Uracil, a key molecule found in all living organisms, has been detected in a near-Earth asteroid.
An international team of experts, led by Professor Yasuhiro Oba in Japan analysed samples from Ryugu, an asteroid around 300 million kilometres from Earth.
The samples were brought to Earth on the Japanese spacecraft Hayabusa2.
The team found uracil, which is a nucleobase – one of the building blocks necessary to form RNA (molecules that contain instructions on how to build and operate living organisms).
The researchers said their findings, published in the journal Nature Communications, add to evidence that important building blocks for life could have been delivered to Earth by meteorites.
Study leader Prof Oba, Associate Professor at Hokkaido University in Japan, said: "Scientists have previously found nucleobases and vitamins in certain carbon-rich meteorites, but there was always the question of contamination by exposure to the Earth's environment.
"Since the Hayabusa2 spacecraft collected two samples directly from asteroid Ryugu and delivered them to Earth in sealed capsules, contamination can be ruled out."
Ryugu belongs to a class of asteroids called carbonaceous, or C-type, asteroids.
As well as uracil, the researchers also detected nicotinic acid - also known as Vitamin B3 or niacin - a key compound involved in metabolism in living organisms, alongside other nitrogen-containing organic compounds.
The researchers extracted these molecules by first soaking the Ryugu particles in hot water.
The team then used various scientific techniques to analyse the chemical composition of these particles.
Prof Oba said that uracil was present in small quantities (between 6–32 parts per billion), while vitamin B3 was more abundant (around 49–99 parts per billion).
He added: "Other biological molecules were found in the sample as well, including a selection of amino acids, amines and carboxylic acids, which are found in proteins and metabolism, respectively."
Last month, findings from the first Ryugu sample also revealed the presence of organic molecules, including amino acids.
However, it did not include any RNA or DNA (molecules that carry genetic instructions in living organisms) components.
Prof Oba and his colleagues believe that the difference in concentrations in the two samples is maybe because they were collected from two different landing sites on Ryugu, which are thought to have different histories.
He said: "The discovery of uracil in the samples from Ryugu lends strength to current theories regarding the source of nucleobases in the early Earth."
Meanwhile, scientists are also waiting to receive samples from another asteroid - Bennu - which is due to return this year as part of NASA's OSIRIS-REx mission.
Commenting on the findings, Professor Mark Sephton at Imperial College London, who was not involved in the study, said: "Uracil is one of the building blocks of life and is found in RNA.
"The presence of nitrogen heterocycles in a fresh sample of the carbonaceous asteroid Ryugu is evidence that the starting materials for life were widespread in the early solar system.
"Similar materials would have rained down on the early Earth and could have helped trigger the start of our planet’s biosphere."