The five informational components of life can form in space

On Earth, all life consists of polymeric molecules called deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). These two building blocks contain all the instructions for every living organism and its many operations. In turn, these consist of five informational components (nucleobases), which are composed of organic molecules (purines and pyrimidines). For decades, scientists have searched meteorite samples for these building blocks.

To date, these efforts have resulted in the detection of three of the five nucleobases in meteorites. However, a recent analysis by researchers at Hokkaido University, Japan (with support from NASA) revealed the two remaining nucleobases that have eluded scientists so far. This finding could help resolve the ongoing debate over whether life on Earth emerged on its own or was assisted by organic compounds deposited by meteorites (aka panspermia).

The research team was led by Yasuhiro Oba, an associate professor at the Institute of Low Temperature Science (ILTS) at Hokkaido University. He was joined by researchers from the Japan Agency for Marine and Earth Science and Technology (JAMSTEC), Tohoku University, Kyushu University and the Solar System Exploration Division (SSED) from NASA’s Goddard Space Flight Center in Maryland. The article describing their findings recently appeared in the journal Communication Nature.

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The peptides could have been transported to early Earth by meteorites, asteroids or comets. Credit: © S. Krasnokutski/MPIA Graphics Department

The five nucleobases that make DNA and RNA include adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U), bases A, G , C and T found in DNA while A, G, C and U are found in RNA. While scientists have found these bases in meteorites before, scientists aren’t sure why more types haven’t been discovered so far. As Oba explained in a recent NASA press release:

“I wonder why purines and pyrimidines are exceptional in that they do not show structural diversity in carbonaceous meteorites unlike other classes of organic compounds such as amino acids and hydrocarbons. Since purines and pyrimidines can be synthesized in extraterrestrial environments, as demonstrated by our own study, one would expect to find a great diversity of these organic molecules in meteorites.

As Oda and his colleagues point out in their study, this newly discovered pair of nucleobases (cytosine and thymine) may have eluded scientists because they degraded before they could be extracted (due to their more delicate structure). In previous experiments, scientists placed grains of meteorite samples in a solution of hot formic acid to extract nucleotides and create a solution (“meteorite tea”) which they would then analyze. As co-author Danny Glavin of NASA’s Goddard Space Flight Center explained:

“We now have evidence that the full set of nucleobases used in life today could have been available on Earth when life first arose. We study these water extracts because they contain the good stuff, ancient molecules organisms that could have been key elements in the origin of life on Earth.

For the purposes of their study, the team relied on a “cold brew” approach rather than a “hot tea” approach. This involved using cold water to extract cytosine and thymine rather than formic acid – which may have destroyed them in previous studies. Second, the team used more sensitive assays than previous studies, allowing them to detect smaller amounts of molecules. This allowed the team to detect the fragile cytosine and thymine in their meteorite tea samples.

While these findings effectively complement the nucleobases that make up all life on Earth, they have yet to settle the debate. At present, scientists still cannot say for sure whether life began in a prebiotic pond billions of years ago or was assisted by organic molecules from outer space. However, detection of the two remaining nucleobases and other molecules found in the sample provided some additional pieces of the puzzle.

For example, the team detected traces of sugars and bases in the sample, indicating that more fundamental biological molecules are found in space. Finally, the team’s research resulted in a new proof-of-concept technique that has proven to be more effective in extracting information from asteroids. This will come in handy when NASA’s OSIRIS-REx mission returns samples from asteroid Bennu next year.

Further reading: NASA, Nature Communication

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