Summary of The Royal Society's Discussion Meeting on Emergence of Life

"Conditions for the emergence of life on the early Earth"

A discussion meeting held at The Royal Society, 6-9 Carlton House Terrace,
London, 13-14 February 2006. This meeting brought together astronomers,
chemists, and biologists to discuss recent research concerning how life
probably emerged on the early Earth.

I had the good fortune to be able to attend this meeting and listen to
many interesting talks on the latest ideas and results related to the
question of abiogenesis and the early emergence of life on Earth.
Conversations during breaks were particularly helpful in aiding this
astronomer in understanding the chemistry and biology in later papers,
though any misinterpretations herein are my own and not those of the
authors concerned. The proceedings will not be published until later in
the summer in a special issue of Proceedings of The Royal Society, so it
is as up to date as it can be. The overall impression gained was that the
gaps in understanding were closing fast.

I especially with to thank Max Bernstein and Jonathan Lunine for explaining
some of the details in their papers and others, during coffee breaks.

Here follows a brief summary of the papers, in the order they were given.

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The first day was devoted to prebiotic chemistry. Pat Thaddeus, an
astronomer from Harvard-Smithsonian Center for Astrophysics, summarised
what is known about prebiotic molecules in space. There are now over 130
molecules identified in interstellar gas and the largest has a carbon
chain with 13 atoms and a molecular weight of 147. The mass of the
smallest interstellar dust grains is only about half an order of magnitude
larger, so positing that grains are just big molecules seems
reasonable--they are a single population of chemically bonded objects.The
largest grains have of order 10^10 atoms (10 to the 10th power)! There is
enough free energy in interstellar gas to allow in principle the assembly
of very complicated structures at T ~ 10 Kelvin. But actually identifying
them is a daunting challenge, because the usual spectroscopic methods used
by astronomers tend to fail at moecular weights of more than a few
hundred. The diffuse interstellar optical and IR bands may be due to such
large molecules, but they remain unidentified. there is evidence from
Ulysses satellite that such IS molecules are entering our solar system and
it may be possible to design a satellite that can capture them using
aerogels and return them to Earth. The ingredients of complex organic
chemistry are already there in the material from which the Solar System
formed.

This was followed by Max Bernstein, of NASA Ames Research Center, on
prebiotic materials from on and off the Earth. The Miller-Urey experiment
in 1953 showed that a reducing atmosphere plus electric discharges could
form complex prebiotic molecules, but it is no longer thought that Earth
started with a reducing atmosphere (i.e., one with methane and ammonia).
More recent experiments can produce some organics from such conditions,
and they can also come from hydrothermal vents and from space. It may be
that different sources provided different molecules, such as amino acids
(found in meteorites), sugars, alcohols, alkanes, and glycine chains.
Even if Earth had been bare rock (which it must have been shortly after
the event that formed the Moon) the shower of organic materials from above
could lead to complex organic chemistry.

Monica Grady (Open University) then looked at the carbon cycle on Earth
and Mars. The terrestrial cycle relies on tectonics to keep the planetary
carbon inventory in balance, while biology in effect upsets the balance.
Our search for signs of life elsewhere should look for both water and
carbon. Mars has CO2, both in the atmosphere and in the polar ice caps.
We infer carbonates in the curst and soil but the amounts are unknown.
Mars definitely once had volcanoes. Igneous carbon has been found
dissolved in martian meteorites. Her colleague Mahesh Anand then
discussed the use of such meteorites to investigate past aqueous activity
on Mars by examining the fractionation of iron isotopes.

After lunch, Jonathan Lunine (University of Arizona) summarised recent
advances in the theoretical study of the formation of the planets. Earth
and other terrestrial planets did not accumulate like snowballs, but were
formed by collisions among larger Moon-sized bodies, not asteroidal sizes.
this process took about 50 million years. However, the formation of
Jupiter was much faster, being completed in about 5 million years. It is
now thought the water originated from bodies that once occupied the region
of the asteroid zone. Organics came from further out, possibly through a
"rain of comets". The giant impact that formed the Moon took place at the
end of the 50 million year period. The early impact dominated period is
known as the Hadean period, when life would have found the going very
tough. By 500 million years, the crust would have been cool enough for
liquid water.

