Page 199 - Introduction to Paleobiology and The Fossil Record

186 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
the 1920s, but nobody tested it seriously until
the 1950s. In 1953, Stanley Miller, then a
student at the University of Chicago, made a
model of the Precambrian atmosphere and
ocean in a laboratory glass vessel. He exposed
a mixture of water, nitrogen, carbon monox-
ide and nitrogen to electric sparks, to mimic
lightning, and found a brownish sludge in the
bottle after a few days. This contained sugars,
amino acids and nucleotides. So, Miller had
apparently recreated step 2 in the sequence
(see Fig. 8.1). However, nowadays most
researchers consider the mixture of gases that
Miller used (with high percentage concentra-
tions of H 2 and CH 4 ) to have been too strongly
chemically reducing to represent a likely
atmosphere for the early Earth. Atmospheric
hydrogen is ultimately replenished from the
mixture of gases released from the solid Earth,
but the geochemistry of the subsurface means
that the mixture generally should contain the
oxidized form of hydrogen (i.e. water vapor,
H 2 O) rather than the large proportion of H 2
in Miller’s atmosphere.
Further experiments in the 1950s and 1960s
led to the production of polypeptides, poly-
saccharides and other larger organic mole-
cules (step 3). Sidney Fox at Florida State
University even succeeded in creating cell-like
structures, in which a soup of organic mole-
cules became enclosed in a membrane (step
4). His “protocells” seemed to feed and divide,
but they did not survive for long.
Could scientists ever show how non-living
protocells could become living? Did this
happen in one jump or was there an interme-
diate stage?
RNA world
Biochemists and molecular biologists have
worried about the transition from non-living
to living; it is hard to see how bacterial cells
could form from non-living chemicals in one
step. What then could have been the transi-
tional form of “precellular” life? The most
widely accepted view today is that RNA is the
precellular entity, and the time between non-
life and life has been termed the “RNA
world”.
RNA, or ribonucleic acid, is one of the
nucleic acids and it has key roles in protein
synthesis. Proteins are manufactured within
the nucleus of eukaryotic cells, and within the
cell mass of prokaryotic cells. The genetic
code, the basic instructions that contain all
the information to construct a living organ-
ism, is encoded in the DNA (deoxyribonucleic
acid) strands that make up the chromosomes.
There are several different forms of RNA that
have different functions: one type acts as the
template for the translation of genes into pro-
teins, another transfers amino acids to the
ribosome (the cell organelle where protein
synthesis takes place) to form proteins, and
a third type translates the transcript into
proteins.
In 1968, Francis Crick (1916–2004), who
co-discovered the double-helix structure of
DNA in 1953 with James Watson, suggested
that RNA was the first genetic molecule. He
argued that RNA must have the unique prop-
erty of acting both as a gene and an enzyme,
so RNA on its own could act as a precursor
of life. When Harvard molecular biologist
Walter Gilbert first used the term “RNA
world” in 1986, the concept was contro-
versial. But the first evidence came soon
after when Sidney Altman and Thomas Cech
independently discovered a kind of RNA
that could edit out unnecessary parts of the
message it carried before delivering it to the
ribosome. Because RNA was acting like an
enzyme, Cech called his discovery a ribozyme.
This was such a major discovery that the two
were awarded the Nobel Prize for Chemistry
in 1989; Altman and Cech had confi rmed part
of Crick’s prediction.
Since 1990, numerous labs have been
chasing evidence for the RNA world. For
example, Jack Szostak and colleagues at Mas-
sachusetts General Hospital in Boston argued
that the first RNA molecules on the prebiotic
(“before life”) Earth were assembled ran-
domly from nucleotides dissolved in rock
pools (Szostak et al. 2001). Among the mil-
lions of short RNA molecules, there would
have been one or two that could copy them-
selves, an ability that soon made them the
dominant RNA on the planet. To take this
forward to create a living cell, Szostak identi-
fied two stages: (i) the production of a proto-
cell by the combination of an RNA replicase
and a self-replicating vesicle; and (ii) the pro-
duction of a cell by the addition of a living
function (Fig. 8.2).
Simply proving that RNA could act as gene
and enzyme was one thing; however, a single