[meteorite-list] Life In The Universe Takes Orders From Space (Carbonaceous Chondrite Meteorites)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:24 2004
Message-ID: <200402192030.MAA02854_at_zagami.jpl.nasa.gov>

http://www.eurekalert.org/pub_releases/2004-02/asu-lit021804.php

Contact: James Hathaway
Hathaway_at_asu.edu
480-965-6375
Arizona State University
February 19, 2004

Life in the universe takes orders from space

A century ago, when biologists used to talk about the primordial soup from
which all life on Earth came, they probably never imagined from how far away
the ingredients may have come. Recent findings have the origins of life
reaching far out from what was once considered "the home planet." Evolution
on the early Earth may have been influenced by some pretty far-out stuff.

In a paper published this week in the journal Science, Arizona State
University Chemistry Professor Sandra Pizzarello claims that materials from
as far away as the interstellar media could possibly have played an active
role in establishing the chemistry involved in the origin of life on this
planet.

In the paper, Pizarello and her co-author Arthur L. Weber of the SETI
Institute show that the exclusive chirality of the proteins and sugars of
life on Earth - their tendency to be left- or right-handed, could in fact be
due to the chemical contribution of the countless meteorites that struck the
planet during its early history. This paper provides a plausible explanation
for how, with a little help from outside, the chemistry of non-life -
characterized by randomness and complexity - becomes the ordered and
specific chemistry of life.

Pizzarello studies meteorites and the chemicals housed within them. A
particular type of meteorite - carbonaceous chondrites - holds particular
interest. Carbonaceous chondrites are very primitive, stony meteorites that
contain organic carbon. These meteorites are rare, but also very exciting
for chemists interested in the origins of life on Earth and in the solar
system. They contain amino acids - the molecules that make up proteins, and
an essential part of the chemistry of life.

According to Pizzarello, it has been known for the last century that there
are large amounts of carbon, hydrogen and nitrogen - the so-called biogenic
elements - in the cosmos. And that it is reasonable to assume that these
elements might have undergone some amount of chemical evolution before life
even began.

According to Pizzarello, who studies meteorites from the collection at ASU
(which has the largest university-owned collection in the world) the
meteorites are the only evidence of chemical evolution scientists have in
hand today. New techniques of meteorite analysis are leading to great
breakthroughs in understanding where these meteorites came from and how they
were formed. Even more exciting, work Pizzarello and her colleagues have
recently published in Science explores what sort of contribution the
chemical evolution represented by meteorites might have had on the early
Earth.

The paper addresses what has been a basic difficulty in relating the
chemical evolution represented by meteorites and the origin of terrestrial
life on Earth. According to Pizzarello, this problem is that chemical
evolution - what we see in meteorites - is characterized by randomness,
while terrestrial life relies on specificity and selection. For example, the
meteorites contain over 70 amino acids. A mere 20 amino acids make up life's
proteins. "There is a fundamental difficulty in trying to figure out how you
go from confusion and randomness to functionality and specificity," said
Pizzarello.

So far, only one trait has been found to be similar, to some extent, between
amino acids in meteorites and biopolymers, that of L-"handedness"
(chirality). Because organic molecules can be asymmetric if they have
different groups attached to a carbon atom, they can arrange spatially in
two ways, like the two hands, and be either left or right handed. All
proteins involved in life on Earth are made up of L-amino acids, while
sugars involved in life have a D structure. Scientists call this
"homochirality."

An overabundance (excess) of the L-form (the chemical name is enantiomer),
has also been found in some amino acids in meteorites. Pizzarello and Weber
devised an experiment to find whether or not the amino acids found with
L-enantiomeric excess in meteorites could have transferred their asymmetry
during organic syntheses on the early Earth . If so, the meteorites could
have provided a constant influx of materials with this excess - especially
during a period early in the solar system's history in which the Earth and
other planets were pummeled heavily by meteorites.

Pizzarello and Weber report in Science that in fact their experiment
succeeded in proving this possibility. In the laboratory, when performing
sugar syntheses in water, using reactions that modeled what may have existed
on the early Earth, the asymmetry in the amino acids led to a similar
asymmetry in the sugars. Pizzarello and Weber thus were able to conclude
that the delivery of material from outer space via meteorites - despite the
seeming randomness and complexity of these materials - could in fact have
"pushed" chemical evolution on Earth toward homochirality.

Pizzarello points out that these findings do not imply that life did not
evolve on Earth, or that the meteorites were the only early source of
enantiomeric excess - only that the steady contribution of these meteorites
might have provided a nudge in the "right" (or, more accurately, "left")
direction.

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Received on Thu 19 Feb 2004 03:30:03 PM PST