More Prebiotic Soup Nonsense

There's another round of nonsense under way, fueled by the discovery that chemical trickery can lead to a slight excess of L-amino acids over D-amino acids. There's a BBC News story about it at: 'Life chemicals' may have formed around far-flung star. The story is reproduced without comment on RichardDawkins.net.

The press reports refer to an article published in The Astrophysical Journal Letters [NON-RACEMIC AMINO ACID PRODUCTION BY ULTRAVIOLET IRRADIATION OF ACHIRAL INTERSTELLAR ICE ANALOGS WITH CIRCULARLY POLARIZED LIGHT]. Here's the 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.

The authors assume that the primodial soup speculation about the origin of life is the most reasonable explanation. According to this widely believed scenario, life originated in a soup of organic molecules that supplied most of the molecules of metabolism such as glucose and amino acids (and nucleotides?). Presumably once life got underway these molecules were used up and only then did metabolic pathways evolve to synthesize these molecules.

The competing hypothesis is Metabolism First [Metabolism First and the Origin of Life]. In this scenario, the first steps involved the establishment of simple oxidation-reduction reactions across a "membrane" using inorganic molecules. Once this supply of energy was in place the first pathways led to synthesis of simple organic molecules like acetate and glycine.

What's wrong with the Primordial Soup model? Well, for one thing, it's awfully hard to imagine how incoming asteroids could supply enough material to make a difference. The maximum concentration of all amino acids in the ocean, for example, could never have been more than 10-100 pM and that's optimistic [Can watery asteroids explain why life is 'left-handed'?].¹

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.

The real problem is not that metabolism firstists such as Bill Martin are right and soupists are wrong—although that's a very real possibility. 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.

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1. We're talking about a primordial soup where the concentration of L-alanine might be 0.50 pM and the concentration of D-alanine might be 0.49 pM. That's supposed to be enough for life based on amino acids to evolve and to lead to the subsequent preference for synthesizing exclusively L-amino acids. How, exactly, does that work?