For decades, astronomers have pondered the idea of panspermia, the theory that life on Earth was delivered here by a meteorite. The concept was once deemed improbable because it raised more questions than answers. But recent close examinations of extraterrestrial objects hint there may be some support for this far-out notion after all.
Researchers from Hokkaido University in Japan have found new evidence that the chemical components necessary to build DNA may have been carried to Earth by carbonaceous meteorites, some of the earliest matter in the solar system, as they report in a study published Tuesday in Nature Communications. Although these kinds of materials make up about 75 percent of all asteroids, they rarely fall to Earth, limiting how often scientists can study them. Yet they are troves of information: Scrutinizing these space rocks can tell stories about unique cosmic locations. Their contents may also help reveal the ancient chemical reactions that made our world a living planet.
Specifically, several meteorites have been found to contain nucleobases. These chemicals, called the building blocks of life, make up the nucleic acids inside DNA and RNA. Of the five major nucleobases, previous meteorite studies detected only three of them, named adenine, guanine, and uracil. But the present research proves for the first time that two more—cytosine and thymine—can exist within space rocks.
“The detection of all primary DNA and RNA nucleobases in meteorites indicates that these molecules have been supplied to the early Earth before the onset of life,“ says Yasuhiro Oba, lead author of the study and an associate professor at Hokkaido University. “In other words, we got information about the inventory of organic molecules related to DNA and RNA before any life arose on the Earth.” One of the oldest specimens in the study clocks in at about 4.6 billion years old, which is even older than the solar system.
Oba’s team used state-of-the-art techniques to analyze three carbon-rich samples from three meteorites that fell to Earth at different times and locations around the world. They investigated the Murray meteorite, which was discovered in the US in 1950; the Murchison meteorite, which residents reported seeing fall to Australia in 1969; and the Tagish Lake meteorite, which was found in Canada in 2000. The researchers then examined each sample’s chemical profiles to determine the concentrations of the building blocks for life. It took about a year to complete their analysis.
Oba says that in addition to the five DNA and RNA nucleobases, about 18 others were found in the meteorites, suggesting that these materials are widespread in space. The Hokkaido team concluded that the organic compounds found in the samples are present within and outside of our solar system.
Of their results, Oba says that he was most surprised by the discovery of cytosine, because the molecule is easily corroded by water and high temperatures. Yet both water and, to an extent, heat are needed to mold organic life. The exact role of these compounds remain murky in astrobiologists’ quest to trace life back to Earth’s original primordial soup.
But there are still some skeptics. Michael Callahan, a chemist at Boise State University in Idaho who was not involved with this report, told ScienceNews that though he believes the study authors positively identified the compounds, “they didn’t present enough compelling data” to convince him that these chemicals are “truly extraterrestrial.”
This isn’t the first time scientists have probed fallen meteorites for the ingredients of life. In 2019, an international team of scientists found ribose and other bio-sugars in two carbon-rich asteroids, one of which was the Murchison meteorite. These sugars are essential to the existence of life, too.
“Other important building blocks of life have been found in meteorites previously, including amino acids,” said Yoshihiro Furukawa, a co-author of the new paper who also led the sugar study, in a statement made to NASA about that research. “But sugars have been a missing piece among the major building blocks of life.”
Evidence of these extraterrestrial sugars proves meteorites can carry organic molecules used on Earth as genetic information. And although DNA-building reactions were commonplace in the universe, whether these space rocks delivered the stuff that eventually became life on Earth isn’t certain yet. In recent years, astrochemistry, or the study of the chemistry of celestial bodies and objects, has inspired many theories and missions to learn about our exotic chemical origins.
To advance his own research, Oba says that scientists need to “analyze wider variations of meteorites and asteroid-return samples,” and perform more experiments to better understand how nucleobases form in extraterrestrial environments.They also suggest that recent samples retrieved from the Japanese Aerospace Exploration Agency’s mission to the Ryugu asteroid, and NASA’s planned mission to the Bennu asteroid, could offer important insights into the evolution of extraterrestrial organic molecules and their role regarding the origins of life on this planet.
Through studying the stars, one of the most important questions about the cosmos we can ask turns out to be one about our own history: Are we truly native to the pale blue dot, or does our chemical makeup reveal alien life exists—and it’s us?