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A Clue To The Origin Of Life

Hello Ron, hello List,

In 1979, there was a highly interesting and informative contribution by
SKY & TEL on this subject for the interested lay-person. Perhaps this
also helps a bit to understand the basic principles underlying
left-handed and right-handedness. Some details may need to be updated
but most, if not everything, still holds true.

Best wishes and good night everybody,

Bernd!

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Organic Clues in Carbonaceous Meteorites
(April, 1979, Sky & Telescope, pp. 330-332)
C.R. Pellegrino and J.A. Stoff, Rockville Centre, New York

On September 28, 1969, an ancient rock mass slammed into the upper
atmosphere somewhere above Australia. It slid, danced, and leaped
through the air, then exploded over the town of Murchison. For several
days thereafter residents and scientists recovered curious shards of
grayish matter from fields, roadsides, and rooftops. The pieces
resembled dried carbon-rich clay and crumbled with similar ease.
Upon closer examination, their matrix appeared to be studded with tiny
glasslike spheres. When these were sectioned and viewed under a
microscope, concentric layers of material, not unlike those distinctive
patterns recognized in pearls, became visible. Further analysis revealed
unexpected traces of water (as high as 10 percent by weight) locked
inside the stony fragments.  The 20th specimen then known of that most
puzzling and sought after of all meteorite types, the carbonaceous
chondrite, had arrived.
Nearly three years later, scientists at NASA's Ames Research Center in
California confirmed the presence of 17 different fatty acids and 18
amino acids in fragments of the Murchison meteorite.  These highly
complex substances are composed of organic elements and, when woven
properly together, comprise the foundations of cellular life. But one
very important question soon arose: were these substances truly
indigenous to the meteorite, or did the meteorite, upon its penetration
into our atmosphere, begin to "breathe in" earthly contaminants? After
all, a mere fingerprint on its surface would have contributed most of
the common amino acids known here on Earth.
During the three-year investigation that followed its arrival, the
Murchison meteorite was examined and compared closely with another
carbonaceous chondrite that had fallen near Murray, Kentucky, 19 years
earlier. The results were impressively similar. Of the 18 amino acids
detected in the two meteorites, the 12 most abundant are seldom if ever
associated with the living tissues of terrestrial plants and animals.
The remaining six (valine, alanine, glycine, proline, aspartic acid, and
glutamic acid) are prominent in earthly proteins, but relatively scarce
in carbonecous chondrites. The first of a long series of paradoxes had
begun to emerge.
The meteorites may have originated in an age when the "dust" of the
solar nebula was falling together into little bodies that became
celestial vacuum cleaners, ever increasing in girth as they continued to
sweep up debris in their path. Some, like our own earth, accumulated
great mass.  Their interiors began to heat up. Gases, steam, and
vaporized rock held fast to their shifting skin: the primordial
atmospheres were born.
Whether the result of a cataclysm involving the collision of ancient
worlds or simply a collection of discarded planetary scraps left hanging
about the sun, a thin belt of solar driftwood - the asteroids -spreads
wide between Mars and Jupiter.  It is from this belt that most
meteorites seem to originate.
The presence of organized elements and hydrocarbons in some of these
meteorites leaves several unanswered questions. These substances seem to
have no business being out there in the first place. If they are native
to the meteorites, then we are faced with a perplexing fact: these
carbon compounds were somehow lifted, against entropy, to a highly
ordered state from vast numbers of random dissociated, inanimate atoms,
and gathered up and arranged in their present condition of seemingly
improbable symmetry. Given only the extreme temperatures, damaging
radiation, and near emptiness of outer space, it is not likely that this
kind of clustering could have proceeded in objects so small as stones,
boulders, or even asteroids (nor that it should be reproduced so
agreeably among individual samples).
Detailed comparisons with earthly tissues seem only to sharpen the
contrasts between terrestrial proteins and the kinds of molecular
ornamentation typically recovered from carbonaceous chondrites. That the
history of these compounds differs from our own is underscored by
important eccentricities in their molecular structure.
It is generally believed by organic chemists that when the earth was
still in its infancy, when its vapors had condensed into newly formed
seas and its shroud of air lacked destructive oxidizing agents, the
first organic acids were probably assembled in two very distinct
varieties. Valine, for example, possibly occurred as mirror images of
itself, much in the same way as your right and left hands are mirror
images, or isomers, of each other. In those days before the dawn of
living self-replicating matter, both "right-handed" and "left-handed"
molecules might have drifted about the Precambrian seas in equal or
near-equal quantities.
When living things finally did take over the earth, the assembly of
proteins was made possible only by the uptake of entirely right-handed
