LIFE × EARTH
Amino Acids in Meteorites
~4.3 billion years ago
Carbon-rich meteorites contain amino acids, traces of ribose and nucleobases, the parts from which proteins, RNA and DNA are built, all assembled in space before any life existed.
Carbonaceous chondrites are meteorites rich in carbon compounds. They contain amino acids, the units from which proteins are built. In far smaller traces, they also contain the sugar ribose, a component of RNA, and nucleobases, the components of RNA and DNA that act as the letters of the genetic alphabet. Every one of these molecules was assembled in space, with nothing alive involved. The chemistry that preceded life therefore does not require life to produce it. Material of the same kind fell onto the young Earth, delivered by asteroids, comets and the dust that drifts between the planets.
Why it matters
A planet does not need to produce all the molecules of life from scratch, because some of them arrive already assembled. The process that formed the first organisms, whatever it was, therefore had a stock of components available. This delivery from space is in no way particular to the Earth.
Dating & uncertainty
Researchers measure these amino acids, sugars and nucleobases in recently fallen meteorites, and two results establish their extraterrestrial origin. First, carbon exists in forms of different mass, called isotopes, and Furukawa and colleagues (2019) found that their proportions in the sugars are incompatible with terrestrial formation. Second, an amino acid exists in two mirror-image forms, called left-handed and right-handed. The amino acids analysed occur in equal proportions of the two forms, a balance that the chemistry of life never maintains. The quantities are minute and very unequal, of the order of a few parts per billion for the rarer amino acids and, for the nucleobases, a thousand times lower still. The proportion of this material that reached the ground intact through the early atmosphere remains an inference.
Earth history as one day
01:16:0722 h 44 min before midnight
if Earth’s 4.54 billion years were compressed into a single day