Evolution
A support site for modules SC0060-3 Evolution
and SCM009-M Molecular Evolution
Page last updated 11/02/02
Prebiotic
evolution
[required for SC0060-3 only]
Prebiotic
evolution can be defined as the processes that led to the first living cells
(ie, the 'origin of life'; ie, evolution prior to life - hence 'prebiotic').
Prebiotic
evolution is
a difficult topic for science, as very limited physical evidence remains, and
there are a
number of considerable physical and chemical obstacles that had to be overcome.
The oldest
fossils so far discovered are simple bacteria-like cells in stromatolites,
which are
rocky stratified
deposits laid down by microbes, eg, ca.3,500,000,000 year old fossilised stromatolites
Western Australia.
The essential requirements for prebiotic evolution are:
A key problem
is that the latter characteristics of a living cell are highly interdependent,
but it is
most unlikely they all evolved at same time - ie, a 'chicken and egg' problem.
The universe
is ca.15,000,000,000 years old, and was initially consisted of ca.75% Hydrogen
and ca.25% Helium, and
all heavier
nuclei formed from subsequent stellar nuclear fusion. Thus was life not
possible
in the early universe until the requisite nuclei
(C, N, O, etc) were formed in stars, dispersed by supernova
explosions and then taken up into planets during new star formation.
Our sun formed ca.4,600,000,000 years ago, and Earth accreted from the heavier atoms.
It is possible
to experimentally stimulate primordial Earth. An 'atmosphere' of water vapour,
hydrogen, nitrogen, carbon monoxide,
carbon dioxide and perhaps hydrogen sulphide (all believed to have been present
in the primordial atomsphere) is subjected to
energy sources such as ultraviolet radiation (ie, to simulate the sun), electrical
discharges (to stimulate lightning)
and radioactivity
(both cosmic and terrestrial radiation were significant on primordial Earth).
Such 'Miller
& Urey' experimental simulations have
produced all twenty of the biological amino acids (the building blocks of proteins)
and several
others in an L/D racemic mix, and
also purine and pyrimidine bases (building blocks of nucleic acids like RNA
and DNA), porphyrins (the ring structure found in
haemoglobin, myoglobin and cytochromes), a number of sugars, and some fatty
acids.
The primordial
atmosphere was 'reducing' in that there was no oxygen present. This is vital
because otherwise any molecules
formed as above would have been rapidly oxidised (which is what would happen
in the modern atmosphere of 21% oxygen).
The above
experiments also produce much hydrogen cyanide (HCN) and formaldehyde (HCHO).
This may be important because
5HCN can give the purine base adenine and nHCHO can give nC-sugar (eg, 5HCHO
gives C5 pentoses, 6HCHO give C6 hexoses, etc)
under UV irradiation. Overall, it is
estimated that ca.3,000,000 tonnes of organic molecules were produced
per year
on young Earth,~
and with no oxygen or organisms to break them down, then lots will have accumulated.
Another
more recent idea is that comets impacting on primordial Earth brought alot of
water and organic molecules. Comets certainly
are largely composed of water and are also rich in organic molecules, and the
young Earth was intensively bomdarded by comets for
the first 1,000,000,000 years or so. In fact, some cosmologists believe most
of the water on Earth is cometary!
Mechanisms
are then required to join the above smaller building blocks together to form
larger biological 'macromolecules'.
Amino acids join to form polypeptides/proteins, purine and pyrimidine bases
join with sugars and phosphate to form
nucleic acids (like RNA and DNA), and fatty acids join with phosphate and other
alcohols to form phospholipids which
then assemble to form the bilayer of biological membranes. An old idea for this
is the 'primordial soup' concept where
organic molecules in aqueous solution condensed to
form macromolecules. However, a major problem with this is that
condensation is not a favourable reaction in aqueous solution - the macromolecules
are more likely to break down!
A more recent
and feasible idea is the 'primordial pizza' concept where organic molecules
in dried and/or semi-dried
(gel) surface phases condensed to form macromolecules.
