Geology Reference
In-Depth Information
ules to the community, or the tendency to catalyze the destruction of competing molecular
species, or even the ability to make copies of itself. The natural world amply rewards such
innovation, and once established, life quickly infested every habitable nook and cranny of
the globe.
But let's take a step back. Why would a collection of molecules spontaneously start
copying itself? The answer lies in the twin evolutionary pillars of variation and selection.
Systems evolve for two reasons. First, they display vast numbers of different possible con-
figurations—that's variation. Second, some of those configurations are much more likely
tosurvivethanothers—that'sselection.Imagineaprebioticcollectionofhundredsofthou-
sands of different molecules, all made of carbon, hydrogen, oxygen, and nitrogen, maybe
with some sulfur or phosphorus thrown in. Prebiotic synthesis (à la Stanley Miller) and
natural samples (for example, David Deamer's meteorite) display this degree of molecu-
lar variation. But not all molecules were created equal. Some molecules were relatively
unstable and decomposed—they were quickly eliminated from the competition. Others
clumped together in useless tarlike masses and floated away or sank to the ocean floor,
where they could play no further role. But some molecules proved to be especially stable,
perhaps even more so when they could bind to others of their kind or to a particularly
tempting mineral surface. These molecules survived, as the molecular broth was relieved
of the least fit.
Molecular interactions further refined the prebiotic mix. Some groups of molecules co-
operatively stuck to mineral surfaces, thus enhancing survival of the clique. Other small
molecules acted as catalysts, enhancing some chemical species by promoting formation of
chemical bonds,orspeedingthedestruction ofotherchemical species bybreakingchemic-
al bonds. The molecular broth was swiftly winnowed, but ultimate security in such a world
was not to be found in eliminating the competition or just hanging on. The ultimate prize
of survival would go to the collection of molecules that learned to make copies of itself.
Three competing models attempt to describe the first self-replicating, quasi-living sys-
tem of molecules. The simplest of these models (and therefore the one many of us prefer)
points to a well-known cycle of a few small molecules—the ubiquitous citric acid cycle.
It starts with acetic acid, which contains only two carbon atoms. Acetic acid reacts with
CO
2
toformpyruvicacid(withthreecarbonatoms),whichinturnreactswithmoreCO
2
to
makethefour-carbonoxaloaceticacid.Otherreactionsproduceprogressivelylargermolec-
ules, up to citric acid, with its six carbon atoms. The cycle becomes self-replicating when
citric acid spontaneously splits into two smaller molecules, acetic acid (two carbon atoms)
plus oxaloacetic acid (four carbon atoms), which are also part of the molecular loop. One
cycle of molecules thus becomes two, two become four, and so on. What's more, many of
life's essential building blocks, including amino acids and sugars, are readily synthesized
by simple reactions with the core molecules of the citric acid cycle. Just add ammonia to
