Self and the Beginning of Life 43“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanityformation of a new layer with identical charge distribution. Clay crystals
attract certain molecules to their surfaces and turn them into complex
organic molecules. Eventually the organic molecules (such as nucleic
acids) acquire the ability to replicate. However, it is not clear how the
last step could be accomplished.^34 (b) Eschenmoser synthesized an
RNA-like molecule called pyranosyl RNA (pRNA) in which the ribose
is replaced by a six-member ring analogue; the artificial molecule can
potentially undergo standard Watson-Crick pairing mechanism of repli-
cation.^35 Others proposed replacing ribose with threose,^36 or ribose with
glycerol.^37 The conversion from these nucleic acid analogues to RNA has
not been demonstrated. (c) Using computer modeling, Nielson designed
a polymer having a protein-like backbone and the usual nucleobase side
chains, called peptide nucleic acid (PNA). Each strand of PNA is poten-
tially capable of serving as template for the construction of its comple-
ment strand.^38 However, not only is the spontaneous synthesis of PNA
questionable, the transition from PNA to RNA has not been explored.^39
(d) In an even more outrageous scenario, Francis Crick hinted that
ready-made RNA (or ribozymes) could have been delivered to Earth
from outer space, as part of his “directed panspermia” hypothesis.^40
3.10 Evidence against RNA: (II) How did Functioning
Ribozymes Emerge from Random-sequenced RNA?
Without artificial selection guided by a highly intelligent experimenter,
it would be next to impossible to randomly pick an RNA chain having
the correct sequence that fits a particular enzyme function. Take, for
instance, an RNA molecule of 100 nucleotides long (a shorter one would
compromise the fidelity of replication). Such a molecule would have 4^100
(or about 10^60 ) possible random sequences. A pool of one copy for each
sequence would weigh 10^13 times the mass of the Earth. The odds of
picking the right copy would be one in 4^100.^39 It is hard to imagine how
this could have happened randomly in nature. Nevertheless, assum-
ing that well-formed RNA enzymes were present pre-biotically, what