In such a state, the increased reactant concentrations and
decreased degrees of freedom make
condensation a much more favourable reaction. Primordial
pizzas could have
risen from pools evaporated by the sun or volcanic activity, or between
the mineral
stacks of clay. Clay consists of
thin mineral layers interspersed with thin water layers, and also contains potentially
catalytic metal atoms.
Cairns-Smith
proposed a key role for clay in prebiotic evolution, but his ideas have recently
lost much credence.
Primordial
pizzas have been experimentally stimulated. For example, anhydrous (water-free)
mixtures of amino acids heated
at 120 oC for one week form polypeptides of formula
weight ca.20,000. These have been called 'proteinoids' by Fox, who has
been able to demostrate that they can show limited enzyme
activities. There is similar data for clays but at lower temperatures
and with
water present.
The next
stage must be aggregation of these macromolecules into larger (but not yet necessarily
living) assemblies. There are a
number of possibilities. 'Coacervates' were discovered some time ago and are
spontaneously formed droplets of polypeptides in
aqueous solution. These are stable, but rather too large with ca.500 micrometre
diameter.
Fox more recently discovered
' microspheres', formed by heating then cooling of aqueous solution of
proteinoid (see above). These are stable, with a more
realistic ca.1-2 micron diameter, and comprise tiny volumes of water enclosed
within a
boundary layer of polypeptide.
Micelles have long been known and are spherical bilayers of phospholipid spontaneously
formed in aqueous solution.
Adsorption into interstices of clays could have provided a nurturing environ
for the above kinds of aggregates.
Micelles
may seem the best idea above, since modern biological membranes are
phospholipid bilayers. However, membrane
bilayers require unbranched fatty acids of chain length
greater than ten carbons, but primordial processes only give branched
fatty acids of
less than ten carbon chain length. Microspheres may be promising, but the boundary
layer is proteinoid not
phospholipid bilayer. At least the above experimental finding show what can
be possible.
Research
has now found that aggregates like those above can display some living characteristics
such as
uptake, retention and
concentration of external molecules (so
producing an internal environment different to the exterior), increase in size
(through the
uptake of external molecules), 'division' (through mechanical shear after size
increase), and
can be even subject to selection!
Nonetheless,
aggregates are certainly not living. The most critical aspect absent is a system
of heredity. Inheritance
in aggregates
is limited to the 'sample' of internal molecules the two daughter aggregates
receive from the splitting parent aggregate. This could,
of course, has been the first heredity system! The chicken and egg kind of problem
is a major obstacle here, in that RNA/DNA
genes are required to produce specific
proteins, yet specific proteins are required to produce DNA/RNA!
A currently
popular idea is that RNA was the first true genetic material (a notion known
as the 'RNA World').
RNA is indeed
produced by primordial processes, whereas DNA is not, so RNA almost certainly
prededed DNA. RNA can adopt
sequence-dependent 3D conformations like proteins do (eg, modern tRNAs and
rRNAs do
this). RNA can give specific
catalytic activities like enzyme proteins do (eg, these 'ribozyme' activities
exist in and are crucial to modern cells -
eg, intron splicing of pre-mRNAs is auto-catalysed by the RNA). RNA
can specifically bind amino acids (eg, modern tRNAs
do this).
Therefore, perhaps RNA preceded DNA and proteins, so solving the chicken and
egg conumdrum. However, proteins
are formed in prebiotic processes, so this idea does not entirely satisfy.
Other ideas
for the first true genetic material include polypeptides specifically
catalysing
their own formation - ie, proteins as
first genetic code! Perhaps some other molecule acted as the genetic code? Structural
patterns in clay minerals were
suggested
by Cairns-Smith as a possible first genetic code. There is little evidence for
any of these ideas, and the RNA world concept
currently holds sway.
Overall,
abiotic (without life) formation of biological macromolecules and then
aggregates
seems both theoretically well
established and supported by good
experimental evidence. However, going from aggregates to living cells
is less clear
and with not much experimental evidence so far.
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University, College Road,
Stoke-on-Trent ST4 2DE, United Kingdom. Tel +44 1782 294613, email a.j.white@staffs.ac.uk