Feb. 20, 2004 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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