Early Earth was a brutally hot, volcanically active, radiation-bathed wasteland. Somehow amid this hostility, the necessary ingredients for life must have appeared, but where did they come from?
Astronomers have been working to answer that question for decades. They have developed several hypotheses, one of which suggests that asteroids and comets delivered the ingredients to Earth over the course of many collisions. A study published today in Nature Astronomy adds to a growing body of evidence to support this idea, finding all five nucleobases in samples of the asteroid Ryugu.
The five nucleobases are the building blocks of DNA and RNA—the genetic material inherent to all life on Earth. “This result further supports the idea that nucleobases could have been present in primitive asteroids and delivered to the early Earth, potentially contributing to the chemical evolution that preceded the origin of life,” co-author Toshiki Koga, a postdoctoral researcher at the Japan Agency for Marine-Earth Science and Technology, told Gizmodo in an email.
Analyzing Hayabusa2’s treasure trove
The Japan Aerospace Exploration Agency (JAXA) launched the Hayabusa2 mission in 2014, sending a spacecraft on a 186-million-mile (300-million-kilometer) journey to Ryugu. One year after reaching the asteroid in 2018, the spacecraft landed on its surface and fired a projectile into it. Hayabusa2’s “catcher” then gathered up the chunks of ejected debris, and the spacecraft carried them back to Earth.
Astronomers have run many analyses of the Ryugu samples since then, but Koga and his colleagues are the first to find all five nucleobases inside them. The researchers studied samples that were collected and curated under strictly controlled conditions and conducted their analysis in a cleanroom to further prevent contamination. They also ran tests to confirm the molecules formed on Ryugu rather than coming from Earth.
“It was not entirely unexpected, but it was still very exciting to detect all five nucleobases in the Ryugu samples,” he said. Previous research found one of the nucleobases, uracil, in the samples, and analyses of other space rocks—such as the asteroid Bennu, the Murchison meteorite, and the Orgueil meteorite—have yielded nucleobases, too.
Microscope images of Ryugu samples collected from the first and second touchdown sites of the Hayabusa2 mission, respectively. © AXA / JAMSTECWhen Koga’s team compared its findings to those obtained from Bennu, Murchison, and Orgueil, it found significant differences in the relative abundances of the nucleobases. Ryugu contains roughly equal amounts of purine nucleobases (adenine and guanine) and pyrimidine nucleobases (cytosine, thymine, and uracil), whereas Murchison mostly contains purines, and Bennu and Orgueil mostly contain pyrimidines.
“The relative abundances of purines and pyrimidines provide clues about the chemical conditions under which these molecules formed,” Koga explained. Interestingly, samples from Ryugu, Bennu, and the Orgueil meteorite that contained more ammonia all tended to have a lower ratio of purines to pyrimidines.
“This relationship suggests that ammonia may have played an important role in shaping the composition of nucleobases in these materials,” Koga said. “Because no known formation mechanism predicts such a correlation, it may indicate that previously unrecognized chemical pathways contributed to the formation of nucleobases in the early solar system.”
Zeroing in on the origin of life
Koga hopes that future research will shed light on the potential connection between ammonia concentration and nucleobase formation. This work will involve analyzing a wider range of meteorite samples and conducting laboratory experiments to test possible nucleobase formation pathways under primitive asteroid conditions, he said.
For now, the fact that all five nucleobases have now been detected in samples from two carbon-rich asteroids—Bennu and Ryugu—suggests that these molecules may have been more widespread across the early solar system than scientists previously thought. This supports the idea that some of the most critical building blocks for life were delivered to Earth by asteroids.
With each new asteroid sample they analyze, scientists are piecing together the chemical history of our solar system. Each discovery they make brings us one step closer to understanding what sparked life on our planet.
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