where most efforts to create synthetic life get bogged down. The challenge is to create a molecule complex enough to carry useful genetic information, which can also replicate. In modern organisms DNA has a whole army of enzymes to help it replicate its genetic information - far too complicated a process for the Bug. Instead, Rasmussen plans to use a molecule called peptide nucleic acid, or PNA. It uses the same "letters" of genetic code as DNA, but has two forms, one soluble only in fat, the other also attracted to water. Rasmussen hopes to put PNA's dual nature to use in a rudimentary form of replication (see Graphic).

The Bug's metabolism has also been pared down to the minimum. The researchers plan to "feed" it with chemicals that can be converted into fatty acids. If enough are produced, the droplet will grow and divide into two. A similar metabolic process turns PNA precursors into functional PNA.

Although most of the design is still on the drawing board or in the earliest stages of experimentation, the team has made most progress with the Bug's metabolism. "If you look at the individual pieces, they are all sort of demonstrated in the lab. But if you put everything together, not yet," says Liaohai Chen, a biochemist at Argonne National Laboratory near Chicago, who heads Rasmussen's experimental team. If all goes according to plan, these three components - container, genome and metabolism - should fit together to provide all the essentials for Darwinian evolution.

In October 2004, Rasmussen landed a large grant from Los Alamos to begin making the Bug a reality. "I can't promise that we'll have it in three years, but I can guarantee that we'll have good progress," he says. The biggest problem may be coordinating the copying of the PNA and the metabolism of the fatty acid precursors so that replication of the genome proceeds at the same pace as the growth of the droplets. "Almost always when you put processes together there are cross-reactions, things that your theories won't tell you about."

Another fledgling research programme, known as Programmable Artificial Cell Evolution, or PACE, could provide the solution to this coordination challenge. Packard and Rasmussen are collaborating with PACE, which is focusing some of its attention on Rasmussen's design. A key idea behind PACE is to deliver precise amounts of particular chemicals to synthetic cells at specific places and times using computers to precisely control the flow of tiny amounts of chemicals. For example, a computer could use sensors to monitor the rates of PNA replication and fatty acid production in Rasmussen's experimental system, then deliver the correct amounts of each precursor. That would let researchers work out the kinks one by one in a controlled, programmable setting, providing something rather like a life-support machine that helps artificial cells through the critical steps towards becoming alive. "Once we have our hybrid unit, then we can successively withdraw the machine to approach a stand-alone cell," says John McCaskill, a chemist at Ruhr University in Bochum, Germany, who heads the PACE programme.

In this way the PACE team plans eventually to evolve its way towards a self-supporting artificial cell. To do that, though, the team will need a way to recognise the system's first tentative steps down the pathway to life. But how do you recognise something faintly lifelike, when it looks nothing like the life we know?

Look for the footprints of adaptation, says Mark Bedau, a philosopher who specialises in the boundary between life and non-life. Bedau is on leave from Reed College in Oregon to work with Packard at ProtoLife. If something is evolving then it should be generating adaptations - novel solutions to the problems of the world. And those new solutions, however subtle and incremental, become the foundation from which evolution takes its next steps. Adaptations which confer some advantage should last longer and spread faster than other variations.

Bedau is developing statistical tests which will pick up these kinds of patterns in unfamiliar life forms. But since the PACE project has not yet begun lab experiments, he does not know whether the tests can detect the glimmerings of real life. However, he has road-tested them on a system that works in a similar way, namely human culture.

In 2002, Bedau and colleague Andre Skusa sifted through more than five years of US patent records, counting the number of times each patent has been cited as a basis for later patents. They found that a few patents - such as the one enabling a web browser to display an ad while loading the main page - were cited far more often than one would expect if the differences found in the number of citations for inventions were random. These key innovations are the equivalent of biological adaptations such as opposable thumbs. "That gives you reason to think it should be possible to do the same kind of thing in chemical systems which are not yet alive but might be on the path to being alive," says Bedau.

Using tests like these, the PACE team hopes to see its hybrid gradually become more and more lifelike. But at what point would it actually become alive? Perhaps at no particular point, says Bedau, who thinks it is quite possible that the living and the non- living are separated not by a clear, distinct line but by a wide grey area in which the Bug is partly but not totally alive. "There are shades of grey, and I imagine measuring how dark the grey is," he says. "Our conception of what life is will evolve as we learn more and acquire the ability to make things that are more and more alive."

