Geology Reference
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pyruvic acid, for example, and you get the essential amino acid alanine. Every living cell
on Earth incorporates the citric acid cycle, so it may well be a primordial characteristic—a
chemical fossil descended from the very first life-form. Such a cycle, by itself, is not alive.
But it does have the potential to replicate the inner circle of molecules at the expense of
less fecund chemicals.
At the opposite extreme of chemical complexity is the self-replicating autocatalytic net-
work, a model championed by Stuart Kauffman, who conducted pioneering theoretical
studies at the famed Santa Fe Institute. The prebiotic broth may have initially incorporated
hundreds of thousands of different kinds of small, carbon-based molecules from varied
sources. We now know that some of those chemicals catalyzed reactions that made new
molecules, while other reactions accelerated the breakdown of their neighbors. An
autocatalyticnetworkconsistsofacollectionofmolecules—perhapsthousandsofdifferent
species working in concert—that speed up the production of themselves, while destroying
any molecule not in the network. It's the molecular equivalent of “the rich get richer.”
Again, as with the citric acid cycle, such a molecular network would not be considered to
be alive, but in a way it does promote the copying of itself, and it is far more complex than
most nonliving chemical systems.
Athirdscenario,theoneprobablyfavoredbythemajorityofbiologicallytrainedorigins
researchers, is the RNA world—a model based on a hypothetical molecule of RNA that
makes copies of itself. To understand why this scenario appeals, we need to take another
step back, to think about life's two most critical functions: metabolism (making stuff) and
genetics (transferring information on how to make stuff from one generation to the next).
ModerncellsusetheladderlikemoleculeDNAtostoreandcopytheinformationneededto
make more proteins, but they use complexly folded protein molecules to make the DNA.
So which came first, DNA or proteins? It turns out that a third kind of molecule, RNA,
plays the central role in both processes.
RNA is an elegant polymer—a long, single-stranded molecule assembled from smaller
individual molecules (called nucleotides) like beads on a string or letters in a sentence.
Fourdifferentmolecular“letters,”designatedA,C,G,andU,canlineupinanyimaginable
sequence, like a coded message. Indeed, these RNA letters hold genetic information (just
like DNA). At the same time, RNA can fold up into complex shapes that have the ability
to catalyze key biological reactions (just like proteins). In fact, RNA molecules facilitate
thesynthesisofallproteins,bothbycarryinggeneticinformationandbycatalyzingprotein
formation. So of all life's varied molecules, RNA is the only one that seems to “do it all.”
The RNA world model rests on the assumption that some as yet poorly understood
chemical mechanism produced vast numbers of different strands of RNA, or perhaps an
information-rich molecule very similar to it. Almost all those varied strands did absolutely
nothing; they simply survived or gradually degraded. However, a select few strands pos-
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