The early Earth was well provided with simple organic molecules, but how were these turned into proteins and nucleic acids? How they were polymerised? This remains unanswered. There is an even more contentious question: which came first, nucleic acids or proteins - or, perhaps, cell
33 This sounds a lot, but the Earth's surface is big. Fifty thousand tons a year works out to about three micrograms (three millionths of a gram) per square metre every day.
membranes? Each of these possibilities has been championed. Each entails considerable difficulties.
During the period between 1952 and the early 1980s, debates about the origin of life often took the following form. If the first big organic molecules made on Earth were proteins, how did they replicate? Proteins are not normally self-replicating (there are exceptions, but they are very special cases). Could they have given rise to nucleic acids that encoded them? If so, how? How did polymers of exclusively "left-handed" amino acids form, and how (since random choice would have been many orders of magnitude too inefficient) did meaningful sequences of amino acids arise, producing functional proteins? And how could proteins and nucleic acids have been held together in the same confined space, so that replication and translation became coherent? On the other hand, if nucleic acids came first, how were they replicated with no protein to act as a replicating enzyme? How were the bases aligned to form meaningful sequences? And once again, how - when the replicating enzyme finally appeared - did they become confined in the same small space? Finally: if the membrane (the confiner of the space) came first, what was it made of? How did it replicate itself? How did it acquire proteins and nucleic acids to replicate inside it?
Turning amino acids into proteins requires a good deal of heat and (normally) a solid surface to act as catalyst. The iron sulphide deposits of hydrothermal vents mentioned in the previous section might have sufficed. But only left-handed amino acids are found in proteins. Therefore, the prebiotic proteins that contributed to the origin of life must presumably have been "left-handed". Would iron sulphide catalysis have been so selective as to polymerise only left-handed amino acids, ignoring the right-handed ones? There is no evidence that it would. What alternative is there? Could proteins have been made without nucleic acids?
At least some nucleotide bases can be made under Miller-Urey conditions, and there was plenty of phosphate on the primitive Earth, so given a prebiotic source of the appropriate sugar (ribose), most ingredients of nucleic acids were available. Quite how base-sugar-phosphate units were assembled and then polymerised is a matter of conjecture, but it happened somehow. It is generally agreed that the first nucleic acids were RNA-like not DNA-like. DNA is chemically more exotic and it probably entered the scene later. But how was it possible to assemble RNAs of reasonable size under prebiotic conditions? Were solid-state catalysts again involved? How was the correct "handedness" imposed? Could RNA have been made without proteins?
At about the time of Haldane's "primaeval soup" conjecture (the 1920s), Aleksander Oparin proposed that the earliest proto-organisms were membrane-bound globules that accumulated ingredients from the environment and "replicated" by random fission. Oparin found that when glucose, a starch-making enzyme, gum arabic and histones were mixed together in solution, self-replicating globules formed. These "coacervates" suggested that membranes might have formed spontaneously and become self-replicating under the right circumstances. The suggestion (see above) that life originated from membrane-bound droplets in the atmosphere is a modern version of Oparin's conjecture and it has circumstantial support. But the questions remain: how and where did such droplets become filled with proteins and nucleic acids, and how and where were those polymers produced?
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