The moment when a blob of molecules becomes a fully living, evolving being is at least several years off. "Even our optimists wouldn't put a time horizon much sooner than 10 years for that kind of achievement," says Packard. Indeed, sceptics wonder whether the Los Alamos Bug and its ilk will ever yield anything useful. "It's certainly interesting from the conceptual point of view," says Pier Luigi Luisi, a biochemist at the University of Rome 3 and an expert on synthetic life. "But nature with nucleic acids and enzymes is so much smarter, because these are products that have been optimised over billions of years of evolution. To pretend to do life with simple chemistry is a nice ambitious idea, but it's probably not going to be very efficient."

Still, if Packard, Rasmussen and their colleagues do someday succeed in creating synthetic life forms, they will have opened the door to a world of new possibilities. "We are breaking the last barriers between us and living technology," says Rasmussen. "That's going to be a very big thing. It's going to happen, no doubt about it."

Among the most obvious payoffs could be organisms custom-designed to break down toxic compounds or produce useful chemicals such as hydrogen fuel. More conventional organisms can be genetically modified to do these tasks, but as Rasmussen points out, "the problem is these guys have evolved for billions of years. They're extremely versatile, and it's very difficult to keep them on task." An artificial organism, on the other hand, could in principle be built to do nothing but the task at hand, yet still have the evolutionary flexibility to adapt to changing conditions.

Packard hopes that this controlled adaptability could lead to even greater things. He envisions living pharmaceuticals that deliver drugs to us in an intelligent, adaptive way, or diagnostic life forms that could roam our bodies collecting information and watching for signs of a problem. The ultimate goal would be machines that repair themselves as living beings do - even computers that can handle incredibly complex calculations while coping with inevitable errors, just as our bodies tolerate errors and failures within our hundreds of billions of cells.

If life is all about the ability to evolve and adapt, then living technologies always have the potential to surprise us with unexpected new strategies that can take them beyond our control. But then again, that risk is nothing new. We already grapple with it when contemplating what would happen if robots or artificial intelligence were to get out of hand and in evaluating the safety of genetically modified fish, crossbred potatoes or even introduced rabbits. Indeed, for the foreseeable future, synthetic life probably poses much less of an escape risk, because the early versions, at least, will be so fragile and require so much life support. That means the safety of synthetic life is something to keep an eye on, not to be frightened of. "There isn't going to be some precipice we're going to fall over," says Bedau. "We'll be slowly inching our way down, and we'll have lots of opportunity to turn around."

As well as concerns about safety, synthetic life raises some profound ethical and religious issues. "Just the fact that you're making life from scratch will give some people pause. They will think that's a prerogative that humans should never take," says Bedau. If humans can create life on their own, doesn't that remove one of the last deep mysteries of existence, in effect prying God's fingers from one of his last remaining levers to affect the world?

Not necessarily, say theologians. "We are fully a part of nature, and as natural beings who are living and creating synthetic life, we are in a sense life creating more life, which is what's been going on in evolution for 4 billion years now," says John Haught, a Catholic theologian at Georgetown University in Washington DC. "And that does not in principle rule out that God would still be creating life using natural causes - namely us - which is the way in which theology understands God as always operating in the world."

One thing seems certain; synthetic life will provide philosophers with plenty to chew on right from the start. Until now, efforts to come up with a good definition of life have been hampered by the fact that we are trying to generalise from just one example, the life that arose here on Earth. Having a second form, completely independent and based on different chemistry, should give a new perspective on this age-old question. And knowing what did or did not work in the lab, may also help us understand the origin of life - the first version, that is - on Earth.

Containment This relies on the fact that oil and water do not mix. The components of each individual Bug are contained by a droplet of fatty acids, suspended in a watery solution enclosed by a test tube. Each fatty acid molecule has a negatively charged head which is attracted to water and which faces out into the watery environment, and a water-hating oily tail facing inward.

Heredity Instead of DNA the Bug has short stretches of peptide nucleic acid, or PNA. Like DNA, PNA is made of two intertwining strands containing the genetic "letters" A, T, C and G. And like DNA, the sequences of letters on these stands complement each other. A pairs up with T and C pairs with G.

