All of life, from simplest to most complex, contains five information-passing compounds that allow the genetic code to work. These nitrogen-based compounds, called nucleobases, are found in all the the DNA and RNA that provide the instructions to build and operate every living thing on Earth.
How these compounds are formed, or where they come from, has long been a key question in astrobiology and the search for the origin of life.
Numerous theories have been advanced to explain their presence, including that they arrived on Earth via meteorites and the infall of dust. But until recently, only three of these nucleobases have been found embedded in meteorites but, puzzlingly, the two others have not been found.
Now an international team centered in Japan has completed the search for nucleobases in meteorites by finding the remaining two, and so it appears possible that all these building blocks of the genetic code could have arrived on very early Earth from afar.
Yasuhiro Oba of the University of Hokkaido, and lead author of the new study in Nature Communications, said that extraterrestrial material arrived in much greater quantities on the early Earth — during what is called the period of “late heavy bombardment” — and so the discovery “of all five primary nucleobases in DNA/RNA indicates that these components should have been provided to the early Earth with such extraterrestrial materials.”
This certainly does not mean that fully formed DNA or RNA was delivered to Earth. Oba said the process of making those nucleic acids from components parts, including nucleobases, is under active study but is not particularly well understood. But it does mean that essential building blocks for the genetic backbone of life clearly did arrive from space for possible use in the life-forming process.
“We don’t know how life first started on the Earth, but the discovery of extraterrestrial nucleobases in meteorites provides additional support for the theory that meteorite delivery could have seeded the early Earth with the fundamental units of the genetic code found in DNA and RNA in all life today,” said co-author Daniel Glavin of NASA’s Goddard Spaceflight Center.
“These nucleobases are highly soluble in liquid water, so over time, any meteorite fragments exposed to water on the early Earth would be extracted from the meteorites into the water and could therefore contribute to the chemical inventory of the prebiotic soup from which life emerged.”
The newly discovered pair of nucleobases, cytosine and thymine, have a more delicate structure than the three found earlier — guanine, adenine and uracil. As a result, they have been elusive in previous analyses and may have degraded when scientists previously extracted samples.
In the earlier experiments, scientists created something of a “meteorite tea,” placing grains of meteorite in a hot bath of formic acid to extract molecules from the sample, forming a solution that was then analyzed to find the molecular makeup of the extraterrestrial broth.
“We study these water extracts since they contain the good stuff, ancient organic molecules that could have been key building blocks for the origin of life on Earth,” said Glavin.
Because the two recently found nucleobases are delicate and had not been found before, the team was initially skeptical to see them in the samples. But two changes in the testing regime seems to have succeeded: the team used cool water to extract the compounds instead of hot formic acid, which is very reactive and could have destroyed these fragile molecules in previous samples. And more sensitive analytics were employed that could pick up on smaller amounts of these molecules.
“This group has managed a technique that is more like cold brew than hot tea and is able to pull out more delicate compounds,” said Jason Dworkin, a co-author of the paper at NASA Goddard. “I was amazed that they had seen cytosine, which is very fragile.”
The finding is by no means proof that life on Earth got an assist from space in the emergence of genetic material. Many hold that the emergence came about exclusively in the prebiotic soup of the planet’s infancy.
But completing the set of nucleobases that make up life today, in addition to all the other important molecules found in the sample, gives scientists working to understand the origins of life more compounds they know were present on early Earth. As a result, they can experiment with them in the lab.
“This is adding more and more pieces; meteorites have been found to have sugars and bases now,” Dworkin said. “It’s exciting to see progress in the making of the fundamental molecules of biology from space.”
The team studied three meteorites — the Murchison, the Tagish Lake, which fell in northern Canada and the Murray meteorite, found in Kentucky. Nucleobases had been found in all three previously. The two newly-identified nucleobases were found in two different samples of the Murchison meteorite and Tagish Lake.
All three meteorites contain thousands of other organic (carbon-based) molecules and these discoveries have over recent decades led scientists to understand that compounds needed for life are found throughout the galaxy, and have consistently fallen to Earth.
