Encyclopedia of Evolution.pdf - Online Reading Center

oth may have been a chemical wonderland, but it could
not have included all the compounds and molecular structures
found in the simplest of present-day microbes.”
A particular problem has been in designing a “one-pot
reaction” in which the scientist puts inorganic chemicals
in and gets living cells out. In order to obtain the different
molecules necessary for life, very different conditions of
acidity and temperature are needed. For example, freezing
conditions are necessary to preserve adenine and guanine,
while for cytosine and uracil warm evaporative conditions
are needed. Biologist Robert Shapiro has been particularly
eager to point out how implausible these prebiotic syntheses
would have been, requiring an unbelievable configuration
of heat and cold, salinity and purity, acidity and neutrality—a
series of unlikely events occurring in just the right
order to produce even a few of the chemical building blocks
of life. The chemical simulations are, according to Simon
Conway Morris, “…highly artificial, if not contrived.” The
outcome of the syntheses depends strongly on the starting
conditions. The reactions would not work at all in a neutral
atmosphere, rather than a reducing atmosphere (see above).
Stanley Miller used a continuous spark plug; but when brief,
stronger blasts of electricity (which simulate lightning) were
used, the results were disappointing. When the order of the
glassware components changed, the results were different.
Some experiments would not have worked without doubly
purified water, available in chemical laboratories but not on
the primitive Earth. So far, there has been little promise from
one-pot reactions in explaining the origin of all of the chemicals
of life, converging in one place at one time.
6. Origin of “handedness” in biological molecules. Organic
molecules can have left- v. right-handed mirror images,
exactly the way human left and right hands are identical but
opposite to one another. Organic molecules produced by
nonliving processes are almost always chiral, an equal mixture
of the two forms. However, proteins in organisms use
left-handed amino acids and right-handed nucleic acids. A
protein made of all left-handed, or all right-handed, amino
acids is more stable than a protein made of a mixture. But,
how did the preponderance of left-handed amino acids ever
get started in the first place? There was great excitement
when it was discovered that meteorites could have an excess
of left-handed amino acids—but this excess was only 7–10
percent. It has been suggested that ultraviolet light polarized
the amino acids before the meteorites reached the Earth. It
is still unclear whether this could have happened on the primordial
Earth. It has been demonstrated that divalent cations
(such as the calcium ion Ca ++ ) can produce an uneven
mixture of the two forms of organic molecules.
7. Synthesis of large molecules from small precursors. The
production of small organic molecules may have been easy
on the primordial Earth, just as in nebulae and comets, but
how could small molecules have assembled into large ones,
such as the proteins and nucleic acids needed by modern
cells? Large molecules tend to dissociate into smaller ones
in water. There are two processes that may have allowed
the formation of large molecules on the primordial Earth:
origin of life 0
• Activated precursors. Regular amino acids do not polymerize
into proteins in water very well, but amino acids in
the amide form do. As biochemist James Ferris explains,
carboxyanhydrides can form into protein-like molecules
in water. When this happens, he points out, the resulting
protein-like molecules form only the correct type
of peptide bonds that characterize biological proteins;
and, moreover, the resulting molecules, up to 10 amino
acids in length, contain only left-handed components.
• Adsorption on mineral surfaces. Small molecules bumping
into one another in watery swirls would be unlikely to
form the complex molecules characteristic of life. In many
chemical reactions, solid surfaces allow the orderly catalysis
of reactions. The catalytic converter of an automobile
uses surfaces of the metal palladium to catalyze the reaction
that eliminates carbon monoxide from auto exhaust.
J. D. Bernal, a British biochemist, first proposed the possibility
that the synthesis of large molecules from small ones
may have occurred on clay mineral surfaces. Clays certainly
provide an enormous amount of surface area for such reactions
to take place. Leslie Orgel calls this possibility “life
on the rocks” and has demonstrated that RNA molecules
of length up to 40 bases can be produced on a mineral surface.
RNA molecules of this size would be long enough to
get the RNA world (see below) started.
B. Assembly of complex chemical systems. Modern life,
even of the simplest bacteria, is too complex to have arisen
directly. In particular, the elegant genetic system of DNA (see
DNA [raw material of evolution]) must represent an
advanced system not found in the first life-forms. In modern
cells, DNA stores genetic information, which is transcribed
into RNA, which directs the formation of proteins. As microbiologist
Carl Woese proposed in 1967 (see Woese, Carl
R.), scientists realize that a simpler genetic system of life must
have preceded that of DNA in the modern cell. Once modern
DNA cells came into existence, they would have outcompeted
the more primitive form which, therefore, no longer exists.
Suggestions (not all mutually exclusive) include:
• Some scientists, such as Stuart Kaufmann, Gunter
Wächtershäuser, and Christian de Duve, have proposed
that metabolic systems (such as glycolysis, which releases
energy from sugar so that organisms can use the energy to
operate) preceded genetic systems. This suggestion has met
with skepticism among most scientists because, at some
point, a genetic system would have to take over the control
of the cell.
• The first genetic system may have been formed of clay mineral
crystals rather than of organic molecules. In this scenario,
proposed by British scientist Graham Cairns-Smith,
organic molecules such as RNA helped the mineral crystals
in their replication. Later, the minerals served as scaffolding
for the reactions of the organic molecules, which later
took place without the scaffolding.
• The first genetic molecule may have been something like
TNA (threonucleic acid) or PNA (peptide nucleic acid).
The formation of such molecules could have occurred more
readily on the early Earth (in particular, TNA contains no