436 CHAPTER 17The Emergence of Life
The simplest things that might be described as “living” must have developed as
complex aggregations of molecules. These aggregations, of course, would have
left no fossil record, so it is only through mathematical theory, laboratory experi-
mentation, and extrapolation from the simplest known living forms that we can
hope to develop models of the emergence of life. This is definitely a work in
progress.
“Life” is difficult to define. It is generally agreed that an assemblage of mol-
ecules is “alive” if it can capture energy from the environment, use that energy
to replicate itself, and thus be capable of evolving. (One may argue whether or
not viruses are alive, as their energy is supplied by a host organism.) In the living
things we know, these functions are performed by nucleic acids, which carry infor-
mation, and by proteins, which replicate nucleic acids, transduce energy, and gen-
erate (and in part constitute) the phenotype. These components are held together
in compartments—cells—formed by lipid membranes.
Although living or semi-living things might have originated more than once,
we can be quite sure that all organisms we know of stem from a single common ancestor
because they all share certain features that are arbitrary as far as we can tell [22,
90]. For example, organisms synthesize and use only l optical isomers of amino
acids as building blocks of proteins; l and d isomers are equally likely to be formed
in abiotic synthesis, but a functional protein can be made only of one type or the
other. d isomers could have worked just as well. The genetic code, the machinery
of replication and protein synthesis, and basic metabolic reactions are among the
other features that are universal among organisms and thus imply that they all
stem from the last universal common ancestor, or LUCA.
The most difficult problem in accounting for the origin of life is that in known
living systems, only nucleic acids replicate, but their replication requires the action
of proteins that are encoded by the nucleic acids. Despite this and other obstacles,
progress has been made in understanding some of the likely steps in the origin of
life [32, 52, 55, 100].
First, simple organic molecules, the building blocks of complex organic molecules,
can be produced by abiotic chemical reactions. Such molecules have been found in
space, carbonaceous meteorites, and comets. In a famous experiment, Stanley
Miller found that electrical discharges in an atmosphere of methane (CH 4 ), ammo-
nia (NH 3 ), hydrogen gas (H 2 ), and water (H 2 O) yield amino acids and compounds
such as hydrogen cyanide (HCN) and formaldehyde (H 2 CO), which undergo fur-
ther reactions to yield sugars, amino acids, purines, and pyrimidines.
Next, some such simple molecules must have formed polymers that could
replicate. Once replication originated, evolution by natural selection could occur,
because variants that replicated more prolifically would increase relative to oth-
ers. The most likely early replicators were short RNA (or RNA-like) molecules.
RNA has catalytic properties, including self-replication. Some RNA sequences (ribo-
zymes) can cut, splice, and elongate oligonucleotides, and short RNA template
sequences can self-catalyze the formation of complementary sequences from
free nucleotides.
The first steps in the origin of life probably took place in an “RNA world,” in
which catalytic, replicating RNAs underwent evolution by natural selection. When Sol
Spiegelman placed RNAs, RNA polymerase (a catalytic RNA isolated from a virus,
phage Qβ), and nucleotide bases in a cell-free medium, different RNA sequences
were replicated by the polymerase at different rates, so that their proportions
changed [87]. In another experiment, a catalytic RNA (RNA ligase) evolved greater
efficiency in ligating an oligonucleotide to itself when it was “grown” in an auto-
mated system with RNA polymerase enzymes and reagents (FIGURE 17.2) [69].17_EVOL4E_CH17.indd 436 3/22/17 1:36 PM