Asteroid fragments upend theory of how life on Earth bloomed

Fragments collected from the asteroid Bennu contain the building blocks for life — all five nucleobases that form DNA and RNA and 14 of the 20 amino acids needed to make known proteins. But there’s a twist: on Earth, amino acids in living organisms tend to have a ‘left-handed’ structure. Those on Bennu, however, contain nearly equal amounts of these structures and their ‘right-handed’, mirror-image forms. This calls into question a hypothesis favoured by many scientists that asteroids similar to this one might have seeded life on Earth.