DNA building blocks found on meteorites are providing more evidence that chemistry in space generated the ingredients for life on Earth.

Adenine and guanine — two of the four building blocks called nucleobases that make up DNA — were found in samples of carbon-rich meteorites analyzed in a study published Monday online in the Proceedings of the National Academy of Sciences.

The meteorites are fragments of rock that fell to Earth from space, where they once formed parts of asteroids.

The chemicals found on the meteorites also included three molecules very similar to nucleobases that are almost never found on Earth.

Meteorites and asteroids Meteorites are pieces of asteroids that fall through the Earth's atmosphere and land on the surface. They have been suggested as possible original sources for some of the molecules necessary for life on Earth. Asteroids, found mainly in a belt between Mars and Jupiter, are chunks of rocky material formed from dust and gas left over from the sun's early history. Their composition depends on what materials came together to form that particular asteroid. Only some contain organic molecules.

However, the researchers found that both the nucleobases and the similar, rare molecules known as nucleobase analogs can be formed in the laboratory from chemical reactions of ammonium cyanide.

This may indicate how the molecules were formed on the asteroid. It also provides evidence that the nucleobases were made in space and did not come from contamination on Earth, the researchers wrote.

"You would not expect to see these nucleobase analogs if contamination from terrestrial life was the source, because they're not used in biology," said Michael Callahan, the NASA astrobiologist who led the study, in a statement.

"However, if asteroids are behaving like chemical 'factories' cranking out prebiotic material, you would expect them to produce many variants of nucleobases, not just the biological ones, because of the wide variety of ingredients and conditions in each asteroid."

Scientists have detected DNA building blocks in meteorites since the 1960s, but were never sure whether they were created in space or came from contamination on Earth.

Callahan and his colleagues tested the Antarctic ice where most of the meteorites were found. They didn't find any of the nucleobase analogs and only very small amounts of the nucleobases.

Amino acids previously found on asteroids

The new discovery means all three kinds of molecules needed to form living cells have now been found in meteorites and appear to have been made there, the researchers wrote:

Nucleobases, used to form nucleic acids that make up genetic material such as DNA.

Amino acids, used to build proteins.

Amphiphilic compounds — which are attracted to water on one end and to oil on the other — that are used to build cell membranes.

"Thus," the paper said, "meteorites may have served as a molecular kit providing essential ingredients for the origin of life on Earth and possibly elsewhere."

The researchers analyzed 12 meteorites for the study, including nine found in Antarctica. All were carbonaceous chondrites, a special type of meteorite known to contain organic molecules, the carbon-based chemicals found in living things. Eleven of the meteorites contained adenine. Three meteorites with a similar geology due to similar exposure to water contained a particularly wide range of nucleobases and nucleobase analogs.

This past June, a Canadian-led study of a B.C. meteorite showed that the types of organic molecules found in meteorites depend on how much water had percolated through the asteroid during its early history, providing strong evidence that they were formed by chemistry on the asteroid itself.