Nucleobases from Space: Evidence from Asteroid Ryugu

Samples collected by the Japanese robotic space probe Hayabusa2 from the asteroid Ryugu have revealed the presence of organic compounds, including all five bases that make up DNA and RNA — the molecules that carry genetic information in all life on Earth, from bacteria and viruses to horses, sequoias, and humans.

Ryugu is an asteroid about 900 meters across. It orbits the Sun on a path that lies closer to the Sun than most asteroids and also intersects Earth’s orbit. In 2019, Hayabusa2 arrived at Ryugu, and at the end of the following year it released a sealed capsule over a barren region in South Australia containing the material it had collected — 5.4 grams of asteroid soil in total. The sample actually consisted of two separate collections by the spacecraft: one taken from the asteroid’s surface and another from a depth of several centimeters.

Ryugu is a C-type asteroid, meaning it is rich in carbon and carbon-bearing compounds. Asteroids of this kind are found mainly in the outer asteroid belt, between Mars and Jupiter, as well as in the Kuiper Belt beyond Neptune, in the far outer reaches of the Solar System. Researchers therefore estimate that Ryugu formed in a more distant region of the Solar System. Based on its structure and composition, scientists think the asteroid was once part of a much larger body that later broke apart — likely in a collision — with the fragment now known as Ryugu eventually migrating into the inner Solar System, where it now follows an orbit relatively close to Earth’s.

Most asteroids formed from the leftover gas and dust from which the Sun, the planets, and later most of their moons took shape about 4.5 billion years ago. But unlike planets and moons, which undergo constant environmental change, asteroids are thought to preserve much of the Solar System’s original composition, including materials formed in its earliest stages. That is one reason asteroids are of such great scientific interest, and why several missions in recent years have set out to collect samples from them.

The Building Blocks of Life

DNA is built from four chemical bases that encode genetic information: adenine, guanine, cytosine, and thymine. Adenine and guanine belong to the purine family and consist of two fused rings of carbon and nitrogen atoms. Cytosine and thymine, by contrast, are pyrimidines and have a single-ring structure. RNA is very similar to DNA, except that thymine is replaced by another pyrimidine, uracil. Together with sugar and phosphate groups, these bases form the nucleotides that make up genetic material on Earth. They also play central roles in other essential biochemical systems. One example is ATP, often described as the energy currency of living cells, which contains adenine, a sugar, and three phosphate groups.

Initial analyses of the Ryugu samples had already revealed the presence of uracil, along with a wide range of other organic compounds, including several amino acids, carboxylic acids, and many additional molecules. Now, in a new study, researchers in Japan report that the asteroid samples contain all five nucleobases found in DNA and RNA on Earth.

These findings do not suggest that life once existed on the asteroid, or in the parent body from which it originated. Rather, they show that relatively complex organic molecules, including the building blocks of life, can form in an abiotic environment.  More broadly, the results again raise the possibility that some of the chemical building blocks relevant to life were delivered to early Earth from space, potentially contributing to the emergence of life here and perhaps elsewhere.

Last year, researchers in the United States reported a remarkable abundance of organic molecules in samples collected from the asteroid Bennu by the American spacecraft OSIRIS-REx. The samples contained all five nucleobases found in DNA and RNA, along with 14 of the 20 amino acids that make up proteins in nature, as well as other molecules. “The findings suggest that broad regions of the early Solar System had conditions suitable for the formation of the molecules needed for life. This did not require a large celestial body such as a planet or moon,” said Tim McCoy, one of the study’s co-authors.

Researchers continue to analyze the samples, hypothesizing that within the asteroid’s interior, some amino acids may have reacted to form simple proteins, or that certain nucleobases combined into a gene-like molecule. However, they do not believe that actual life could have emerged on the ancient asteroid within its limited timeframe of a few million years in a warm environment. “I don’t think it went that far,”  concluded McCoy. “I think it went somewhere down the path towards life.”

Last year, researchers in the United States reported a rich abundance of organic molecules in samples collected from the asteroid Bennu by the American spacecraft OSIRIS-REx. The samples contained all five nucleobases found in DNA and RNA, along with 14 of the 20 amino acids used to build proteins, as well as many other organic compounds. “The new findings indicate that much of the early solar system had the necessary conditions for the formation of life’s essential molecules. It doesn’t take something like a planet or a big moon,” said Tim McCoy, one of the study’s co-authors.

Meaningful Ratios

Organic compounds, including some of the chemical building blocks of life, have also been found in meteorites that landed on Earth, such as the Murchison meteorite, which fell in Australia in 1969, and the Orgueil meteorite,which fell in southern France in 1864. Studies carried out over the years have shown that both contain amino acids as well as nucleobases. Unlike asteroid samples, which were collected and sealed in space, meteorites landed and cooled in a biologically active environment, making terrestrial contamination a greater concern. Even so, researchers now have methods to rule out such contamination and show that these molecules were indeed formed in space,  in part by identifying molecular structures and chemical signatures that are unknown on Earth, or at least have not yet been observed here.

A possible clue to the link between these molecules and the emergence of life lies in their relative abundances. During the race to uncover the nature of hereditary material, the biochemist Erwin Chargaff discovered that in every living cell, regardless of the organism, the total amount of purines equals the total amount of pyrimidines. More specifically, the amount of guanine equals that of cytosine, and the amount of adenine equals that of thymine. This insight was an important step toward uncovering the double-helical structure of DNA, and it may also offer clues in the search to understand the origin of life in the universe.

The Japanese researchers compared the concentrations of nucleobases in samples from the two asteroids and found that in Ryugu, purines and pyrimidines were present in roughly similar amounts. Bennu, by contrast, was relatively richer in pyrimidines, as was the Orgueil meteorite. In the Murchison meteorite, however, purines were more abundant. The researchers suggest that these differences in relative abundance reflect different chemical pathways of formation, likely under different conditions. In their view, the chemical composition of asteroids and meteorites — including the relative amounts of purines and pyrimidines — could provide a useful tool for classifying such bodies and reconstructing their histories.

“The universal reliance of metabolism on adenine-based cofactors and energy currencies further indicates that this molecular architecture reflects ancient chemical constraints. Accordingly, understanding the sources and availability of nucleobases on the early Earth remains a central question in origins-of-life research,” the researchers conclude.

Translated with the assistance of ChatGPT. Edited, revised, and reviewed by the editorial staff of the Davidson Institute of Science Education.