The strands have a peptide backbone which does not carry an electrical charge, so will dissolve in fat. This means that the molecules of PNA prefer to face the inside of the fatty acid droplet, like crumbs embedded in the surface of a piece of chewing gum.

This gives the molecule unusual mobility. In its usual double-stranded form, with its two peptide backbones facing outwards, a PNA molecule is completely fat-soluble, so it will sink into the oily centre of the Bug's droplet. But above some critical temperature, the two strands of the PNA double helix separate spontaneously. When this happens, the bases, which bear a slight charge, are exposed and attracted to the Bug's watery environment.

So these single-stranded PNA molecules should then migrate to the edge of the droplet where the backbone can remain in the oil while the bases interact with the water outside.

This mobility provides the handle needed to control replication. The plan is to supply the Bug with short bits of single-stranded PNA precursors, just half the length of its tiny genome. If a single-stranded PNA gene on the Bug's surface encounters two of these "nutrient" PNAs with the right base sequences, it will pair with them to form a double-stranded PNA molecule. This should then sink down into the droplet, where conditions favour the joining-up of the two "nutrient" fragments into a whole strand.

Eventually, the double-stranded molecule will dissociate once again and its two strands drift back to the surface where each can pick up new partners - a rudimentary form of replication.

Metabolism The third essential part of the Bug's life - metabolism - has also been pared to its barest minimum. The researchers plan to "feed" the Bug with fatty acid precursors. These will have photosensitive molecules attached their charged "head" ends. These photosensitive caps mask the charged head, making the molecules completely fat soluble. This means they will tend to collect within the Bug's droplets.

When light strikes the photosensitive cap, it breaks off, exposing the negatively charged fatty acid head, which migrates back to the surface of the droplet. Eventually, so many new fatty acids will be produced that they will not all fit on the surface and the droplet will split in two to create a larger surface area.

The Bug will also be supplied with inactive PNA precursors bound to a photosensitive molecule. Once again, when light strikes this photosensitiser, it breaks off to release the active PNA fragment.

Effective metabolism also requires one more step to prevent the photosensitive molecule, once broken off, from re-sticking to the fatty acid or PNA and so deactivating it once again. The PNA genetic material prevents this by acting as a rudimentary wire, conducting electrons to neutralise the photosensitiser. In this way, the Bug's "genome" plays an active role in the metabolic process.

Evolution If all goes according to plan, these three components - container, genome, metabolism - should fit together to provide all the essentials for Darwinian evolution. As the Bugs grow and reproduce, corralled in a test tube, natural selection should favour PNA base sequences that pair up and split off fastest, and also conduct electrons most efficiently to the photosensitisers.

Synthetic slaves Artificial organisms could be custom-built for particular tasks:

# act as "living pharmaceuticals", delivering drugs in the body in an adaptive way

# be tiny diagnosticians, roaming our bodies, collecting information and checking for problems

THE Los Alamos Bug has some stiff competition in the race to be the first artificial life form, especially since some of the entrants are taking much more conventional routes to that goal.

At the Institute for Biological Energy Alternatives in Rockville, Maryland, Craig Venter, leader of the private group that sequenced the human genome, and colleague Hamilton Smith are trying to create a new life form by extracting the genome from an existing bacterium and replacing it with a synthetic genome stripped down to a bare minimum of genes (New Scientist, 31 May 2003, p 28).

Because this approach leaves most of the cell's machinery intact, Venter's team is widely expected to be the first to succeed, perhaps within a few months or years. (Uncharacteristically, Venter is not talking to the press about this project.) However, Venter's new organism will end up looking very much like existing life.

And at the University of Rome 3, Pier Luigi Luisi is working on the "minimal cell project". Starting with a simple membrane-bound vesicle, Luisi's team plans to gradually add in off-the-shelf enzymes and other cellular components until they assemble the simplest possible working cell.

Across the Atlantic at Harvard University, Jack Szostak has been working on a synthetic life form just as simple as Rasmussen's Los Alamos Bug, but using more familiar chemistry. Szostak's design calls for a tiny membrane-bound vesicle containing little more than an RNA or RNA-like molecule with a special talent: that of catalysing its own replication.

The problem is that no one has yet developed an RNA capable of replicating more than just a small part of itself. Szostak predicts success is probably 10 or 20 years off. "I've been saying that for the last 10 or 20 years," he says, "and it's still true."

