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All this should be very disappointing to 

 those who hold that proteins arose sponta- 

 neously, but proteins are so fundamental 

 to life that the idea still has a few support- 

 ers. Another theory, superficially attrac- 

 tive, holds that life arose somewhere else 

 in the universe — reaching this planet via 

 comets or dust grains. This doesn't really 

 solve the problem, however; it just shifts it 

 elsewhere and creates new dilemmas. Any 

 cometary debris that might have borne the 

 seeds of life would have been subjected to 

 extremes of temperature, and the all-im- 

 portant enzymes are very temperature sen- 

 sitive, only working in a narrow tempera- 

 ture range from about -14° F to 212° F 

 and being irreversibly destroyed at higher 

 temperatures. Proponents of this theory 

 must explain not only how life arose but 

 also how it operated at the extremes of 

 temperature found in space. And how did 

 it then adapt to the earth, where the major- 

 ity of proteins function most efficiently at 

 about 80° F? The more ambitious theorists 

 would have life arising around other plan- 

 ets or even stars. But to escape, the primi- 

 tive organism would have had to over- 

 come a gravitational force so strong that 

 only the fiery impact of a large comet or 

 asteroid could blast it into space — an 

 event also likely to desfiroy it. Scientists 

 proposing that life arose either on other 

 planets or in other solar systems must ex- 

 plain not only how the chemistry to form 

 life happened but also how it got here. 



Back with our feet firmly on the ground, 

 we need to look again at the origin-of-life 

 problem. The "protein first" argument 

 fails primarily because even if proteins did 

 manage to assemble themselves success- 

 fully, they had no way of copying them- 

 selves so that their success could be 

 recorded and amplified. (The order in 

 which the amino acids are strung together 

 is crucial because it determines how the 

 chains will fold and twist into the three-di- 

 mensional shapes of individual proteins.) 

 Only one group of biological molecules 

 can copy themselves. These are the two 

 slightly different nucleic acids, ribonucleic 

 acid (RNA) and deoxyribonucleic acid 

 (DNA). Like enzymatic protein mole- 

 cules, nucleic acids are long and un- 

 branched, formed from subunits. The 

 building blocks are called nucleotides and, 

 like amino acids, can be assembled fairly 

 easily from simple, inorganic molecules. 

 The nucleotides also have side chains, or 

 "bases," but there are only four common 

 ones and they are divided into two pairs, 

 each of which has a special affinity for the 

 other. As the bases in a chain of RNA at- 



tract then- partners from solution, a com- 

 plementary chain is built that separates 

 from the original (A). When this comple- 

 mentary chain attracts its own comple- 

 ment, a copy of the original RNA se- 

 quence emerges (B). The really exciting 

 thing that points to RNA as the first "living 

 molecule" is that not only does it replicate 

 but it can also act as a catalyst. 



The majority of scientists working on 

 the origin of life now believe that there 

 was a time when RNA was the only bio- 

 logical molecule. But the range of reac- 

 tions that RNA can accelerate is small and 

 usually involves only the joining or split- 

 ting apart of RNA molecules. Such a "liv- 

 ing" system is extremely limited. It has not 

 overcome the hurdle of protein produc- 

 tion, which would extend the range of 

 RNA so that a much broader range of re- 

 actions can occur. The solution offered by 

 the "RNA world" proponents is that small 

 segments of RNA — called adapter mole- 

 cules — go off and find the correct amino 

 acid and bring it back to the parent RNA 

 for assembly into a protein. This is really 

 an enormous logistical exercise, and one 

 that introduces a lot of problems. How, for 

 example, do the little RNA molecules rec- 

 ognize an amino acid? How do they join to 

 it? Why do they come back "full" and not 

 "empty"? How do they give up the amino 

 acid to growing protein chains? But the so- 

 lutions to these questions make the system 

 more and more complicated. 



One of the guiding principles of science 

 is Occam's razor, which suggests that the 

 most likely explanation is the simplest. 

 Making functioning proteins requires both 

 the information that specifies the sequence 

 of amino acids and the amino acids them- 

 selves. The information is encoded on 

 RNA (there is no other plausible candi- 

 date) and is carried in sequences of three 

 bases. The simplest theory would be that 



three bases on the RNA recognize the 

 amino acids. This was investigated by 

 some biologists in the 1960s. The re- 

 searchers put short segments of RNA of 

 known base sequence in solution to see if 

 they could capture specific amino acids. 

 The results were negative, so the theory 

 gained little support, and the more com- 

 plex theories became popular. 



But did those early experiments give the 

 simplest theory a fair shake? Let's go back 

 to the fish analogy. If you wanted to catch 

 fish, you could use a net. But if you just 

 throw a piece of loose netting into the sea, 

 you will probably fail. Similarly, free- 

 floating RNA molecules cannot capture 

 the amino acids in a solution, because the 

 RNA will be buffeted by all sorts offerees 

 and will drift about wrapping itself up ran- 

 domly, just as loose netting would. To suc- 

 ceed in catching the fish, the net needs to 

 have a rigid support. 



In the past forty years, the nucleic acids 

 have been studied intensively. One of the 

 most exciting techniques has been "gene 

 probing," which involves extracting nu- 

 cleic acids and attaching them to a variety 

 of solids by their backbone, so that the 



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12 Natural History 6/94 



