Geology Reference
In-Depth Information
sessedsomekindofself-beneficial function;theyfoldeduptobecomemorestable,orthey
latched on to a secure mineral surface, or perhaps they destroyed their rivals—just another
example of molecular competition in the broth.
The key assumption of the RNA world hypothesis is that one of these myriad strands
learned the remarkable trick of how to make copies of itself: it became a self-replicating
molecule. This idea isn't so far-fetched. After all, RNA is a lot like DNA, which is able
to make copies of itself. What's more, RNA is easily mutated. So the first self-replicat-
ing RNA molecule, however inefficient or sloppy, would soon have found itself compet-
ing with lots of slight variants of itself, some of which pulled off the copying trick a little
faster,orwithalittlelessenergyexpenditure,orperhapsinslightlydifferentenvironments.
Such a precocious RNA molecule would seem to fulfill all the requirements for life: it is a
self-sustaining chemical system capable of incorporating novelty and undergoing Darwini-
an evolution—in this case, molecular evolution.
Perhaps it took a long time for that first functional, crudely self-replicating molecular
system to emerge, whether it was a citric acid cycle, or an autocatalytic network, or self-
replicating RNA. But unimaginable numbers of molecular combinations were being tried
on trillions of trillions of mineral surfaces, across almost two hundred million square miles
of Earth's surface, for many millions of years. And one of those inconceivably immense
numbers of molecular combinations, someplace, sometime, worked. It learned to self-rep-
licate and to evolve. And that invention changed everything.
Experiments in the Boston-based laboratory of Harvard biologist Jack Szostak demon-
strate the power of selection in molecular evolution. In many of their experiments, Szos-
tak's team starts with a mixture of one hundred trillion different RNA sequences, each of
which consists of a random hundred-letter string of A, C, G, and U. That immense collec-
tion of diverse RNA strands, each of which folds up differently, is then confronted with a
task:forexample,bondingtightlytoanotherdistinctivelyshapedmolecule.Szostak'steam
pours a solution with all hundred trillion strands into a beaker with little glass beads, each
of which has been coated with that distinctively shaped target molecule. These target mo-
lecules dangle out into the RNA-rich solution like little hooks. The vast majority of RNA
molecules don't respond; they have the wrong shapes to interact. But a tiny fraction of fol-
ded RNAs latch on to the targets and stick tight.
That's where the fun begins, because Szostak's coworkers pour out the old solution
(along with almost one hundred trillion different nonfunctional RNA strands) and recover
the few strands that by virtue of their fortuitous shapes happen to stick to the coated glass
beads.Then,usingstandardtricksofgenetictechnologythatmimicplausibleprebioticpro-
cesses, they prepare a new batch of one hundred trillion RNA strands, but this time all the
strands are sloppy copies—each a mutant of one of those few original functional strands.
Repeating the above steps produces a new population offunctional RNA strands, but some