I find it amusing that these people are attempting to create life using the creation model rather than the evolutionary model. IOW, they are attempting to "create" life rather than to have it evolve naturally.

You need to read the article again -- they're going to get a primitive replicator going, and *then* let it evolve.

Of course that is the only way life could possibly have come about in the first place, since the most important ingredient in the creation of life was and is.... intelligence.

Just how intelligent do cyanobacteria seem to you?

And what exactly do you find wrong with the following scenarios (which are based on a huge amount of research concerning the earliest kinds of life):

(See for example chapter 2 entitled "Phylogeny from Function: The Origin of tRNA Is in Replication, not Translation", in the online book "Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson".)

The Path from the RNA World Anthony M. Poole, Daniel C. Jeffares, David Penny: Institute of Molecular Biosciences, Massey University
Abstract: We describe a sequential (step by step) Darwinian model for the evolution of life from the late stages of the RNA world through to the emergence of eukaryotes and prokaryotes. The starting point is our model, derived from current RNA activity, of the RNA world just prior to the advent of genetically-encoded protein synthesis. By focusing on the function of the protoribosome we develop a plausible model for the evolution of a protein-synthesizing ribosome from a high-fidelity RNA polymerase that incorporated triplets of oligonucleotides. With the standard assumption that during the evolution of enzymatic activity, catalysis is transferred from RNA M RNP M protein, the first proteins in the ``breakthrough organism'' (the first to have encoded protein synthesis) would be nonspecific chaperone-like proteins rather than catalytic. Moreover, because some RNA molecules that pre-date protein synthesis under this model now occur as introns in some of the very earliest proteins, the model predicts these particular introns are older than the exons surrounding them, the ``introns-first'' theory. Many features of the model for the genome organization in the final RNA world ribo-organism are more prevalent in the eukaryotic genome and we suggest that the prokaryotic genome organization (a single, circular genome with one center of replication) was derived from a ``eukaryotic-like'' genome organization (a fragmented linear genome with multiple centers of replication). The steps from the proposed ribo-organism RNA genome M eukaryotic-like DNA genome M prokaryotic-like DNA genome are all relatively straightforward, whereas the transition prokaryotic-like genome M eukaryotic-like genome appears impossible under a Darwinian mechanism of evolution, given the assumption of the transition RNA M RNP M protein. A likely molecular mechanism, ``plasmid transfer,'' is available for the origin of prokaryotic-type genomes from an eukaryotic-like architecture. Under this model prokaryotes are considered specialized and derived with reduced dependence on ssRNA biochemistry. A functional explanation is that prokaryote ancestors underwent selection for thermophily (high temperature) and/or for rapid reproduction (r selection) at least once in their history.

And:

On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells William Martin and Michael J. Russell
Abstract: All life is organized as cells. Physical compartmentation from the environment and self-organization of self-contained redox reactions are the most conserved attributes of living things, hence inorganic matter with such attributes would be life’s most likely forebear. We propose that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyse the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments, which furthermore restrained reacted products from diffusion into the ocean, providing sufficient concentrations of reactants to forge the transition from geochemistry to biochemistry. The chemistry of what is known as the RNA-world could have taken place within these naturally forming, catalyticwalled compartments to give rise to replicating systems. Sufficient concentrations of precursors to support replication would have been synthesized in situ geochemically and biogeochemically, with FeS (and NiS) centres playing the central catalytic role. The universal ancestor we infer was not a free-living cell, but rather was confined to the naturally chemiosmotic, FeS compartments within which the synthesis of its constituents occurred. The first free-living cells are suggested to have been eubacterial and archaebacterial chemoautotrophs that emerged more than 3.8 Gyr ago from their inorganic confines. We propose that the emergence of these prokaryotic lineages from inorganic confines occurred independently, facilitated by the independent origins of membrane-lipid biosynthesis: isoprenoid ether membranes in the archaebacterial and fatty acid ester membranes in the eubacterial lineage. The eukaryotes, all of which are ancestrally heterotrophs and possess eubacterial lipids, are suggested to have arisen ca. 2 Gyr ago through symbiosis involving an autotrophic archaebacterial host and a heterotrophic eubacterial symbiont, the common ancestor of mitochondria and hydrogenosomes. The attributes shared by all prokaryotes are viewed as inheritances from their confined universal ancestor. The attributes that distinguish eubacteria and archaebacteria, yet are uniform within the groups, are viewed as relics of their phase of differentiation after divergence from the non-free-living universal ancestor and before the origin of the free-living chemoautotrophic lifestyle. The attributes shared by eukaryotes with eubacteria and archaebacteria, respectively, are viewed as inheritances via symbiosis. The attributes unique to eukaryotes are viewed as inventions specific to their lineage. The origin of the eukaryotic endomembrane system and nuclear membrane are suggested to be the fortuitous result of the expression of genes for eubacterial membrane lipid synthesis by an archaebacterial genetic apparatus in a compartment that was not fully prepared to accommodate such compounds, resulting in vesicles of eubacterial lipids that accumulated in the cytosol around their site of synthesis. Under these premises, the most ancient divide in the living world is that between eubacteria and archaebacteria, yet the steepest evolutionary grade is that between prokaryotes and eukaryotes.

