Evolution__3rd_Edition

530 PART 5 / Macroevolution
Early life may have used RNA for
inheritance
Fossil cells exist earlier than 3
billion years ago
years ago. Many of the molecular building blocks of life (such as amino acids, sugars,
and nucleotides) can be synthesized from a solution of simpler molecules, of the sort
that probably existed in the prebiotic seas, if an electric discharge or ultraviolet radiation
is passed through it. Once the molecular building blocks exist, the next crucial step
is the origin of a simple replicating molecule.
Although we do not know what the earliest ancestral replicating molecule was, several
lines of evidence suggest that RNA preceded DNA. For instance, single-stranded
RNA is simpler than DNA, which is always double-stranded. DNA needs enzymes to
“unzip” the two strands in order to read or replicate the nucleotide information. DNA
always takes on the structure of a double helix. RNA, by contrast, can interact directly
with its environment. It can be read or replicated directly. Also, RNA can take on many
different structures, depending on its nucleotide sequence. In some of those structural
forms, RNA will act as an enzyme (or “ribozyme”), catalyzing biochemical reactions.
RNA molecules are known that act as RNA polymerases, catalyzing the replication of
RNA. However, no one has yet discovered an autocatalytic RNA that could catalyze
its own replication. Such a self-replicating molecule would be one of the simplest
imaginable living systems. Some other small lines of evidence also suggest that RNA
preceded DNA, such as “prebiotic soup” experiments that have more readily yielded
the nucleotide U than T.
The (hypothetical) early stage of life, when it used RNA as the hereditary molecule,
is called the “RNA world.” Life came to use DNA later in history. One reason for the
transition from RNA to DNA may have been that RNA-based life was limited by the
relatively high mutation rate of RNA. (This reasoning is similar to the argument in
Section 12.2.1, p. 320, about the evolution of sex.) Asexual life forms cannot exist with a
total deleterious mutation rate of more than about one.
Modern RNA viruses such as HIV have a mutation rate of about 10 4 per nucleotide.
This limits their coding capacity to about 10 4 nucleotides, or about 10 genes. More
complex life forms could not evolve until the mutation rate reduced. The evolution of
DNA would have reduced, or led to a reduction of, the mutation rate.
The fossil record tells us little about the origin of life, because those events were on a
molecular scale. However, the record does tell us something about timing, and leads us
to the next stage. The Earth itself is about 4.5 billion years old. For the first few hundred
million years, Earth was bombarded by huge asteroids that vaporized any oceans.
Temperatures were too high to allow life. Life probably could not have originated
before about 4 billion years ago.
The oldest known rocks are at a site at Isua, Greenland, and are 3.8 billion years old.
These rocks contain chemical traces that may or may not be chemical fossils of life
forms (van Zuilen et al. 2002). Chemical evidence of this kind is inevitably uncertain,
because it could have been produced by a non-biological process. Some biologists and
geologists tentatively accept it as evidence of life, but few place strong trust in it. The
rocks have undergone too much metamorphosis to have any chance of retaining fossil
cells a if cells existed at that time. Fossil evidence of cells comes from various sites in the
period 3–3.5 billion years ago. The earliest fossil cells were until recently thought to
come from 3.5 billion-year-old rocks from the Apex Chert in Western Australia (Schopf
1993). However, Brasier et al. (2002) have argued that the alleged fossils in these rocks
are artifacts and not fossils. Other evidence for fossil cells exists from the 3–3.5 billion-
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