"You say that the soup is but one possibility. I am curious which
origin of life scenarios do not require a soup in the earth's early
oceans. Only two that I know of, Cairns-Smith's clay mineral scenario
and the hydrothermal vent scenario, and both of these are highly
speculative."
Or the 'it came from space' scenario. But let me ask you, what do you mean
by primordial soup ?
"But I think even these require the soup. At some point
CS's clay life will have to serve as some sort of template or
something to bridge the gap to organic life. Where do the organic
compounds come from and on what do the first critters subsist until
some type of metabolism evolves? Same problem for the
hydrothermal vent creatures when they leave home to take over the
oceans. The soup seems to be the goo that holds all origin of
life scenarios together. Somewhere along the line they all seem to
need it, if only as a source of building blocks or food."
I thought your argument was more about what the primordial soup consisted
of that whether or not it existed, that's why I ask about your definition
of primordial soup. Your question now is where did metabolism come from, I
do not know if metabolism is a necessary requirement for 'life' nor am I
aware of any truely satisfactory definition of such.
Now the question about metabolism, is metabolism necessary or could it
have evolved ? What if 'life' started simple with duplicating molecules ?
And what if these molecules were more likely to 'survive' and duplicate if
they were surrounded by a membrane ? Would a self replicating structure
like this be considered 'life' ? Initial the energy sources for these
structures are simple, directly or indirectly from the sun, perhaps some
volcanic heat.
So the first step would have to be the formation of monomers. This is the
Miller-Urey experiment, which showed that a synthesis of most of the
amino-acids was possible under sufficiently reducing circumstances.
Adenine, which plays a central role in the energy of cells has been shown
to form under prebiotic earth conditions by Oro. It requires an aqueous
ammonia solution containing large enough amounts of HCN. Other possible
pathways for the production of adenine have been selected as well.
The next step, nucleotides is less straightforward and perhaps there was a
precursor to the RNA world. What we need is monomers forming polomers and
duplication of such polymers. The presence of clay or metals could have
helped with the reaction and duplication. Orgel's work suggest that Zinc
ions are excellent catalysts for formation of certain nucleatides (GC) of
length of about 100.Fox's protocells would allow the duplication to take
place away from the outside environment. Formation of an energy storage
and conversion mechanism would again provide the 'cell' with an advantage
over its competitors.
Then there is the theory of hypercycles (Eigen). Eigen has shown that a
set of integrated autocatalytic cycles can lead to increased catalytic
efficiency So before the existance of a metabolic system, this could
have produced molecules capable of replication. Such hypercycles tend to
increase in complexity and order. Eigen also mentions some findings from
experiments by Spiegelman and Sumper which show that selection and
evolution are inevitable consequences of self-replication. (Eigen1980, the
origin and evolution of life at the molecular level, pp 119-139, Aspects
of chemical evolution, advances in chemical physics vol LV, eds. Prigogine
and Rice, Wiley Interscience, NY).
Kaufman has made a step further in suggesting that the road was not a long
one with many small steps but an abruipt formation of a complex system.
When a system reaches a sufficient level of complexity, it can generate
order out of disorder. Kaufman claims that under sufficient conditions the
formation of a more complexly ordered structure is all but inevitable.
Kaufman has 'proven' this in computer simulation models and we have yet to
see a comparable 'proof' in chemistry although the fact that order can
form 'spontaneously' out of chaos is not new (Prigogine).
A synthesis of all these ideas and possibilities will require some time
but science appears to make significant leeway in understanding how life
could have arosen on this earth.
>Pim> But the experiments are not to show how it happened as much as
>that it could have happened.
"Of course, what you are getting at I guess is that it could
have happened in a particular way. But what is gained by
showing it could have happenned a particular way if that
particular way requires a primordial soup and there is no
evidence that that soup ever existed?"
I am confused about what you mean by primordial soup ? The Urey Miller
experiment is based on the formation of aminoacids from a simple set of
chemicals under circumstances similar to what we perceive were the
circumstances on this earth a few billion years ago.
Evidence of a prebiotic soup are long gone so evidence of its existance
has to be indirect.
"Ok, let me briefly outline the reasons (from memory, so some
details may be a little off):"
"1) The modern scientific view is that the earth's early atmosphere
was at most very mildly reducing and more likely neutral."
Yes, hardly as reducing as Miller-Urey's experiment
"2) Miller experiments in these type atmospheres yield only small
amounts of amino acids, at least two orders less than in reducing
atmospheres. Just as importantly, but not as widely recognized,
the *variety* of amino acids is also greatly reduced. If I
remember correctly, only one in other than trace amounts."
Oro passed an electric discharge through a mixture of CO, N2 gas and water
and claims to have found amino acids and adenine/guanine. Others based on
computer models predict far less reducing circumstances (Pinto) but
calculate that sunlight would cause enough CO2 and H to form formaldehyde
which could within 10 million years or so, form enough concentrated
mixture to polymerize into more complex molecules.
Miller reported that under mildly reducing and non-reducing circumstances
the yields were lower and the rangeof products smaller.
Now depending on the type of atmosphere different yields and products were
found which is why hydrothermal vents might have been suggested for a more
reducing atmosphere, or the influx of extraterrestrial material.
"3) Relatively rapid circulation of the earths early oceans through
hydrothermal vents would have destroyed what little organics
formed."
That depends, we do know that bacteria and other creatures live in and
around such vents. It depends on the distance to the vents, the speed of
the outflow at the vents.
"4) Meteorite impacts would have made the origin of life impossible
for the first few hundred million years. This impact frustration
subsided to the point where life might form about 3.8 to 4.1
billion years ago. Yet life was well established on Earth
about 3.8 billion years ago. You have at most a few hundred
million years."
If life formed in the oceans then meteorite impacts would be far less
important. Especially in the hydrothermal vent cases, which are at the
ocean bottom. The meteorites could even have been the cariers of the
aminoacids as shown in analyses of meteorites (I believe Nature recently
showed that the mix was also slightly favored towards left handed
aminoacids).
And as Kaufman put it, once the conditions were right, in his scenario it
does not take much time for the next step to happen.
My references claim that the first life happened about 3.5 billion years
ago.
"5) There is no abiotic carbon in 3.8 million year old rock
samples (the soup vanished without a trace)."
What would 'abiotic carbon' look like ? There is little data helping us
with the prebiotic conditions.
I know that some 3-4 billion year old rocks have been dated. It would be
interesting to see what they contained. I have not found any information
on the presence of 'abiotic carbon' but I am also waiting for a definition
of such.
I found some reference to rock 3.8 billion years old from Greenland, but
the rocks were apparantly not well enough preserved to infer details of
the atmosphere at that time.
These rocks did contain carbonate chemicals that could have formed if the
atmospheric carbon had been CO2 and not CH4 (Kerr 1980, Science 210:42043)
I am not suggesting that all that needs to be known is known and all that
has to be found has been found but chemical evolution, however young, has
made some significant advances from the early Miller-Urey experiments. Has
life been created out of non-living matter ? Not yet. Will it ever happen
? Perhaps. It appears more and more likely but given the problems with are
limited knowledge of the prebiotic conditions there is a lot of guess work
involved. And the idea that we can analyze all the possible reactions that
could have taken place on a prebiotic earth appears pretty hopeless.
Nevertheless the simple and elegant experiment shows that things are not
that hopeless, combined with the meteorite evidence which found very
similar ratios to the Miller experiment suggest that scientists are on the
right way, the road however can be longer than expected.
Regards
Pim