For the first time ever, all four major components of DNA, the biological blueprint of living things, have been detected in rocks from outer space, a discovery that suggests the building blocks of life may have been delivered to Earth by ancient extraterrestrial objects, according to a new study.
DNA is a helical structure made of so-called “nucleobases”—the compounds adenine, guanine, cytosine, and thymine—which combine in motley permutations to write the source code for life on Earth, including humans. Though adenine and guanine were found in meteorites about 50 years ago, the presence of cytosine and thymine in these extraterrestrial objects has remained elusive, despite evidence that the compounds might have existed in the primordial interstellar dust that gave rise to our solar system some 4.6 billion years ago.
Now, scientists led by Yasuhiro Oba, a professor at Hokkaido University, have at last detected trace amounts of cytosine and thymine in three carbonaceous (carbon-rich) meteorites, a finding that bolsters the notion that extraterrestrial impacts ”contributed to the emergence of genetic properties for the earliest life on Earth,” according to a study published in Tuesday in the journal Nature Communications.
The two newly detected nucleobases belong to a group called the pyrimidines, whereas adenine and guanine are categorized as purines. In addition to discovering the remaining compounds inside DNA, Oba and his colleagues also found traces of another pyrimidine called uracil, which is used by RNA, a simpler sister molecule of DNA, instead of thymine. Though uracil has been identified in meteorites before, the discovery of all three pyrimidines in the space rocks sheds new light on the puzzling scarcity of these nucleobases in meteorites, compared to the purines adenine and guanine.
“The lack of pyrimidine diversity in meteorites remains a mystery since prebiotic chemical models and laboratory experiments have predicted that these compounds can also be produced from chemical precursors found in meteorites,” Oba’s team said in the study. “Here we report the detection of nucleobases in three carbonaceous meteorites using state-of-the-art analytical techniques optimized for small-scale quantification of nucleobases down to the range of parts per trillion (ppt).”
“In addition to previously detected purine nucleobases in meteorites such as guanine and adenine, we identify various pyrimidine nucleobases such as cytosine, uracil, and thymine,” they added. “This study demonstrates that a diversity of meteoritic nucleobases could serve as building blocks of DNA and RNA on the early Earth.”
The team achieved this breakthrough by analyzing samples from the Murchison, Murray and Tagish Lake meteorites, which landed in Australia, Oklahoma, and British Columbia, respectively. The research follows a number of related studies of these meteorites, and others, that have found proteins, nitrogen, water, organic compounds, and other key ingredients for life in extraterrestrial objects that ended up on Earth, potentially planting the seeds of habitability on our infant planet.
It’s even possible that nascent lifeforms could have been transferred between worlds—such as Earth and Mars—through a process known as panspermia, in which organisms are able to survive interplanetary voyages by hitchhiking on meteorites ejected by impacts on their home worlds.
While it’s still not clear why the purines are more readily detectable in the space rocks, the researchers think that all these nucleobases could have been formed by photochemical processes in the interstellar medium, “suggesting that these classes of organic compounds are ubiquitously present in extraterrestrial environments both inside and outside the solar system,” according to the study.
In other words, the new findings not only help to unravel the tale of our own origins as Earthlings, they may also inform our search for alien life elsewhere in the universe. The advent of sample-return missions such as NASA’s OSIRIS-REx, which will deliver pristine material from a carbonaceous asteroid to Earth next year, will further constrain these most essential human questions of how life began on our planet, and whether it exists elsewhere.