Meteorites could have brought DNA to Earth
Key building blocks of DNA have been found in space rocks, suggesting cosmic impacts might have helped deliver these vital ingredients of life to ancient Earth.
DNA is made of four main building blocks: adenine (A), thymine (T), cytosine (C) and guanine (G). RNA also uses A, C and G, but swaps out thymine for uracil (U). Scientists wondering whether meteorites might have delivered these compounds to Earth have looked for nucleobases in space rocks, but until now scientists had only detected A and G in space rocks, and not T, C or U.
Nucleobases have two flavours: purines and pyrimidines. The nucleobases previously seen in meteorites are both purines, which are each made of a hexagonal molecule fused with a pentagonal molecule. The ones missing until now are pyrimidines, smaller structures each made of just a hexagonal molecule. It was a mystery why only purines were seen in meteorites. Prior lab experiments simulating conditions in outer space suggested that both purines and pyrimidines could have formed during light-triggered chemical reactions within interstellar molecular clouds, and that the compounds could then have been incorporated into asteroids and meteorites during the formation of the Solar System. Such chemical reactions may have also happened directly within the space rocks.
Now scientists have finally detected all the pyrimidines and purines found in DNA and RNA in meteorites that made it to Earth. “The presence of the five primary nucleobases in meteorites may have a contribution to the emergence of genetic functions before the onset of life on the early Earth,” study lead author Yasuhiro Oba, an astrochemist at Hokkaido University in Japan, said.
Researchers employed state-of-theart analytical techniques to detect tiny amounts of nucleobases, down to the range of parts per trillion – more sensitive than prior methods that attempted to detect pyrimidines in meteorites. The scientists analysed samples from three carbonaceous meteorites that prior work suggested could have hosted the kinds of chemical reactions that created nucleobases: the Murchison, Lake Murray and Tagish Lake meteorites.
They detected T, C and U at levels of up to a few parts per billion within the meteorites, present at concentrations similar to those predicted by experiments replicating the conditions that existed prior to the formation of the Solar System. In addition to the crucial T, C and U compounds, the scientists also detected other pyrimidines not used in DNA or RNA that further show meteorites’ ability to carry these compounds.