Alan Schwartz (Nijmegen Univ) then discussed geologically plausible
sources of phosphorous, an important ingredient in biological materials
such as nucleotides and polynucleotides. At present, apatite is the only
significant source on Earth, but it isn't very soluble or reactive. What
other sources might have existed? It turns out that much more reactive
phosphites are produced volcanically, and Earth was much more acitve in
the Hadean than it is now. There may also be extraterrestrial sources or
reactive phosphorous as well.

James Kasting (Penn State Univ) concluded the day with a discussion of
experimental geological data from isotopes of oxygen. This is directly
relevant to the question of the way climate changed with time from the
earliest Earth (the Archaen, right after the Hadean). It is very
difficult to draw any firm conclusions about early climate because there
are possible mechanism that would reconcile the data. The Earth was
"warm" (20-40 C) but not "hot" (55-85 C). Seawater slowly changed in
isotopic composition with time. If so, this would upset the
straightforward use of O-18 isotopes as a past thermometer. There were
two well documented early glaciation eras or "Snowball Earth" episodes 2.3
billion years ago, and again about 750 million and 600 million years ago.
There may have been another 2.9 billion years ago, but the evidence is
less clear.

The second day was on the development of biology (from chemicals). W. R.
Taylor led off with "Stirring the primordial soup". He discussed
modelling RNA evolution and ist survival. RNA can't reproduce on its own
the way DNA can, because it is a single strand. But clusters of RNA can
form blobs and the molecules can, in a sense, help one another to make
copies, by RNA directed RNA polymerase reactions. The size of such
molecules could be small, initially 200 base pairs. (I didn't follow all
the specialist organic chemistry that followed.) Amino acids can in some
way "poison" the polymerase reaction, leading to cross linking and
eventually to hybird replicase. This can lead to DNA eventually and what
is referred to as Dyson's parasitic take-over, when the RNA world shifts
gear and becomes a DNA world.

How this initial formation of RNA might happen was discussed next by James
Ferris (Rensselaer Polytechnic Institute). Clay-catalyzed formation of
RNA oligomers is a fascinating idea and one that has been around for some
time (e.g., JD Bernal, 1949). Certain clays, specifically (so far) the
clay mineral montmorillonite (from weathering of volcanic ash) can
catalyze the formation of chains of prebiotic ribose into oligomers of up
to 30-50 monomer units. In other words, the usual "objection" that the
chance alignment and connection of chains of amino acids into RNA would
take forever, because of the low probability of such reactions at random
in a soup, is no objection at all if such chains are formed naturally by
catalyzed reactions. There is some selectivity for chirality (preference
of molecular handedness) in such reactions, and the interesting
possibility that some lipophile molecules could form membranes in small
spherical cavities and the ribose chain formation could take place inside
the membrane--sort of an early primitive version of a cell.

Gunter Wachtershauser then spoke on the Chemautotrophic origin of life in
a volcanic iron-sulphur world. A model for the "pioneer organism" could
be thought of as an inorganic substructure and an organic superstructure.
The earliest emergent biology may have involved metallo-peptides.
Evolution could then be thought of as a "search" for better ligands or
bonds. Even today, there are life forms that get energy from H2S rather
than O2.

