As the Urey-Miller model of abiogenesis has grown weaker with time, interest in extra-terrestrial sources of amino acids has increased. The phrase "building blocks of life" is well-used: in 2005, space.com referred to amino acid precursors formed "in the winds of dying stars and spread all over interstellar space"; in 2008, National Geographic used the phrase when reporting on the detection of a precursor of glycine in the galaxy Arp 220. In December 2010, Nasa reported the presence of 19 amino acids in a carbon-rich meteorite and commented: "Finding them in this type of meteorite suggests that there is more than one way to make amino acids in space, which increases the chance for finding life elsewhere in the Universe." Clearly, these sources are promoting the idea that finding amino acids provides a significant part of an abiogenesis solution. This is also the message picked up by the world's media, which dutifully (and uncritically) passes on the hype.
One of the first challenges to be faced when assembling the "building blocks of life" comes from chirality. Of the 20 amino acids found in the biosphere, all except glycine can exist as a left-handed form or a right-handed form. Natural processes generate racemic mixtures, where left-handed and right-handed forms are in equal quantities. In living things, however, only the left-handed forms are found: the L-isomer. This phenomenon (known as an enantiomeric excess) has long been a puzzle to abiogenesis researchers. Some have concluded it was an accident: there is no reasoned explanation of the cause. Those favouring a source of amino acids from space have noted that some meteorites have yielded non-racemic mixtures. Many have not found the argument convincing: Moran provides some reasons. First, the flux of amino acids today is far too small to act as a foundation for a theory of origins. Second, even allowing favourable figures for higher quantities of amino acids in the past, the conclusion is unchanged. Third, racemisation occurs with time, resulting in 50/50 mixtures of left-handed and right-handed forms. Moran cites a paper Bada published in 1991, concluding that chance must be invoked.
"Because of the problem of racemization, it is likely that only after biotic protein synthesis became an efficient process in the evolution of early life could the chirality of amino acids be maintained in proteins. Instead of amino acid chirality preceding the origin of life, it may have developed after life was well established, and possibly in close association with the origin of protein biosynthesis. As to why the protein amino acids consist only of the L-enantiomers, it is probably just a matter of chance."
Notwithstanding all this, papers continue to be published that seek natural causes for producing non-racemic amino acids. The latest reports "the first abiotic cosmic ice simulation experiments that produce species with enantiomeric excesses". For some time, people have anticipated that circularly polarized ultraviolet light might disturb the equilibrium between L- and D-forms. This has now been confirmed. The paper's conclusions are related directly to models of abiogenesis:
"This result, directly comparable with some L excesses measured in meteorites, supports a scenario in which exogenous delivery of organics displaying a slight L excess, produced in an extraterrestrial environment by an asymmetric astrophysical process, is at the origin of biomolecular asymmetry on Earth."
The paper is accompanied by a press release and the media have taken an interest. BBC News carried a report.
"This excess is pretty cool," Dr Glavin [from Nasa] told BBC News. "You've got to break the symmetry somehow, this is critical. But how do you break it? That's one of the most important questions: did life just randomly choose one type over another? It's starting to look like Nature helped a bit."
A welcome critique has come from Larry Moran, who describes their abiogenesis scenario as "nonsense". His first objection to the Primordial Soup model is that amino acid concentrations are far too low (already noted above). Secondly, he asks - what happens next? This is not a trivial question! What processes are going to turn this sterile "broth" into proteins and more? People have spent years working on possible routes and have come up with nothing convincing.
"Instead of trying to prove that asteroids could carry a slight excess of L-amino acids, I wish these workers would apply a bit of healthy skepticism to the subsequent steps of the scenario. It's not reasonable to assume that minute quantities of amino acids could ever fuel the origin of life. Incidentally, the Primordial Soup Hypothesis also imagines that early cells used exogenous glucose as a fuel. This implies that the glycolysis pathway is more primitive that the gluconeogenesis pathway for synthesis of glucose. Unfortunately the data disproves this prediction. Gluconeogenesis is more ancient and glycolysis evolved later [Aldolase in Gluconeogenesis & Glycolysis]. A nasty little fact."
Maybe Moran is prepared to use tough word ("nonsense") because he is growing more confident with the Metabolism First Hypothesis. Certainly, more and more evolutionary biologists are giving positive comments about this alternative approach. However, the majority are still giving lip service to the Primordial Soup and that gets fed to the public. What gets unsaid is that there are natural causes producing amino acids and other organic molecules in space (go here), and there are natural causes creating non-racemic mixtures of these organic molecules, but the findings relate to extraterrestrial chemistry and have no relevance to the origin of life on earth. The situation reveals an unacceptable face of science. Moran has it exactly right when he says:
"The problem is that most scientists are not thinking critically about the origin of life. There are several possibilities and none of them are particularly convincing. However, the Primordial Soup Hypothesis has a number of glaring weaknesses that need to be addressed honestly and it doesn't do anyone any good if scientists sweep these weaknesses under the rug."
Non-Racemic Amino Acid Production By Ultraviolet Irradiation Of Achiral Interstellar Ice Analogs With Circularly Polarized Light
Pierre de Marcellus, Cornelia Meinert, Michel Nuevo, Jean-Jacques Filippi, Gregoire Danger, Dominique Deboffle, Laurent Nahon, Louis Le Sergeant d'Hendecourt, and Uwe J. Meierhenrich.
The Astrophysical Journal Letters, January 2011, Volume 727, Number 2, L27 | doi: 10.1088/2041-8205/727/2/L27
Abstract: The delivery of organic matter to the primitive Earth via comets and meteorites has long been hypothesized to be an important source for prebiotic compounds such as amino acids or their chemical precursors that contributed to the development of prebiotic chemistry leading, on Earth, to the emergence of life. Photochemistry of inter/circumstellar ices around protostellar objects is a potential process leading to complex organic species, although difficult to establish from limited infrared observations only. Here we report the first abiotic cosmic ice simulation experiments that produce species with enantiomeric excesses (e.e.'s). Circularly polarized ultraviolet light (UV-CPL) from a synchrotron source induces asymmetric photochemistry on initially achiral inter/circumstellar ice analogs. Enantioselective multidimensional gas chromatography measurements show significant e.e.'s of up to 1.34% for (13C)-alanine, for which the signs and absolute values are related to the helicity and number of CPL photons per deposited molecule. This result, directly comparable with some L excesses measured in meteorites, supports a scenario in which exogenous delivery of organics displaying a slight L excess, produced in an extraterrestrial environment by an asymmetric astrophysical process, is at the origin of biomolecular asymmetry on Earth. As a consequence, a fraction of the meteoritic organic material consisting of non-racemic compounds may well have been formed outside the solar system. Finally, following this hypothesis, we support the idea that the protosolar nebula has indeed been formed in a region of massive star formation, regions where UV-CPL of the same helicity is actually observed over large spatial areas.
Moran, L. More Prebiotic Soup Nonsense, Sandwalk (27 January 2011)
Palmer, J. 'Life chemicals' may have formed around far-flung star, BBC News (26 January 2011)
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