How Did the Ingredients of Life Arise

Let us assume, for the moment, that the many intricate steps leading from the first proteins and early nucleic acids through to LUCA are, if not inevitable, at least capable of being understood using well-known physical and chemical processes. We are still left with the question: how did the first proteins and nucleic acids come into existence? If the step from inorganic chemistry to DNA and proteins is a rare phenomenon, then we have a resolution of the Fermi paradox. For without these large molecules, evolution cannot begin the step to LUCA and then to the variety of life we see around us. Life, at least as we know it, cannot exist.

The basic building blocks of the vital macromolecules appear to be easily synthesized. We find amino acids, for example, both in interstellar space217 and in experiments that attempt to mimic the chemistry of early Earth.218 In 1953, Stanley Miller performed a classic experiment in which he passed an electric discharge through a vessel containing a mixture of water, methane and ammonia. The experiment was intended to investigate the effects of electric currents passing through the atmosphere of the early Earth. At the end of his experiment, Miller found many organic compounds in the vessel. Other scientists have disagreed with Miller's choice of model atmosphere, but the results were unarguably dramatic. It seems probable that amino acids could have formed on Earth soon after our planet cooled; amino acids are almost an inevitability of organic chemistry and the marvelous associative properties of carbon. Similarly, sugars, purines and pyrimidines — the components from which nucleic acids develop — can form in Miller-type experiments (although it must be admitted that yields are often low).

Although the details have yet to be determined, we need not suppose that the basic chemical building blocks required for life are in any way exceptionally rare. We can be less confident, however, about the probability of natural processes successfully linking these components into the molecules of life — nucleic acids and proteins. Indeed, it is at this point many creationists (and a few scientists) claim life on Earth is unique: they argue the probability of random processes creating a nucleic acid or a protein is tiny.

Consider, for example, serum albumin (an average-sized protein produced in the liver and secreted into the bloodstream, where it performs several necessary tasks). Serum albumin contains a chain of 584 amino acids, which are curled up into a sphere. In our bodies, the synthesis of the molecule is under the direction of nucleic acids. But imagine a time before DNA existed, so that a molecule of serum albumin had to be synthesized by adding one amino acid at random to the end of a growing chain. The chances are negligible — just 1 in 20584 — that random processes would produce the protein. Similarly, "genesis DNA" — a primitive chain of nu-cleotides that some scientists propose as being necessary for life to start — has a low probability of being created by chance.219

Making a Protein Through Random Processes

Since there are 20 amino acids from which to choose, at each step the probability that the correct amino acid is chosen to add to the end of a growing chain is 1 in 20. Therefore, for serum albumin, which has 584 amino acids, the probability that every amino acid is chosen in the correct order is 1 in 20584 — which is the same as 1 in 10760. This is an incredibly small probability. There is essentially zero chance that this protein can be synthesized by such a random process. Even a small protein like cytochrome c, which consists of just over 100 amino acids, has only a 1-in- 10130 chance of being synthesized at random. Again, for practical purposes, this number is indistinguishable from zero.

The beginning of life seems to suffer from a "chicken and egg" paradox: DNA contains the instructions necessary for the assembly of amino acids into proteins, but every DNA molecule requires the help of enzymes (in other words, proteins) to exist. DNA makes proteins makes DNA makes proteins. Which came first?

Although these criticisms seem to be fatal to the claim that life arose by chance, biochemists have in recent years made great progress in countering them. The details are not yet complete, but there is no reason to suppose the problems are insurmountable. Begin with the combinatoric arguments against the primordial synthesis of proteins. There is indeed essentially no chance of cytochrome c, for example, somehow coming together by acci dent. But if we allow for a period of prebiotic molecular evolution, then proteins could be synthesized through the workings of chance.

For example, imagine a lake somewhere on the still-young Earth. Suppose that in this lake there were only 10 different amino acids capable of forming peptides; and suppose that a peptide with a length of 20 amino acids displayed some catalytic function making it favored by natural selection. Then Nature only needed to try out 1020 combinations to hit on this peptide — still an enormous number, but a number that could comfortably be accommodated in the timescales available. Once the peptide was created, natural selection would ensure the amount of peptide in the lake increased in volume. Suppose that 1000 different "useful" peptides, each 20 amino acids in length, were created in the lake. If two such peptides could join to form a single chain, then 1 million different peptides with a length of 40 amino acids could be formed. Again, Nature would have plenty of time to try out all the combinations. In the same way, pep-tides containing 60 amino acids could be synthesized, and 80, and 100 ... in short, there was time for proteins to arise in that ancient lake. And there were many millions of lakes on the early Earth. (The particular proteins that arose would surely have been an historical accident. Replay the tape of history, and the proteins we use might be very different.)

Similar sorts of argument involving prebiotic molecular evolution can be used to counter the claim that "genesis DNA" was a miraculous fluke. However, such arguments may be unnecessary. It seems increasingly plausible that the original self-replicating molecule was not DNA, but one of the varieties of the much simpler RNA molecule. Furthermore, RNA provides an answer to the "chicken and egg" paradox. In the early 1980s, Sidney Altman and Thomas Cech demonstrated that some types of RNA molecule could also act as catalysts; they could play the role of enzymes. These RNA enzymes — or ribozymes — led to the idea of the "RNA world" — a time in the early history of life when catalytic RNA enabled all the chemical reactions to take place that are necessary for primitive cellular structures. In a sense, neither the chicken nor the egg came first: catalytic RNA acted both as genetic material and as enzymes.220

There seems to be no fundamental reason to suppose that the basic molecules of life could not arise through natural processes that had a reasonable chance of occurring. (Although, in all honesty, one has to concede that the chemical pathways leading to the first RNA molecules are still murky. The subsequent evolution of cellular structures up to LUCA is just as unclear. There are several competing scenarios, each with their advantages and drawbacks. Furthermore, several questions — such as why life uses only the left-hand form of amino acids, and whether the genetic code is inevitable or simply one of a whole raft of possible codes — are outstanding.

But progress in these fields is rapid, and we can expect the picture to have more clarity within a few years. Even if life turns out to have a completely different origin from that sketched above — and there are several other competing hypotheses — we are not yet driven to the hypothesis that life was some bizarre fluke.) There is, however, one last argument to consider regarding the probability of the early Earth being the site of the genesis of life: paradoxically, life seems to have arisen here too easily!

figure 69 Stromatolites, similar to the one pictured here, are the oldest known fossils. The oldest of them are 3.5 billion years old.

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