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033108-1[1] - The origin of life In the beginning How life...

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Unformatted text preview: The origin of life In the beginning... How life on Earth got going is still mysterious, but not for want of ideas EVER make forecasts, especially about the future. Samuel Goldwyn’s wise advice is well illustrated by a pair of scien- tific papers published in 1953. Both were thought by their authors to be milestones on the path to the secret of life, but only one has so far amounted to much, and it was not the one that caught the public imagination at the time. James Watson and Francis Crick, who wrote “A structure for deoxyribose nucleic acid”, have become as famous as rock stars for asking how life works and thereby starting a line of inquiry that led to the Hu- man Genome Project. Stanley Miller, by contrast, though lauded by his peers, lan- guishes in obscurity as far as the wider world is concerned. Yet when it appeared, “Production of amino acids under possi- ble primitive Earth conditions" was ex- pected to begin a scientific process that would solve a problem in some ways more profound than how life works at the mo- ment—namely how it got going in the first place on the surface of a sterile rock 150m km from a small, unregarded yellow star. Dr Miller was the first to address this question experimentally. Inspired by one of Charles Darwin‘s ideas,thattheingredi— ents of life might have formed by chemical reactions in a “warm, little pond“, he mixed the gases then thought to have formed the atmosphere of the primitive Earth—methane, ammonia and hydro- gen-in a flask half-full of boiling water, and passed electric sparks. mimicking lightning, through them for several days to see what would happen. What happened. as the name of the paper suggests, was amino acids, the building blocks of pro- teins. The origin of life then seemed within graSp. But it has eluded researchers ever since. They are still looking, though, and this week several of them met at the Royal Society. in London, to review progress. The origin of pieces The origin question is really three sub— questions. One is, where did the raw ma- terials for life come from? That is what Dr Miller was asking. The second is, how did those raw materials spontaneously assem- ble themselves into the first object to which the term “alive” might reasonably be applied? The third is, how, having once come into existence, did it survive condi- tions in the early solar system? The first question was addressed by Patrick Thaddeus, of the Harvard-Smithso- nian Centre for Astrophysics, and Max Bernstein, who works at the Ames labora- tory, in California. part of America‘s space agency. NASA. As Dr Bernstein succinctly put it, the chemical raw materials for life, in the form of simple compounds that could then be assembled into more complex bio- molecules, could come from above. below or beyond. The Economist February 18th 2006 75 Also in this section 76 Climate change and the world's rivers 77 Climate change in Greenland The “above” theory—ie, that the raw materials were formed in the atmosphere. and which Dr Miller‘s original experiment was intended to investigate—has fallen out of favour. That is because it depends on the atmosphere being composed of chemi- cals rich in hydrogen, which methane and ammonia are. Dr Miller thought this was likely because it was known in the 19508 that Jupiter's atmosphere contains these gases. Modern thinking. though, favours an early terrestrial atmosphere rich in car- bon dioxide. as is found on Venus and Mars. Such an atmosphere is no godd for making amino acids. The "beyond" theory is that the raw materials were formed in space, and came to Earth either while it was being formed, or in the form of a later chemical “top up" from comets and interstellar dust. Dr Thaddeus waxed eloquent in support of this, pointing out that radio astronomy has now identified 135 different molecules in outer space {each gives out a specific pat- tern of radio waves when its atoms are shaken, allowing it to be identified from afar). Moreover, these molecules tend to be concentrated in the sorts of nebulae in which stars and their associated planetary systems are known to form. Sadly, though, few of the 135 chemicals found so far resemble any important building block of life. That leaves the “be- low“ hypothesis, which is the one Dr Bernv stein favours. His theory is that the crucial raw materials were built up in hydrother- mal vents like those found today in the deep ocean. These do, indeed, leak chemi- cals of the sort that Dr Miller used, though they provide reaction—encouraging energy in the form of heat alone, with no electric- ity. Nevertheless, modern vents do seem to produce not only simple amino acids but also short aminovacid chains—in other » 76 Scienceand technplbfi_ > words.tiny proteins. Going from the raw materials to the fin- ished product. though, is a big step. In this case, the definition of"finished product“ is something that is recognisably the ances- tor of life today. Such an ancestor would store information in DNA. or a molecule similar to it. that was able to replicate. and thus breed. It would also use that informa- tion to make proteins. And it would proba- bly do all this inside a membrane made of fatty molecules. In other words. it would be a living cell. Worlds without end The favoured theory at the moment is that the first genetic material was not DNA. but its cousin RNA. In the wake of the Watson and Crick paper. a series of experiments showed that RNA acts as a messenger for the DNA, and as a fetcher and carrier of amino acids for the factories in which pro— teins are made. Until recently, therefore. it was seen as a rather humble substance—a molecular hewer of wood and drawer of water for the presiding DNA genius in the cell nucleus. But it is also an important component of the protein factories