This discovery adds to the evidence that the progenitors of life came from space.
More ingredients for life have been found in meteorites.
Space rocks that fell to Earth during the last century contain five bases that store information in DNA and RNA, scientists reported on April 26 in Nature Communications .
These “nucleotide bases”—adenine, guanine, cytosine, thymine, and uracil—combine with sugars and phosphates to make up the genetic code of all life on Earth. It is still unknown whether these basic ingredients for life first came from outer space or instead formed in the warm soup of Earth’s chemistry. But the discovery adds to the evidence that the progenitors of life first came from space, the researchers say.
Scientists have discovered pieces of adenine, guanine and other organic compounds in meteorites since the 1960s. Researchers have also seen hints of uracil, but cytosine and thymine have remained elusive until now.
“We’ve completed a 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, Maryland.
A few years ago, geochemist Yasuhiro Oba of Hokkaido University in Sapporo, Japan, and his colleagues invented a technique to gently extract and separate various chemical compounds in liquefied meteorite dust and then analyze them.
“Our detection method has orders of magnitude higher sensitivity than that used in previous studies,” says Oba. Three years ago, researchers used this same technique to detect ribose, a sugar essential for life, in three meteorites.
In the new study, Oba and his colleagues joined forces with NASA astrochemists to analyze one of these three meteorite samples and three additional ones, looking for another type of the most important ingredient for life: nucleobases.
The researchers believe that their milder extraction technique, which uses cold water instead of the usual acid, keeps the compounds intact. “We found that this extraction approach is very convenient for these fragile nucleobases,” says Glavin. “It’s more like a cold brew than a hot tea.”
Using this technique, Glavin, Oba, and their colleagues measured the content of bases and other compounds associated with life in four meteorite samples that fell decades ago in Australia, Kentucky, and British Columbia. In all four, the team identified and measured adenine, guanine, cytosine, uracil, thymine, several compounds related to these bases, and several amino acids.
Using the same technique, the team also measured the chemical content of soil collected in Australia and then compared the measured meteorite values with the soil data. For some of the compounds detected, the meteorite values were higher than the surrounding soil, suggesting that the compounds entered Earth in these rocks.
But for other detected compounds, including cytosine and uracil, the content in soil is 20 times higher than in meteorites. This could indicate terrestrial contamination, says astrochemist Michael Callahan of Boise State University in Idaho.
“I think that [дослідники] positively identified these compounds,” says Callahan. But “they haven’t provided enough convincing data to convince me that they really are extraterrestrial.” Callahan previously worked at NASA and collaborated with Glavin and others to measure organic materials in meteorites.
But Glavin and his colleagues point to several specific chemicals found to support the hypothesis of an interplanetary origin. In the new analysis, the researchers measured more than a dozen other compounds associated with life, including isomers of nucleobases, Glavin says. Isomers have the same chemical formulas as their related bases, but their ingredients are arranged differently. The team found some of these isomers in meteorites, but not in soil. “If there was contamination from the soil, we should also see these isomers in the soil. And we didn’t do that,” he says.
Going directly to the source of such meteorites—pristine asteroids—could clarify this question. Oba and his colleagues are already using their extraction technique on pieces of the surface of the Ryugu asteroid, which Japan’s Hayabusa2 mission brought to Earth in late 2020. NASA’s OSIRIS-REx mission is expected to return in September 2023 with similar samples from asteroid Bennu.
“We’re really excited about what kind of stories these materials can tell,” says Glavin.