And:

The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front M. J. RUSSELL & A. J. HALL: Department of Geology and Applied Geology, University of Glasgow
Abstract: Here we argue that life emerged on Earth from a redox and pH front at c. 4.2 Ga. This front occurred where hot (c. 150)C), extremely reduced, alkaline, bisulphide-bearing, submarine seepage waters interfaced with the acid, warm (c. 90)C), iron-bearing Hadean ocean. The low pH of the ocean was imparted by the ten bars of CO2 considered to dominate the Hadean atmosphere/hydrosphere. Disequilibrium between the two solutions was maintained by the spontaneous precipitation of a colloidal FeS membrane. Iron monosulphide bubbles comprising this membrane were inflated by the hydrothermal solution upon sulphide mounds at the seepage sites. Our hypothesis is that the FeS membrane, laced with nickel, acted as a semipermeable catalytic boundary between the two fluids, encouraging synthesis of organic anions by hydrogenation and carboxylation of hydrothermal organic primers. The ocean provided carbonate, phosphate, iron, nickel and protons; the hydrothermal solution was the source of ammonia, acetate, HS", H2 and tungsten, as well as minor concentrations of organic sulphides and perhaps cyanide and acetaldehyde. The mean redox potential (ÄEh) across the membrane, with the energy to drive synthesis, would have approximated to 300 millivolts. The generation of organic anions would have led to an increase in osmotic pressure within the FeS bubbles. Thus osmotic pressure could take over from hydraulic pressure as the driving force for distension, budding and reproduction of the bubbles. Condensation of the organic molecules to polymers, particularly organic sulphides, was driven by pyrophosphate hydrolysis. Regeneration of pyrophosphate from the monophosphate in the membrane was facilitated by protons contributed from the Hadean ocean. This was the first use by a metabolizing system of protonmotive force (driven by natural ÄpH) which also would have amounted to c. 300 millivolts. Protonmotive force is the universal energy transduction mechanism of life. Taken together with the redox potential across the membrane, the total electrochemical and chemical energy available for protometabolism amounted to a continuous supply at more than half a volt. The role of the iron sulphide membrane in keeping the two solutions separated was appropriated by the newly synthesized organic sulphide polymers. This organic take-over of the membrane material led to the miniaturization of the metabolizing system. Information systems to govern replication could have developed penecontemporaneously in this same milieu. But iron, sulphur and phosphate, inorganic components of earliest life, continued to be involved in metabolism.

And:

Obcells as Proto-Organisms: Membrane Heredity, Lithophosphorylation, and the Origins of the Genetic Code, the First Cells, and Photosynthesis (Journal of Molecular Evolution, Volume 53 - Number 4/5, 2001)
N-Carbamoyl Amino Acid Solid-Gas Nitrosation by NO/NO_x: A New Route to Oligopeptides via alpha-Amino Acid N-Carboxyanhydride. Prebiotic Implications (Journal of Molecular Evolution, Volume 48 - Number 6, 1999
Chemical interactions between amino acid and RNA: multiplicity of the levels of specificity explains origin of the genetic code (Naturwissenschaften, Volume 89 Number 12 December 2002)
The Nicotinamide Biosynthetic Pathway Is a By-Product of the RNA World (Journal of Molecular Evolution, Volume 52 - Number 1, 2001)
On the RNA World: Evidence in Favor of an Early Ribonucleopeptide World
Inhibition of Ribozymes by Deoxyribonucleotides and the Origin of DNA
Genetic Code Origin: Are the Pathways of Type Glu-tRNAGln to Gln-tRNAGln Molecular Fossils or Not?
Johnston WK, Unrau PJ, Lawrence MS, Glasner ME, Bartel DP.RNA-catalyzed RNA polymerization: accurate and general RNA-templated primer extension. Science. 2001 May 18;292(5520):1319-25.
Ferris JP. (1999 Jun). Prebiotic synthesis on minerals: bridging the prebiotic and RNA worlds. Biol Bull , 196, 311-4.
Levy M, and Miller SL. (1999 Jun). The prebiotic synthesis of modified purines and their potential role in the RNA world. J Mol Evol , 48, 631-7.
Unrau PJ, and Bartel DP. (1998 Sep 17). RNA-catalysed nucleotide synthesis [see comments] Nature , 395, 260-3.
Roth A, and Breaker RR. (1998 May 26). An amino acid as a cofactor for a catalytic polynucleotide [In Process Citation] Proc Natl Acad Sci U S A , 95, 6027-31.
Jeffares DC, Poole AM, and Penny D. (1998 Jan). Relics from the RNA world. J Mol Evol , 46, 18-36.
Poole AM, Jeffares DC, and Penny D. (1998 Jan). The path from the RNA world. J Mol Evol , 46, 1-17.
Wiegand TW, Janssen RC, and Eaton BE. (1997 Sep). Selection of RNA amide synthases. Chem Biol , 4, 675-83.
Di Giulio M. (1997 Dec). On the RNA world: evidence in favor of an early ribonucleopeptide world. J Mol Evol , 45, 571-8.
Hager AJ, and Szostak JW. (1997 Aug). Isolation of novel ribozymes that ligate AMP-activated RNA substrates. Chem Biol , 4, 607-17.
James KD, and Ellington AD. (1997 Aug). Surprising fidelity of template-directed chemical ligation of oligonucleotides [In Process Citation] Chem Biol , 4, 595-605.
Lohse PA, and Szostak JW. (1996 May 30). Ribozyme-catalysed amino-acid transfer reactions. Nature , 381, 442-4.
Lazcano A, and Miller SL. (1996 Jun 14). The origin and early evolution of life: prebiotic chemistry, the pre- RNA world, and time. Cell , 85, 793-8.
Ertem G, and Ferris JP. (1996 Jan 18). Synthesis of RNA oligomers on heterogeneous templates. Nature , 379, 238-40.
Robertson MP, and Miller SL. (1995 May 5). Prebiotic synthesis of 5-substituted uracils: a bridge between the RNA world and the DNA-protein world [see comments] Science , 268, 702-5.
Robertson MP, and Miller SL. (1995 Jun 29). An efficient prebiotic synthesis of cytosine and uracil [published erratum appears in Nature 1995 Sep 21;377(6546):257] Nature , 375, 772-4.
Breaker RR, and Joyce GF. (1995 Jun). Self-incorporation of coenzymes by ribozymes. J Mol Evol , 40, 551-8.
James KD, and Ellington AD. (1995 Dec). The search for missing links between self-replicating nucleic acids and the RNA world. Orig Life Evol Biosph , 25, 515-30.
Bohler C, Nielsen PE, and Orgel LE. (1995 Aug 17). Template switching between PNA and RNA oligonucleotides [see comments] Nature , 376, 578-81.
Connell GJ, and Christian EL. (1993 Dec). Utilization of cofactors expands metabolism in a new RNA world. Orig Life Evol Biosph , 23, 291-7.
Nielsen PE. (1993 Dec). Peptide nucleic acid (PNA): a model structure for the primordial genetic material? Orig Life Evol Biosph , 23, 323-7.
Lahav N. (1991 Aug 21). Prebiotic co-evolution of self-replication and translation or RNA world? J Theor Biol , 151, 531-9.
Ekland EH, Szostak JW, and Bartel DP. (1995 Jul 21). Structurally complex and highly active RNA ligases derived from random RNA sequences. Science , 269, 364-70.

Let me know when you get done reading those, and I'll give you some more.

Or if you'd like the Cliff-Notes version, here's a schematic:

Any other questions?

Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.