them- selves. Indeed. these factories are known as ribosomes because of it. And the past few years have seen the discovery of more and more roles for RNA. including some in which it acts as a chemical catalyst—a job that had previously been thought to be re- stricted to protein—based enzymes. This ubiquity. combined with the fact that RNA can catalyse chemical reactions. has led to the idea of an RNA world that preceded the modern DNAIprotein world-and it seems very likely that RNA did. indeed. precede DNA. if only because it is the more chemically stable of the two. But that does not explain either where the RNA came from in the first place, or how the RNAi‘protein interdependence came aboutwa question known as the "break- out"problem. There are several ideas for how large molecules such as RNA (and also early pro- teins) might have been generated out of the chemical raw materials that came from above or below or beyond. Two of the most persistent. though. are that clay was the catalyst. and that iron and nickel sul- phides were the catalysts. The clay theory is widely held. but needs tightening up.James Ferris of Rens- selaer Polytechnic Institute, in New York state. explained to the meeting that his re- search on a type of clay called montmoril- ionite showed that it catalysed the forma- tion of RNA molecules up to so units long. (A unit. in this context. is one of the four chemical bases that make up the alphabet of the genetic code. attached to some sugar and phosphate.) He also showed that the process was selective. with the same rela- tively small set of RNA molecules emerg- ing every time. That is important. because A crucible of life? if all possible permutations of the four bases were equally likely. none of them would ever become common enough for anything interesting to happen. The ironrnickeltsulphur model is the brainchild of Gunter wachtershauser of Munich University. It. too. relies on cataly- sis, though in this case the best-tested chemical pathways generate amino acids and proteins. rather than RNA. Unfortu- nately. neither the clay route nor the iron! nickel route answers the breakout ques- tion. But a third. and novel. model de- scribed at the meeting might. This was devised by Trevor Dale of Car- diff University. in Wales. He has come up with a way that proteins and RNA might catalyse each other‘s production. The protein involved would crystallise in the form of long, and easily formed. fi- bres called amyloid. (This is the form that proteins take in brain diseases such as Alz- heimer‘s and Creutzfeldt-lakob.) The amy- loid fibres would then act as surfaces on which RNA molecules could grow. Crucially. RNA forming on a fibre this way would grow as double strands, like the DNA in a cell nucleus. rather than as the single strands in which the molecule normally comes. when the strands sepa- rated. each would act as a template for a new double-stranded molecule. just as happens when a DNA molecule divides. The protein. meanwhile, would grow because the protruding end of the RNA would act as a catalyst. adding amino ac- ids on to the end of the amyloid fibre. when the fibre grew toolongto be stable,it would break in two. Thus both RNA and protein would replicate. Such a system. Dr Dale thinks. could be the ancestor of the ribosome and. if wrapped in a fatty membrane. of the cell. And. as David Deamer, of the University of California. Santa Cruz, told the meeting. such membranes will assemble spontane- ously in certain conditions. Dr Dale's idea is certainly chemically plausible. though it has yet to be tested in a The mnmurebgiisigiath 2006 laboratory. But he is conducting tests at the moment. and hopes to have the results later this year. The third sub-question—of how life managed to get going at all in the hastile arena of the early Earth, was neatly ad- dressed by Charles Cockell. of Britain‘s Open University. The perceived problem is that for the first soom years of its exis- tence. the planet was being bombarded by bits of debris left over from the formation of the solar system. Yet chemical signa- tures in the few rocks left over from this period suggest that life—presumably in the form of bacteria-was well established by the end of it. How, then. did that life sur- vive the constant rain of asteroids? Beginnings are such delicate times Dr Cockell turned the question neatly on its head by showing that impact craters are ideal places for life to get going. The heat generated by an impact produces local hy- drothermal springs. These start off hot. thus favouring the formation of amino ac- ids and RNA-forming bases. They then cool over the centuries to the point where these individual molecules can get to- gether in more complex chains. And they also have lots of microscopic nooks and crannies with space for micro-organisms to breed. and interesting chemicals in them for bugs to feed on. The biggest irony of all, then, might be that the conditions once thoughtanear-in- superable obstacle to the emergence of life on Earth may actually have enabled it to comeabout. l Climate change (I) . Full to burstin Rising levels of carbon dioxide will dump even more water into the oceans I-IE lungs of the planet. namely green- leafed plants that breathe in carbon di- oxide and breathe outoxygen.also put wa- ter vapour into the atmosphere. Just as people lose water through breathing (think of the misted mirror used to check for vital signs). so. too. do plants. The ques- tion is. what effect will rising concentra- tions of carbon dioxide have on this? The answer. published in this week‘s Nature by Nicola Gedney of Britain’s Meteorologi- cal Office and her colleagues. would ap- pear to be. less water in the atmosphere and more in the oceans. Measurements of the volume of water that rivers return to the oceans show that. around the world. rivers have become fuller over the past century. In theory. there are many reasons why this could be » ...
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