The origins of life on earth have been theorized and theologized for millennia. But perhaps the answer is more Sagan than Sanger—recently, scientists have identified all five nucleobases found in DNA and RNA in carbonaceous meteorites [1,2].
While there is a diverse array of nucleobases found on earth, only five primary nucleobases compose nucleotides, the individual monomers that make up nucleic acid polymers. They are further categorized into two groups based on the type of nitrogenous base they contain: purines (adenine, guanine) and pyrimidine (cytosine, thymine, and uracil) [3].
Extraterrestrial nucleobases were first detected in the 1960s, when researchers found these molecules on the Orgueil meteorite [4]. At the time, only adenine and guanine were identified. Since then, identifying pyrimidines has proven to be challenging: in fact, only one pyrimidine indigenous to meteors has ever been detected. Yet, the introduction of meteor parent bodies to artificial recreations of the interstellar molecular cloud leads to the presence of many different nucleobases [5]. Why, then, have they remained elusive?
Scope, primarily. In one study, researchers based out of Japan’s Kyushu University used high-performance liquid chromatography along with electrospray ionization high-resolution mass spectrometry (HPLC/ESI-HRMS) to identify these compounds. HPLC is an analytical chemistry technique which separates a complex mixture of compounds into discrete molecules [6]. Once the molecules are separated, the mixture in solution is pushed through a narrow capillary and ultimately ionized using an electrical field. This results in the solution being misted, where each droplet can be measured in a mass-to-charge ratio and ultimately identified [1]. The analytical parameters used by Oba et al. allows them to observe molecules in parts-per-trillion (ppt), whereas previous studies have only been able to capture parts-per-billion (ppb) scale. Almost like a laboratory LASIK procedure, they are able to see the molecular composition of a mixture in sharper resolution, increasing the sensitivity of their net detection.
Murchison meteorite at the The National Museum of Natural History (Washington). Source: https://commons.wikimedia.org/wiki/File:Murchison_crop.jpg
Samples collected from three separate meteorites were used in this study: Murchison, a meteorite that hit Victoria, Australia (1969); Tagish Lake, which landed in British Columbia, Canada (2000); and Murray, which struck earth in modern-day Oklahoma approximately 100 million years ago [7–9]. In this study, both purines and pyrimidines were found in all three samples, though not uniformly—uracil was found in neither Tagish Lake nor Murray meteorites, and thymine was likewise not found in Murray.
Importantly, one might assume that a meteorite flaming out and crashing into Earth’s crust may compromise the cleanliness of the samples detected. To account for this, samples were also taken from the soil surrounding Murchinson and examined under similar conditions. The team found that the soil was much more nucleobase-rich, and more homogenous for nucleobase density than the meteor itself. This bolsters the claim that the nucleobases detected on the meteor are extraterrestrial in origin.
Meteors are rich in many types of organic molecules, from amino acids (protein monomers) to imidazole to, apparently, nucleobases and their chemical isomers [1]. This understanding spurred recent sample collection journeys to space by the Japanese Aerospace Exploration Agency, which will bring to earth uncontaminated asteroid samples for chemical analysis. A more robust understanding of which organic compounds are extraterrestrial in origin will allow scientists to deduce which ones are strictly home-grown. Only then can we begin to explore the space between the origins of life on earth and the basic building blocks for which it is required.
Edited by Yasmeen Saad
References
1. Oba, Y. et al. Identifying the wide diversity of extraterrestrial purine and pyrimidine nucleobases in carbonaceous meteorites. Nat. Commun. 13, 2008 (2022).
2. Martins, Z. The Nitrogen Heterocycle Content of Meteorites and Their Significance for the Origin of Life. Life 8, 28 (2018).
3. Jensen, K. F., Dandanell, G., Hove-Jensen, B. & WillemoËs, M. Nucleotides, Nucleosides, and Nucleobases. EcoSal Plus 3, (2008).
4. Orgueil Meteorite: Organic Nitrogen Contents. https://www.science.org/doi/10.1126/science.146.3649.1291 doi:10.1126/science.146.3649.1291.
5. Meierhenrich, U. J. et al. Precursors of Biological Cofactors from Ultraviolet Irradiation of Circumstellar/Interstellar Ice Analogues. Chem. – Eur. J. 11, 4895–4900 (2005).
6. Rekhi, H., Rani, S., Sharma, N. & Malik, A. K. A Review on Recent Applications of High-Performance Liquid Chromatography in Metal Determination and Speciation Analysis. Crit. Rev. Anal. Chem. 47, 524–537 (2017).
7. Meteoritical Bulletin: Entry for Tagish Lake. https://www.lpi.usra.edu/meteor/metbull.php?code=23782.
8. Meteoritical Bulletin: Entry for Murray. https://www.lpi.usra.edu/meteor/metbull.php?code=16882.
9. Matson, J. Meteorite That Fell in 1969 Still Revealing Secrets of the Early Solar System. Scientific American https://www.scientificamerican.com/article/murchison-meteorite/.
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