The Origin 

 of Life 



Which came first , proteins 

 orRNA? 



^ by Anthony Mellersh 



O 



. ,_H The origin of life remains one of the 



^— > Great Questions. The presence of fossil 

 ^ bacteria in rocks 3.8 billion years old sug- 



^~~* gests that very soon after the earth cooled, 

 O life arose from the simple organic chemi- 

 J> cals present in the primordial soup. But 



["tI how did small molecules organize and 

 begin to replicate, transforming a sterile 

 planet into a hving world? By necessity, 

 the answers so far have been speculative, 

 but we can make some educated guesses 

 about certain steps in the early chemical 

 evolution of hfe. 



Underlying all living systems is a com- 

 plex web of chemical reactions orches- 

 trated by enzymes. These biological cata- 

 lysts, almost all of which are proteins, 

 deliver the right chemicals at the right time 

 and at the right place, insuring that the en- 

 ergy and building blocks are brought to- 

 gether for each cellular function. These re- 

 actions, essential to life, would be unlikely 

 to occur in the absence of enzymes. If we 

 are to explain how the earliest organisms 

 arose, we must figure out how these and 

 other proteins can be made from scratch. 



Proteins are long, unbranched mole- 

 cules made up of subunits called amino 

 acids. Amino acids and other small or- 

 ganic chemicals almost certainly came 

 from a variety of sources, hi 1953 Stanley 

 L. Miller, a graduate student at the Univer- 

 sity of Chicago, found that by passing 

 electric sparks through gases, he could 

 create amino acids, but the conditions 

 found around thermal vents at the bottom 

 of the ocean could also have produced 

 them. These simple chemicals are also 

 present in many of the meteorites that fall 

 to the earth's surface. 



But getting irom amino acids to pro- 

 teins is another story, and one fraught with 

 problems. Amino acids aren't likely to link 

 up with one another — a problem related to 

 their chemical construction. All are 

 formed from a few elements and have the 

 same central structure, a chain of three 

 atoms. The first is a nitrogen (N), holding 

 three hydrogens (H); the middle is a car- 

 bon (C), with one hydrogen and one vari- 



10 Natural History 6/94 



able side chain attached; the third is a car- 

 bon bound to two oxygens (O). 



With a sideways squint, and a lot of 

 imagination, we can see the molecule as a 

 fish, an analogy that is useful for explain- 

 ing how hard it must have been for amino 

 acids to form proteins. To make a protein, 



many "fish" must link up with one another 

 in long chains. To do so, each must lose a 

 molecule of water before the positively 

 charged nitrogen atom at its head can at- 

 tach to the negatively charged carbon-oxy- 

 gen at another's tail. If the head (N) con- 

 tributes two hydrogens and the tail an 



Another likely outcome is that the side 

 chain will react with nearby molecules, 

 producing a branched molecule (bottom, 

 right). Neither of these reactions would be 

 very helpful in the formation of proteins, 

 which are long and unbranched. 



The number of other, unwanted mole- 

 cules that can react with a growing chain 

 of amino acids further complicates the 

 problem of protein synthesis. Just as there 

 are a lot of different fish in the sea, there 

 would have been lots of different amino 

 acids in the primordial soup. Some may 

 have had longer central chains or any of 

 hundreds of possible side chains. And if 

 that isn't complicated enough, most amino 

 acids can exist in two forms, left-handed 

 and right-handed; but only the left-handed 

 one is used for building proteins. All in all, 

 thousands of possibilities. Yet all proteins 

 are made from only twenty kinds of amino 

 acids. Add to the confusion any number of 

 headless or tailless fish — miscellaneous, 

 highly reactive molecules — that would 



oxygen, water (H2O) is released, the tails 

 are shed, and the fish are linked together. 



Amino acids aren't very likely to form 

 such chains on their own. The hydrogen 

 and oxygen needed to create the water are 

 fairly tightly bound, so the reaction rarely 

 happens spontaneously. If conditions are 

 hot and dry enough, however, the water 

 molecule is lost and amino acids can join 

 together; but, following the path of least 

 resistance, as soon as they join, the head of 

 the first is most likely to bond with the tail 

 of the second, forming a circle (below). 



have readily reacted with a free head or 

 tail, preempting the chain formation re- 

 quired for protein synthesis. 



Even if by some enormous stroke of 

 luck, all these hurdles were overcome, and 

 a single copy of an effective protein spon- 

 taneously arose, it would be a dead end. It 

 could pass on this fortunate accident to 

 posterity only if it could replicate itself, 

 but proteins cannot. 



Illustrations by Joe LeMonnier. after Anthony Mellersti 



