Artist concept credit: NASA's Goddard Space Flight Center/Chris Smith
By grinding up and analyzing samples of twelve carbon-rich meteorites, nine of which were recovered from Antarctica, a research group from NASA's Goddard Space Flight Center, in Greenbelt, Md, found both adenine and guanine; two of the primary nucleobases contained within our DNA, as well as hypoxanthine and xanthine; neither of which are used in the construction of DNA, but are utilized by other biological processes.
In two of the meteorites, the team also discovered for the first time, trace amounts of three molecules related to nucleobases (nucleobase analogues), purine, 2,6-diaminopurine, and 6,8-diaminopurine; two of which rarely appear in biology. According to Dr. Michael Callahan, lead author of a paper on the discovery appearing in Proceedings of the National Academy of Sciences of the United States of America;
"You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they're not used in biology, aside from one report of 2,6-diaminopurinecyanophage S-2L,"
While the presence of molecules not known to commonly appear in biology was a strong indication that the findings were not the result of contamination. Callahan's group further confirmed this fact by analyzing a sample of ice taken from Antarctica, where most of the meteorites in the study were found. The result showed much smaller concentrations of the nucleobases, as well as xanthine and hypoxanthine, all of which appeared in parts per trillion in the ice sample, as oppose to the parts per billion generally found within the samples taken from the meteorites. None of the Nucleobase analogues were discovered in the sample. Analysis of the soil taken from the surrounding area where one of the meteorites used in study fell in Austraila, also failed to detect those analogs.
In one final experiment designed to rule out terrestrial contamination and confirm the extraterrestrial origin of the molecules. Callahan's team was able to successfully generate all of the nucleobases and analogues found within the meteorite samples in a completely non-biological chemical reaction in the lab, using hydrogen cyanide, ammonia, and water. Showing not only that the find was almost certainly not the result of contamination, but more importantly, that chemical processes taking place within certain types of asteroids could potentially produce all of the molecules detected in the study.
If confirmed by further research, the findings of Callahan and his team could be a major step towards understanding the true origins of life on Earth. Particularly when considered along with previous research conducted by the team which detected amino acids, the molecules responsible for building proteins, within similar samples; as well samples taken from the Comet Wild 2 during NASA's stardust mission. All of which, adds to the growing body of evidence supporting the theory that the chemistry taking place within comets and asteroids is capable of generating the basic building blocks of essential biological molecules, and that life may owe it's existence, at least in part, to materials delivered from space via meteorite and comet impacts.
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Source:
NASA
Image credit: NASA's Goddard Space Flight Center/Chris Smith
For a deeper explanation of the Nucleobases and nucleobase analogues, as well as the findings of his research, check out the following video from NASA, featuring Dr. Callahan.
