In 1952, the Miller-Urey experiment showed that an electrical discharge in a
gas-phase mixture of ammonia, hydrogen, methane, and water produced five amino
acids.
When the experiment was conducted, evidence indicated that this mixture was
representative of the Hadean (early Earth) atmosphere. The experiment was
repeated in the presence of jets of hot steam, simulating Hadean volcanic
eruptions and producing an even larger variety of amino acids.
A. Consider the following criticisms of the 鈥渙rganic soup鈥� model and justify
the selection of data that other experiments might provide regarding the
origin of life on Earth.
鈥� Biopolymers on Earth have a left-hand symmetry at the carbon adjacent to the
carboxylic acid carbon, and these experiments produced mixtures of both
leftand right-hand symmetries.
鈥� No peptide bonds between amino acids were mobserved.
鈥� Early Earth鈥檚 atmospheric oxygen concentration is known to have been very
low, implying the absence of an ozone layer to filter high-energy ultraviolet
(uv) radiation.
鈥� Ammonia decomposes when it absorbs high-energy uv radiation, but diatomic
nitrogen does not.
Models of the abiotic synthesis of biomolecules suffer from a 鈥渃hicken and
egg鈥� dilemma. Proteins are needed to synthesize DNA and RNA, and DNA and RNA
are needed to synthesize proteins. Which molecules came first?
B. In light of the following observations, evaluate the hypothesis that
nucleotides arose from a prebiotic mixture.
鈥� Nuclei acids are not found in experiments like those of Miller and Urey.
鈥� Purines and pyrimidines decompose at high temperature, and Earth was
bombarded by meteors and comets during the Hadean eon.
鈥� Bonds in the purine and pyrimidine rings of nucleic acids are broken by
high-energy uv radiation.
鈥� Carl Sagan and colleagues synthesized ATP from a mixture of adenosine,
ribose, and phosphate when exposed to uv radiation.
鈥� Ribose has never been synthesized in experiments like those conducted by
Miller and Urey.
鈥� Ribose has a left/right symmetry, and the righthanded form occurs in Earth
organisms.
Continuing with the analogy, if neither the chicken nor the egg came first,
then both must have arisen together. Some regard simultaneous innovations in
both catalysis and information storage and retrieval as too improbable. In
samples of meteorites, both amino acids and nucleic acids have been found. The
amino acids are mixtures of left- and right-handed symmetries, although some
have shown a significant bias toward the left-handed form (J. Elisa et al.,
ACS Central Science, 2016). The arrival from space of the seeds of
biomolecules is called panspermia. Carl Sagan (1966) and Francis Crick (1973),
one of the first to describe the structure of DNA, regarded panspermia as the
only plausible origin of life on Earth. In fact, their belief was in directed
panspermia, the intentional seeding by intelligent aliens.
C. Describe the questions that must be addressed for panspermia to be a
scientific hypothesis about the origin of life on Earth and describe the
reasons for the directed panspermia revision of this hypothesis. To avoid the
conflicting chicken-and-egg claims that 鈥減rotein catalyst was first鈥� and 鈥淒NA
information storage was first,鈥� two alternatives have emerged regarding the
origin of life on Earth. Consider two simple ideas: 1) water blocks uv
radiation, and cracks in the ocean floor (hot vents) provide a temperature
difference that generates a source of entropy; and 2) ribosomes are composed
of RNA.
D. Describe one of the following as a hypothesis concerning the origin of life
on Earth:
鈥� Reactions among molecules in the vicinity of hot vents became organized in
space and time, eventually developing structures that foreshadow the proton
gradient upon which metabolism is based. This alternative is the basis for
what is referred to as the metabolism-first hypothesis.
鈥� The catalytic properties of the ribosome reflect the self-catalytic
polymerization of nucleotides with sequential structures conserved in modern
DNA, the catalytic properties conserved in proteins, and the catalytic
properties of the ribosome whose core structure is RNA. This alternative is
the basis for what is referred to as the RNA-first hypothesis.
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