Chemical Evolution Theory of Life’s Origins
1. the synthesis and accumulation of small organic molecule
s, or monomers, such as
amino acids and nucleotides.
Production of glycine (an amino acid)
3 HCN + 2 H
N + CN
Production of adenine (a base):
Production of ribose (a sugar):
2. the joining of these monomers into polymers, including pr oteins and nucleic acids.
Bernal showed that clay-like materials could serve as sites
3. the concentration of these molecules into droplets, call
ed protobionts, that had
chemical characteristics different from their surroundin
gs. This relies heavily on
the formation of a semi-permeable membrane, one that allows
materials to flow one way or the other through it. Droplet form ation requires a
liquid with a large surface tension, such as water. Membrane formation naturally
occurs if phospholipids are present.
4. The origin of heredity, or a means of relatively error-fre e reproduction. It is widely,
but not universally, believed that RNA-like molecules were
the first self-replicators
— the RNA world hypothesis. They may have been preceded by ino rganic
Lattimer, AST 248, Lecture 13 – p.1/20
Acquisition of Organic Material and Water
In the standard model of the formatio of the solar system, vol atile materials are
concentrated in the outer solar system. Although there is as much carbon as
nearly all other heavy elements combined in the Sun and the bu lk of the solar
nebula, the high temperatures in the inner solar system have lead to fractional
amounts of C of
of the average.
Ices are similarly much more abundant in the outer solar syst em.
Meteorite and comet impacts could deliver much of the Earth’ s volatile material,
especially C and H
O. At present-day rates, a billion years is needed to deliver the
C in Earth’s biosphere, but 4 billion years ago, the delivery rate was much larger.
Some simple organic materials would have been included in th is delivered
material, as indicated by their presence in the Murchison me teorite.
A reducing atmosphere on the early Earth would generate more organics.
Lattimer, AST 248, Lecture 13 – p.2/20
Self-organization leads to more complex structure
Crucial questions which did not have experimental answers u
p until now, but new
evidence has become evident:
Synthesis of nucleotides
Polymerization of nucleotides
Incorporation of a self-copying gene into single cells upon
selection could act
Primitive metabolism provides environment for later emerg
ence of RNA
replication. Example: Wächtershäuser’s iron-sulfur worl
d theory, De Duve
thioester theory. But can’t explain the high specificity of c hemical
reactions. Thermosynthesis world, involving thermal cycl
ing, suggests an
ATP-like enzyme that promotes peptide bonds: the “First Pro tein”.
Origin of homochirality
Genesis of the protein translation mechanism
Pieces are now coming together to support plausibility of sp ontaneous generation
Minimal number of genes seems to be about 206 in theory, in exp eriment there
seem to be 387 essential genes
Evidence suggests that this complexity has evolved, step by
step, from very simple
Lattimer, AST 248, Lecture 13 – p.3/20
Step 1 is possible in the early Earth’s atmosphere if it was wa s highly
Miller & Urey experiment).
Later research cast doubt on the existence of a reducing atmo sphere and pointed
to a neutral atmosphere dominated by CO
. More recent evidence is that H
escaped very slowly on early Earth and its abundance wasn’t n egligible after all....
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