Darwin's Dangerous Idea - Evolution and the Meaning of Life

158 PRIMING DARWIN'S PUMP Molecular Evolution 159
replicator that could somehow serve as a temporary scaffolding to hold the
protein parts or nucleotide bases in place until the whole protein or macro
could get assembled. Wondrous to say, there is a candidate with just the right
properties, and more wondrous still, it is just what the Bible ordered: clay!
Cairns-Smith shows that in addition to the carbon-based self-replicating
crystals of DNA and RNA, there are also much simpler (he calls them "lowtech")
silicon-based self-replicating crystals, and these silicates, as they are
called, could themselves be the product of an evolutionary process. They
form the ultra-fine particles of clay, of the sort that builds up just outside the
strong currents and turbulent eddies in streams, and the individual crystals
differ subtly at the level of molecular structure in ways that they pass on
when they "seed" the processes of crystallization that achieve their selfreplication.
Cairns-Smith develops intricate arguments to show how fragments of
protein and RNA, which would be naturally attracted to the surfaces of these
crystals like so many fleas, could eventually come to be used by the silicate
crystals as "tools" in furthering their own replication processes. According to
this hypothesis (which, like all really fertile ideas, has many neighboring
variations, any one of which might prove to be the eventual winner), the
building blocks of life began their careers as quasi-parasites of sorts, clinging
to replicating clay particles and growing in complexity in the furtherance of
the "needs" of the clay particles until they reached a point where they could
fend for themselves. No skyhook—just a ladder that could be thrown away,
as Wittgenstein once said in another context, once it had been climbed.
But this cannot be close to the whole story, even if it is all true. Suppose
that short self-replicating strings of RNA got created by this low-tech process.
Cairns-Smith calls these entirely self-involved replicators "naked
genes," because they aren't for anything except their own replication, which
they do without outside help. We are still left with a major problem: How did
these naked genes ever come to be clothed? How did these solipsistic selfreproducers
ever come to specify particular proteins, the tiny enzymemachines
that build the huge bodies that carry today's genes from generation
to generation? But the problem is worse than that, for these proteins don't just
build bodies; they are needed to assist in the very process of self-replication
once a string of RNA or DNA gets long. Although short strings of RNA can
replicate themselves without enzyme assistants, longer strings need a retinue
of helpers, and specifying them requires a very long sequence—longer than
could be replicated with high-enough fidelity until those very enzymes were
already present. We seem to face paradox once again, in a vicious circle
succinctly described by John Maynard Smith: "One cannot have accurate
replication without a length of RNA of, say, 2000 base pairs, and one cannot
have that much RNA without accurate replication" (Maynard Smith 1979, p.
445).
One of the leading researchers on this period of evolutionary history is
Manfred Eigen. In his elegant little book, Steps Towards Life (1992)—a good
place to continue your exploration of these ideas—he shows how the macros
gradually built up what he calls the "molecular tool-kit" that living cells use
to re-create themselves, while also building around themselves the sorts of
structures that became, in due course, the protective membranes of the first
prokaryotic cells. This long period of precellular evolution has left no fossil
traces, but it has left plenty of clues of its history in the "texts" that have been
transmitted to us through its descendants, including, of course, the viruses
that swarm around us today. By studying the actual surviving texts, the
specific sequences of A, C, G, and T in the DNA of higher organisms and the
A, C, G, and U of their RNA counterparts, researchers can deduce a great
deal about the actual identity of the earliest self-replicating texts, using
refined versions of the same techniques the philologists used to reconstruct
the words that Plato actually wrote. Some sequences in our own DNA are
truly ancient, even traceable (via translation back into the earlier RNA
language) to sequences that were composed in the earliest days of macro
evolution!
Let's go back to the time when the nucleotide bases (A, C, G, T, and U)
were occasionally present here and there in varying amounts, possibly congregated
around some of Cairns-Smith's clay crystals. The twenty different
amino acids, the building blocks for all proteins, also occur with some
frequency under a wide range of nonbiotic conditions, so we can help
ourselves to them as well. Moreover, it has been shown by Sidney Fox (Fox
and Dose 1972) that individual amino acids can condense into "protein-oids,"
protein-like substances that have a very modest catalytic ability ( Eigen 1992,
p. 32). This is a small but important step up, since catalytic ability— the
capacity to facilitate a chemical reaction—is the fundamental talent of any
protein.
Now suppose some of the bases come to pair up, C with G, and A with U,
and make smallish complementary sequences of RNA—less than a hundred
pairs long—that can replicate, crudely, without enzymatic helpers. In terms
of the Library of Babel, we would now have a printing press and a bookbindery,
but the books would be too short to be good for anything except
making more of themselves, with lots of misprints. And they would not be
about anything. We may seem to be right back where we started—or even
worse. When we bottom out at the level of molecular building blocks, we
face a design problem that is more like construction out of Tinker Toy than
gradual sculpting in modeling clay. Under the rigid rules of physics, either
the atoms jump together into stable patterns or they don't.
Fortunately for us—indeed, fortunately for all living things—scattered in
the Vast space of possible proteins there happen to be protein constructions
that—if found—permit life to go forward. How might they get found? Somehow
we have to get those proteins together with the protein-hunters, the