or entirely left-handed amino acids. The geometry of long-chain carbon
compounds allowed no room for random associations of both right-handed
and left-handed components in their construction. On Earth, it was the
left-handed variety that won acceptance. Hence, terrestrial proteins,
whether they be derived from trees or mosquitoes or men (except for a
special class of single-celled organisms, which utilize right-handed
amino acids in their cell walls), are composed entirely of left-handed
amino acids.
Using a beam of plane-polarized light, it is possible to determine the
right-handedness or left-handedness of a set of molecules. A
right-handed molecule will twist or rotate the plane of  polarization to
the right, whereas the left-handed variety will twist it to the left.
When homogenous mixtures of amino acids from the Murray and Murchison
meteorites were examined in this manner, no such rotation was observed,
indicating that both forms were present in equal quantities.  These
findings are reminiscent of ratios presumed to have existed in
Precambrian seas prior to the emergence of cellular enzyme activity, and
strongly suggest an origin held, not in the biology of cells, but in the
chemistry of atoms.
The nature of meteoritic amino acids is different from those on your
fingertips, to be sure. Equal distributions of both molecular
configurations would seem to cast serious doubts on the feasibility of
their ever having been generated by any kind of cellular activity or by
life as we know it.  Nevertheless, comparison with other
carbonaceous-chondrite meteorites occasionally leads to points of
confusion. A meteorite that in 1864 fell in Orgueil, France, and a 1938
fall in lvuna, Tanganyika, contain greater traces of right-handed than
left-handed amino acids. Not only does this mixture run counter to amino
acids found in earthly proteins, but their tendency toward one polarity
points to a possible origin in cells.
We can already provide a good explanation for discrepancies of
amino-acid ratios among carbonaceous chondrites. Given irradiation by
light, heat, X-ray, or other energy sources, it is possible to
interconvert amino acids from one form to another. For example, a
solitary left-handed molecule of valine, impelled by a constant input of
energy, would eventually flip over to a right-handed configuration. An
entire vial of left-handed valine exposed to the radiations of the sun
would, given enough time, undergo total interconversion. One should not,
however, expect to recover a vial filled only with right-handed
molecules since, once produced, they are as likely to flip back to the
left-handed variety as left-handed molecules become right-handed.
The situation is analogous to laying amillion pennies heads up (to
represent left-handed molecules) in a large tray. By randomly tossing
handfuls of them into the air - our application of energy - more and
more of the pennies would land heads down until the distribution of
heads and tails was nearly equal. Probabilities being what they are,
from this point on one would always expect to find about the same number
of heads and tails no matter how many more handfuls wer tossed.
Thus, a vial of pure left-handed valine suspended in space and
irradiated (but not fried) for a million years or so would ultimately
turn up as an optically inactive mixture of left-handed and right-handed
molecules, in spite of its initially pure form.
The Murray and Murchison meteorites are among the lightest and least
densely packed of the nearly 40 carbonaceous chondrites known today. If,
during their long passage through the solar system, they were ever part
of a large asteroidal body, then surely they resided on or very near its
surface. Consequently, their contents were left naked to the raw
energies of space, and amino acids recovered from these meteorites are
presumed to have undergone many "flips of the coin" - they have become
utterly randomized. A meteorite originating in the depths of a parent
body would have received more adequate shielding against such energy.
The lvuna and Orgueil specimens reveal compression of their matrix,
suggesting the operation of mild gravitational forces exerted by
overlaying rock in their respective parent bodies. Mixtures of amino
acids extracted from these meteorites deviate sharply from the
half-and-half composition of lighter specimens (which include the Murray
and Murchison meteorites). Skewness among the denser carbonaceous
chondrites infers an initial sample consisting largely or entirely of
right-handed amino acids.
Although the origin of these substances is still a matter of
speculation, most planetary geologists and organic chemists are in
agreement that they were contained in the meteorites prior to any
contact with our atmosphere. If we assume the least glamorous
hypothesis, then some manner of preliving chemical evolution, perhaps
advancing in the direction of molecules that would one day be able to
reproduce themselves, appears to be preserved or fossilized in
meteorites.
These celestial vagrants offer the alluring possibility that the
universe is not such a lonely place in which to live. Clouds of
formaldehyde (HCHO) spread across various parts of the galaxy seem to
exemplify the trend: wherever carbon and hydrogen and their associated
counterparts lie scattered and heated at the right temperature, it is a
fair bet that they will coalesce into compounds of higher order. That
you are alive and reading these words is evidence that such reactions
can and do occur.


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