All of the bases in DNA and RNA have now been found in meteorites

All of the bases in DNA and RNA have now been found in meteorites

More ingredients for life have been discovered in meteorites.

Scientists reveal in Nature Communications that space rocks that fell to Earth during the last century contain the five bases that store information in DNA and RNA.

Adenine, guanine, cytosine, thymine, and uracil are the “nucleobases” that mix with sugars and phosphates to form the genetic code of all life on Earth. It is still unknown whether these basic ingredients for life originated in space or developed in a warm soup of earthly chemistry. But the discovery adds to evidence that suggests life’s precursors originally came from space, the researchers say.

Scientists have detected bits of adenine, guanine and other organic compounds in meteorites since the 1960s. Although uracil has been detected, cytosine and thymine have remained elusive until now.

“We’ve completed the set of all the bases found in DNA and RNA and life on Earth, and they’re present in meteorites,” says astrochemist Daniel Glavin of NASA’s Goddard Space Flight Center in Greenbelt, Md.

A few years ago, geochemist Yasuhiro Oba of Hokkaido University in Sapporo, Japan, and colleagues came up with a technique to gently extract and separate different chemical compounds in liquified meteorite dust and then analyze them.

“Our detection method has orders of magnitude higher sensitivity than that applied in previous studies,” Oba says. This identical technique was used three years ago to identify ribose, a sugar necessary for life, in three meteorites.

Oba and colleagues worked with NASA astrochemists to analyze one of those three meteorite samples, as well as three others, in search of another type of essential element for life: nucleobases.

A 2-gram chunk from this rock — a piece of the meteorite that fell near Murchison, Australia, in 1969 — contains two crucial components of DNA and RNA now identified for the first time in an extraterrestrial source, researchers say. NASA

The researchers believe their gentler extract, which employs cold water instead of acid, keeps the compounds intact. “We’re finding this extraction approach is very amenable for these fragile nucleobases,” Glavin says. “It’s more like a cold brew, rather than making hot tea.”

Glavin, Oba, and their colleagues used this technique to determine the abundances of bases and other existence compounds in four samples from meteorites that fell decades ago in Australia, Kentucky, and British Columbia. The team found and measured adenine, guanine, cytosine, uracil, thymine, various chemicals related to those bases, and a few amino acids in all four.

The team also analyzed chemical abundances in soil collected from the Australia site using the same approach, and then compared the measured meteorite values to those of the soil. For some detected compounds, the meteorite values were greater than the surrounding soil, which suggests that the compounds came to Earth in these rocks.

Other detected compounds, such as cytosine and uracil, have soil abundances that are up to 20 times higher than in meteorites. According to cosmochemist Michael Callahan of Boise State University in Idaho, this might indicate earthly contamination.

“I think [the researchers] positively identified these compounds,” Callahan says. But “they didn’t present enough compelling data to convince me that they’re truly extraterrestrial.” Callahan previously worked at NASA and collaborated with Glavin and others to measure organic materials in meteorites.

However, Glavin and his colleagues point to a few specific detected chemicals as evidence for an interplanetary origin. According to Glavin, the researchers measured more than a dozen other life-related chemicals in the current study, including isomers of the nucleobases. Isomers and bases contain the same chemical formulas, but their ingredients are ordered differently. Some of such isomers were discovered in meteorites but not in soil. “If there had been contamination from the soil, we should have seen those isomers in the soil as well. And we didn’t,” he says.

Going directly to the source of such meteorites — pristine asteroids — might solve the problem. Oba and colleagues are already using their extraction approach on pieces of the surface of the asteroid Ryugu, which Japan’s Hayabusa2 mission will bring to Earth in late 2020. The OSIRIS-REx mission of NASA is scheduled to return to Earth in September 2023 with similar samples from the asteroid Bennu.

“We’re really excited about what stories those materials have to tell,” Glavin says.

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