Life’s Origins 163On the other hand, many scientists have suggested that this is a highly unparsimoni-
ous and implausible hypothesis: build a genetic code of proteins first, then replace it with
another more complex one. Instead, they argue, it makes more sense to evolve the genetic
code in nucleic acids from the very beginning, even if nucleic acids are harder to produce in
chemical reactions than are proteins. Thus, we have a classic “chicken or the egg” problem.
Which came first: the protein replication system or the nucleic acid replication system?
Fortunately, there is a way to resolve this conundrum. In 1968, DNA co-discoverer Francis
Crick first suggested that the earliest protocell was a strand of RNA. In the early 1980s Tom
Cech and other scientists discovered certain types of RNA, known as ribozymes, that perform
multiple functions. It was such a momentous discovery that they won the 1989 Nobel Prize in
Chemistry for it. These molecules act not only as a genetic code, but also catalyze reactions and
bond together proteins. In fact, the functional part of the ribosome in the cell, which translates
the RNA into proteins, is a ribozyme. Thus, ribozymes perform not only their familiar role as
replicators, but also the role that proteins play. Further research led to the idea that the simplest
scenario for the origin of living, self-replicating systems would be an “RNA world” (a term
first proposed by Walter Gilbert in 1986, but Francis Crick, Leslie Orgel, Carl Woese, and oth-
ers first argued that it was plausible back in the late 1960s). The very first self-replicating form
of life would be a single-stranded RNA, perhaps enclosed in a lipid bilayer membrane, and
perhaps using simple carbohydrates for food storage. Using both its replication powers and
enzymatic powers, it would make more copies of itself and perform the role of the proteins as
well until later more complex reactions involving many different proteins could evolve.
Every year, more discoveries are made that add details to our understanding of the ori-
gin of life and the RNA world. For example, coding sequences of amino acids are easily built
on small RNA templates in normal prebiotic conditions (Lehmann et al. 2009). Experiments
show that the first ribozymes in RNA world were much longer and more stable (Santos et al.
2004; Kun et al. 2005). Other experiments have shown that nucleotides easily merge in water
to form RNA over 100 nucleotides long (Costanza et al. 2009). Pino et al. (2008) demonstrated
that RNA molecules link up into long chains easily under normal earth conditions. And
finally, a range of experiments have shown that new genes have been produced repeatedly
by evolution (Long 2001; Long et al. 2003; Patthy 2003).
The “RNA world” hypothesis is now accepted as the most likely scenario for the ori-
gin of the first self-replicating system that can be truly called “life,” although there are still
additional conundrums that are being worked on: How did the RNA world get replaced by
the DNA world of today? And what preceded the RNA world? Could it have been (as some
suggest) a PNA world (peptide-nucleic acid) system that had amino acids in the nucleic acid
chains instead of the sugar ribose? Or something else? Like any good scientific problem, the
solution of one mystery then leads to additional new and more interesting problems to solve.
This is how science should operate.
What scientists don’t do is point to a complex system, say they can’t imagine how it
could have arisen by natural causes, and throw up their hands in surrender as creationists
do. Instead of claiming the origin of life is impossible to solve, and falling back on untest-
able, unscientific, god of the gaps arguments, scientists have made enormous progress show-
ing how life must have arisen. We may never watch life evolve from non-life in a test tube
(although we are coming close), but we certainly have good experimental evidence about
how nearly all the steps took place, so the problem does not require any supernatural inter-
vention or other cop-out to solve.