The genetic code of all living things is based on only five different bases – four occur in DNA, a fifth only in RNA. However, how and where these letters in our genome originated is unclear. Now, analyzes of samples from the asteroid (162173) Ryugu show that its material contains all five nucleobases. This supports hypotheses according to which the first building blocks of life did not originate on Earth, but at least some of them came from space. Impacts from meteorites and comets could therefore have brought the DNA bases, but also other biomolecules and their precursors, to the young Earth. The Ryugu samples also show that there are differences between different asteroids with regard to these chemical gifts.
Where did the first building blocks of life on earth come from? It was long assumed that more complex organic molecules such as DNA building blocks, peptides or amino acids originated on the early Earth – in undersea vents, hydrothermal pools or fluid-filled pores in the rock. But now another scenario is increasingly coming into focus. Accordingly, the impacts of comets and asteroids could have brought the first building blocks of life or their chemical precursors to Earth. A possible indication of this is the detection of organic molecules in the dust and ice of such celestial bodies and in meteorite samples. In the latter, all five nucleobases have already been discovered over the last few years – the bases that form the genetic code in DNA and RNA. “This suggests that the molecular requirements for life not only exist on Earth, but can also arise elsewhere in the solar system as natural products of chemical evolution,” explain Toshiki Koga from the Japan Agency for Marine and Earth Sciences and Technology (JAMSTEC) and his colleagues.
Five in one go
With the meteorite samples, however, there was always a residual uncertainty as to whether these molecules had only gotten into the samples after the meteorites had hit. Samples that the Japanese space probe Hayabusa 2 collected from the asteroid (162173) Ryugu and brought back to Earth in 2020 now provide more clarity. This almost one kilometer wide, strikingly diamond-shaped asteroid is around 4.5 billion years old and is therefore a relic from the early days of our solar system. Similar to the asteroid (101955) Bennu sampled by NASA’s OSIRIS-REx mission in 2020, Ryugu is rich in volatiles and carbon and may come from the inner asteroid belt. Both contain relatively many organic molecules, as initial analyzes of the asteroid samples revealed. Koga’s team has now specifically analyzed samples from the asteroid Ryugu again for the presence of the DNA bases adenine, guanine, cytosine and thymine as well as the RNA base uracil.
The analyzes showed that all five nucleobases as well as various precursor molecules and derivatives of these bases can be detected in the material of the asteroid Ryugu. This makes this asteroid the second after Bennu to contain these important building blocks of life. This suggests that the presence of such building blocks of life on asteroids was and is not a rare exception. And it supports the assumption that such molecules could also have reached the young Earth through impacts from celestial bodies. “The universal detection of all five canonical nucleobases in samples from Ryugu and Bennu underscores the potential contribution that such space-derived molecules may have made to prebiotic evolution on early Earth,” the team writes.
Ratio of purine and pyrimidine bases not the same
The analyzes also show interesting differences between the two asteroids Ryugu and Bennu. For one thing, the concentration of nucleobases in the Ryugu samples was only half as high as in the samples from the asteroid Bennu, as Koga and his colleagues found. On the other hand, both also differ in the ratio of the two main groups of these bases. These are divided based on their chemical structure: Guanine and adenine are based on two interconnected rings of carbon and nitrogen atoms, derived from the molecule purine. The three remaining nucleobases, thymine, cytosine and uracil, are based on pyrimidine, an organic compound with only one such ring. In their comparative analyses, Koga and his team found that both nuclease base groups are represented in almost equal proportions in the Ryugu material. In Bennu, on the other hand, the pyrimidine bases predominate, while in some meteorites found on Earth it is the purine bases.
According to the researchers, these differences could be due to the history of their formation and the amount of precursor substances on these asteroids. For example, the comparison showed a strong correlation between the ammonia content in the various asteroids and their ratio of purine to pyrimidine bases. “This correlation suggests that ammonia availability is a key factor in the formation pathways of these nucleobases,” the team writes. The ratio of the different nucleobases in celestial bodies could therefore provide information about the cosmic environments in which they were formed.
Source: Toshiki Koga (Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka) et al., Nature, doi: 10.1038/s41550-026-02791-z