Previous analyses of the Murchison meteorite and samples from asteroid Bennu had detected some nucleobases — the molecular letters of DNA and RNA — but never all five. Murchison was enriched in purines (adenine, guanine) but depleted in pyrimidines (cytosine, thymine, uracil). Bennu showed the opposite bias. Neither asteroid carried the full alphabet.
Koga and colleagues analyzed two samples from asteroid Ryugu, returned by JAXA's Hayabusa2 mission. Both samples contain all five canonical nucleobases — adenine, guanine, cytosine, thymine, and uracil. Not just detected: present in roughly equal proportions of purines and pyrimidines, unlike the skewed ratios in meteorites and other asteroid samples.
The balance matters more than the completeness. If asteroids delivered prebiotic molecules to early Earth, a biased delivery — too many purines, not enough pyrimidines — would require Earth-based chemistry to make up the deficit. Ryugu's balanced inventory means a single carbonaceous asteroid could, in principle, deliver a proportionate chemical starting kit. No supplementary synthesis required. The delivery contains what you need in the ratios you need it.
The deeper structural point: the question was never “do asteroids contain nucleobases?” — that's been known since Murchison in 1969. The question was whether any single source carries the full set in useful proportions. Individual detections don't demonstrate sufficiency; the ratio between components determines whether a delivery is a raw material or a contamination. Five letters arriving in wildly unequal amounts is chemically less useful than three letters in proportion.
Completeness is not the same as functionality. Sufficiency requires proportion.