Even the simplest life-form is orders of magnitude more complicated than anything on display at Legoland. Nevertheless, like the Legoland structures, life molecules are assemblages of a relatively small set of comparatively simple molecules. Origin-of-life researchers understand that without some available rich reservoir of these simple molecules there is no hope for any possible naturalistic explanation of life’s origin. Thanks to a recent paper published in the Astrophysical Journal1, the hope for such reservoirs has now become dim indeed and, thus, the scientific case for a supernatural explanation of life’s origin continues to grow stronger.
The three most complex kinds of life molecules are DNA, RNA, and proteins. DNA molecules are comprised of double-stranded chains of four different nucleobase molecules that are joined together by phosphates and right-handed versions of five-carbon sugars called deoxyribose. RNA molecules are comprised of three of the DNA nucleobase molecules plus a fifth kind that are all joined together by phosphates and right-handed versions of five-carbon sugars called ribose. Meanwhile, proteins are assemblages of exclusively left-handed versions of 19 different amino acids plus a twentieth (glycine) that lacks a duality of configuration.
Lego building blocks can be purchased at any toy store and, if desired, one can buy virtually unlimited quantities from the manufacturer. Such is not the case for the building blocks of life molecules. In spite of exhaustive searching over several decades, scientists have failed to find evidence for any kind of terrestrial reservoir of non-biologically derived amino acids, nucleobases, or sugars that are relevant to life chemistry. What they have found instead are sound physical and chemical reasons for why such reservoirs are impossible on Earth2.
The lack of any possible terrestrial reservoirs of nonbiologically derived amino acids, nucleobases, or five-carbon sugars is what fuels the search for such reservoirs in outer space. A hint of where such reservoirs might exist is indicated by chemical analysis of the Murchison meteorite, the largest known carbonaceous chondrite. This meteorite contains a very low abundance of a few of the biologically significant amino acids, a trace of three of the nucleobases, and no five-carbon sugars. 3 An isotope analysis of the Murchison amino acids shows that at least some of them have an interstellar heritage (the remainder might arise from terrestrial contamination).
A team of five American astronomers examined possible chemical pathways for the formation of amino acids in the one interstellar source where such pathways are possible, namely dense molecular clouds in the Milky Way Galaxy’s core and spiral arms. The team noted that at least a few of the different amino acids important for biology can be synthesized when “dirty” ice crystals in dense molecular clouds are exposed to ultraviolet radiation or to fast-moving electrons mimicking cosmic rays4. They discovered, however, that different physical and chemical conditions are needed to produce the various amino acids. That is, it appears that no single process is capable of generating all the biologically significant amino acids.
The recognition that many different mechanisms are required to assemble the biologically important amino acids presents a serious challenge to naturalistic explanations of life’s origin. It is unlikely, to say the least, that all the required mechanisms would be operating simultaneously in one part of any molecular cloud. This conclusion is borne out by the observation that no meteorite has ever been found to harbor all 20 of the amino acids required for protein assembly. The Murchison meteorite has the most and it contains only five, and 40 percent of the total amino acid abundance is glycine, the simplest of the amino acids.
An even more significant challenge to a naturalistic origin of life is the fact that the same ultraviolet radiation and fast-moving electrons that make the amino acids also destroy them. Under molecular-cloud conditions all amino acids manifest short lifetimes. Such short lifetimes likely explain why so few of the biological amino acids show up in meteorites and why the abundance levels are so very low. Such short lifetimes also explain why astronomers have been unsuccessful in detecting anywhere in the Milky Way Galaxy the spectral line signature for any of the biological amino acids. A tentative claim for the detection of glycine5 has since been shown incorrect6.
The origin-of-life research community for nearly 50 years has been engaged in the fruitless pursuit of finding realistic natural pathways to assemble amino acids into proteins. Such a pursuit is irrelevant, however, if no concentrated reservoirs of amino acids exist. The new research study confirms that lack of such naturally occurring reservoirs.