These ingredients for life appear to be produced via chemical reactions in asteroids, including photochemical reactions involving ultraviolet photons and cosmic rays. (A 2019 Nature Communications paper by the Oba group reported that photochemical reactions in the interstellar medium may contribute to the presence of nucleobases in the Tagish Lake meteroite.)
Scientists classify nucleobases into two categories. One is called pyrimidine bases, which include cytosine, thymine, and uracil. These molecules have a six-sided ring with two nitrogen atoms in their structure. Without them, the double helix structures of DNA and RNA cannot be made.
According to Oba, the new study found all three pyrimidines in Murchison meteorites, whereas previous studies had only detected uracil. This means a “diversity of meteoritic nucleobases” could have reached a young Earth and been the building blocks of DNA and RNA, according to the study.
The other category is the purines, including the nucleobases adenine and guanine, which are aromatic organic compound that consists of two rings fused together. They are more robust and have been found more commonly in carbon-based meteorites.
The study scientists were excited not only about the discovery of all the nucleobases contained in DNA and RNA in the meteorites, but also about the ever-growing collection of molecules needed at the origin of life — amino acids, sugars, phosphates — that have been found in meteorites as well.
And the creation of a more effective technique for extracting chemical information comes as a perfect moment, when samples from the asteroids Ryugu and Bennu are either in the early phases of analysis or soon will be arriving to Earth.
The Japanese space agency JAXA safely landed its Ryugu sample on Earth in 2020. NASA’s Bennu sample — which was collected by the OSIRIS-REx spacecraft in 2020 — is expected to arrive on Earth in 2023.
These returned, or soon to be returned, samples have the potential to push the field ahead significantly, giving scientists pieces of asteroids are old and representative of the early solar system.
Both missions explored carbonaceous asteroids, which are thought to be the rocky building blocks of the early solar system. These asteroids could help scientists better understand how the solar system formed, and how life later emerged.
The Ryugu sample comes from an asteroid about 200 million miles from Earth. The surface of the asteroid has been dated at 8.9 million years old, plus more or minus 2.5 million years. But it comes from an asteroid family much older that broke apart from a major impact. The first paper, in Nature Astronomy, describing the sample reported that it was “among the most primordial materials available in our laboratories.”
Oba from Hokkaido University said that he and colleagues “highly expect we can detect nucleobases in the asteroid return samples from Bennu and Ryugu,” as well as other necessary-for-life compounds.
Under negotiated agreements, both the Ryugu and Bennu samples are being shared, or will be shared, by the two space agencies and a number of international organizations. Analysis of the Ryugu sample has begun in earnest and NASA Goddard’s Glavin said that a major paper is now under review and should be public in the not too distant future, Glavin said
Bennu and is roughly the same size as Ryugu at about about 1/3 of a mile in diameter. It orbits the Sun about 160 million miles from Earth.
Like Ryugu, Bennu was originally part of a much larger parent body. Bennu’s basic mineralogy and chemical nature would have been established during the first 10 million years of the solar system’s formation, where the carbonaceous material underwent some geologic heating and chemical transformation inside a much larger proto-planet. The asteroid is known to be water rich and has a range of minerals formed only in the presence of water.
Asteroids such as Bennu and Ryugu are are very close in chemical composition to the Sun and the primitive solar nebula , minus hydrogen, helium and other volatiles that can be readily vaporized.
Impacts on Bennu boulders indicate that the asteroid has been in near Earth orbit (separated from the main asteroid belt between Mars and Jupiter) for some 1–2.5 million years.
The amount of ample collected on Ryugu by Hayabusa2 was on the upper end of expectations, and the same is the case for OSIRIS-REx on Bennu.
“The good news is there should be plenty of Bennu sample mass available for organics analyses,” said Glavin. “So I am very optimistic about our chances of also being able to detect nucleobases and other organic compounds fundamental to life in the Bennu samples, if they are there.”