The five RNA and DNA nucleobases were first discovered in meteorites. These chemicals are essential ingredients for life on Earth and several purines as well as two pyrimidines – thymine and cytosine – not previously detected in meteorites have been found.
The results come from an analysis of three carbonaceous meteorites, known to be rich in organic molecules. The finding “provides further support for the theory that the delivery of these compounds to Earth by meteorites may have played a role in the emergence of genetic functions for early life,” according to astrobiologist Daniel Glavin of the NASA, which provided one of the meteorites.
Previously, seven purine bases and one pyrimidine base had been detected in meteorites. But the experiments had relied on a method of hot formic acid extraction to release the nucleobases from the meteorite powders. This “could have led to the destruction of some nucleobases,” suggests Glavin.
The Japan-based group used a gentler extraction method and a new high-resolution mass spectroscopy technique to quantify nucleobases down to parts per trillion. Liquid chromatography has also been used to identify structural isomers of nucleobases.
“The detection of cytosine is very surprising because it was considered to be very weak against aqueous processes possibly encountered on the meteorite’s parent body,” notes Yasuhiro Oba, an astrochemist at Hokkaido University in Japan, who performed sample analysis.
These DNA and RNA building blocks were extracted from three meteorites: Murchison, arguably the most famous meteorite for the study of extraterrestrial organic molecules, which fell in 1969 in Australia; the Murray meteorite, which exploded at high altitude in 1950 and fell east of Murray, Kentucky; and the Tagish Lake meteorite, a large object that struck a frozen lake in Canada in 2000.
Similar concentrations of nucleobases in these meteorites “have also been synthesized in laboratory experiments simulating photochemical reactions in the interstellar medium, providing further evidence that nucleobases can form in space,” Glavin says.
Chemist Uwe Meierhenrich from the University of Nice Sophia Antipolis in France comments that “this discovery confirms that the molecular selection involved in the origins of life took place on Earth, and not before”. He adds that “this further confirms our models on the molecular origins of life on Earth, as the same hypothesis seems to hold for amino acids.” Over 90 amino acids have been found in meteorites, while biological organisms employ 21, suggesting that there was extraterrestrial input of a wide range of organic molecules.
Based on experiments and laboratory models, some astrobiologists suggest that the composition of the early Earth’s atmosphere was too oxidative to easily form nucleobases through natural processes, Glavin notes. “Therefore, delivery of nucleobases by a meteorite to Earth would have provided a source of nucleobases for the emergence of life.”
Not everyone is convinced. “The paper did not provide enough evidence to firmly establish that cytosine, uracil, and thymine were of extraterrestrial origin,” comments Michael Callahan, an analytical chemist at Boise State University in the USA. Idaho. Earth’s soil can contain higher concentrations of cytosine, uracil and thymine, so it’s difficult to determine how much is extraterrestrial versus terrestrial, he adds.
Additionally, the pyrimidines were present at extremely low concentrations. “If these results are representative of typical pyrimidine concentrations in meteorites, then geochemical synthesis on early Earth would likely have been responsible for the emergence of genetic material rather than inputs from extraterrestrial inputs,” Callahan says.