David Deamer (UC Santa Cruz) followed with "Modeling self-assembly
processes in the prebiotic environment". How might prebiotic chemistry
started the self-assembly and replication processes, leading eventually to
cells that we would recognise as living things? The choice of sites is
limited by requirements for liquid water, a source of organic compounds,
and a source of energy to drive reactions. Clays are a likely location
for polymerization. There is also a requirement for sufficient
concentration to drive reactions. Another requirement was that the
chemistry had to involve soluble molecules, which were monomers, that
could undergo polymerization into linear polymers. At the same time,
other compounds were amphiphilic, allowing self-assembly into
membrane-bounded compartments that could encapsulate large molecular
components, becoming microscopic reactors that isolate large molecules
from their surroundings and enhance their probability of inteaction. He
went on to describe some actual attempts to fabricate artificial life
forms using these processes. This report concluded with a description of
attempts to do this in a high-acidity hot pond on the flanks of a volcano
in Kamchatka, Siberia. The experiment did not work, probably because the
conditions were too extreme.

After lunch, Donald Canfield (U Southern Denmark) discussed the eraly
evolution of life in a world without free oxygen. However, volcanic
emanations and atmospheric reactions would have produced a wide variety of
electron acceptors and electron donors for organic reactions. We know of
many anaerobic metabolisms today, and the reactions would be similar.
Examples include ecosystems with cycling of hydrogen, methane, and a
variety fo iron, sulphur and nitrogen compounds. Such a biosphere would
have been much less active than that known today. The implication is that
without oxygenic photosynthesis, life as we know it could not have
diversed and become so widespread.

Karl Stetter (U. Regensberg) then described "Hyperthermophiles in the
history of life". The word means super-heat-loving. These bacteria and
archaea are found in volcanic ponds, underground, and in submarine
high-temperature environments. They survive at temperatures above 80 C
and can reproduce well up to 113 C. Some types can survive autoclaving
for an hour (a standard medical sterilising technique) and freezing at
-140 C indefinitely. They get thir energy form inorganic redox reactions
employing compounds like H2, CO2, H2S and ferric or ferrous iron. They
could have lived in the early Earth environment of 3.9 billion years ago,
as the era of heavy bombardment drew to a close. The earliest archaeal
phylogenetic lineage is represented by the remarkably tiny Nanoarchaeum
equitans, which thrives in hot submarine vents and is a parasite on other
hyperthermophiles. It has the smallest genome of any known living
organism, at 490,885 base pairs.

Formal presentations closed with "The origin and emergence of life under
impact bombardment" by Charles Cockell (Open University). Craters formed
by numerous comet and asteroid impacts during the Hadean era may have
provided ponds and lakes of heated water, diverse clays and zeolites which
could act as templates for prebiotic reactions, non-acidic hydrothermal
conditions, fracturing of rocks to provide large surface areas for
reactions, and iron form iron meteorites to provide chemistry for some
forms of reactions. Even today, microbial communities are found in the
shattered rocks that underlie recent impact craters.

The major themes of the meeting can be summarised as follows:

1. The basic chemical ingredients of life, including amino acids, are
found in abundance in meteorites and in interstellar space (dust
particles). They can also be formed in prebiotic chemistry on or beneath
the surface of the primitive Earth during the "Hadean" era of heavy
bombardment from space, and during the early volcanic phases that
followed.

2. Experiments have shown that pre-biotic monomers can be polymerized and
catalyzed in certain clays (though not in others), confirming a long-held
hypothesis. The length of such chains can get up to 50 monomers. This
goes a long way to overcoming creationist objections that the
probabilities of molecules accidentally coming together to form long
chains is too low for this to have happened. If you add a few 50-monomer
chains together, you can soon get a long enough chain that can be copied
in an environment containing many such polymer chains. This environment
also provides a way to get primitive "cell walls" through amphiphilic
lipids coming together to form spherical chambers in which polymer chains
might form ribose chains leading to the first RNA.

3. The chemistry of the earliest life was probably powered by reactions
involving compounds of carbon, sulphur, and iron. The scarcity of
phosphorus was once thought to be a problem but recent work shows it is no
longer an obstacle.

4. The earliest archaea were probably similar to the hyperthermophiles
found today in extreme environments.

--
Mike Dworetsky

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