A meteorite study has strengthened evidence that life on Earth began in space.
Many experts believe biological raw ingredients were carried to Earth in lumps of asteroid rock.
A key clue lies in the molecular structure of amino acids, the building blocks of proteins and living organisms.
The molecules come in two mirror-image varieties, known as left and right-handed. But only left-handed amino acids are found in nature.
Recently, scientists at the American space agency NASA reported finding the amino acid isovaline in samples of meteorites that came from asteroids.
Most of it was in the left-handed form, suggesting that left-handed life may have had a kick-start in space.
Now the same scientists have discovered left-handed isovaline in a much wider variety of carbon-rich asteroids.
"This tells us our initial discovery wasn't a fluke; that there really was something going on in the asteroids where these meteorites came from that favours the creation of left-handed amino acids," said Dr Daniel Glavin, of NASA.
The scientists believe that early in its history, Earth was bombarded with meteorites containing left-handed amino acids.
The bias towards left-handedness would have continued as the material was incorporated into emerging life.
Evidence suggests that the presence of liquid water amplifies levels of left-handed isovaline (L-isovaline) in asteroids.
"Liquid water seems to be the key," said Dr Glavin. "We can tell how much these asteroids were altered by liquid water by analysing the minerals their meteorites contain.
"The more these asteroids were altered, the greater the excess L-isovaline we found. This indicates some process involving liquid water favours the creation of left-handed amino acids."
Radiation may be responsible for initially tipping the balance towards left-handedness, the scientists believe.
Cosmic rays encountered by the solar system in its youth may have led to slightly more left-handed amino acids being created, or slightly more right-handed molecules being destroyed.
Life in other star systems with different early conditions could have built up around right-handed amino acids, said Dr Glavin.