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All four of the key DNA building blocks have now been found in meteorites, suggesting that space rocks may have delivered the compounds to Earth, contributing to the origin of life.

DNA has a spiral-staircase structure, in which each step consists of pairs of molecules called nucleobases. Two of these four nucleobases 鈥� adenine and guanine, which belong to a group of chemical compounds called purines 鈥� were first detected in meteorites in the 1960s.

Now, at Hokkaido University in Japan and his colleagues have discovered the remaining two DNA nucleobases, cytosine and thymine, known as pyrimidines, in several meteorites.

The team found the nucleobases in about 2 grams of rock from three meteorites: the Murchison, Murray and Tagish Lake meteorites. The Murchison and Murray meteorites, which hit Earth in the mid-20th century, are thought to date to at least 5 billion years ago. Like Earth, the Tagish Lake meteorite probably formed 4.5 billion years ago, and it hit our planet about two decades ago.

Oba鈥檚 team ground each rock sample into a powder that was added to water, before using ultrasound waves to separate the particles into layers. The group then used mass spectrometry to identify compounds according to their molecular weight.

鈥淭here was a reason why cytosine and thymine in meteorites were never reported until now鈥娾�︹�塼hese compounds are in very trace amounts, which required a method with the capability to measure such small amounts,鈥� says at Boise State University in Idaho.

Could the compounds have come from contamination? In soil around the Murchison meteorite landing site in Australia, the relative amounts of nucleobases differ substantially from those in the meteorite, suggesting that the rock鈥檚 nucleobases came from space.

鈥淚 am convinced that the data is not reflective of terrestrial contamination,鈥� says at the Naval Research Laboratory in Washington DC.

Rocks containing nucleobases may have hit Earth between 4 and 3.8 billion years ago, in the Late Heavy Bombardment. This precedes the earliest known undisputed microbe fossils, which are about 3.4 billion years old.

Oba鈥檚 team also detected a higher concentration of nucleobases in the soil the Murchison meteorite fell onto than in the meteorite.

鈥淚f these results are representative of typical pyrimidine concentrations in meteorites,鈥� says Callahan, 鈥渢hen [nucleobases present on] Earth would likely have been responsible for the emergence of genetic material rather than inputs from聽extraterrestrial delivery.鈥