Conceptual image of meteoroids delivering nucleobases to ancient Earth. The nucleobases are represented by structural diagrams with hydrogen atoms as white spheres, carbon as black, nitrogen as blue and oxygen as red. (NASA Goddard/CI Lab/Dan Gallagher)

All of life, from simplest to most complex, contains five information-passing compounds that allow the genetic code to work.  These nitrogen-based compounds, called nucleobases, are found in all the the DNA and RNA that  provide the instructions to build and operate every living thing on Earth.

How these compounds are formed, or where they come from, has long been a key question in astrobiology and the search for the origin of life.

Numerous theories have been advanced to explain their presence, including that they arrived on Earth via meteorites and the infall of dust.  But until recently, only three of these nucleobases have been found embedded in meteorites but, puzzlingly, the two others have not been found.

Now an international team centered in Japan has completed the search for nucleobases in meteorites by finding the remaining two, and so it appears possible that all these building blocks of the genetic code could have arrived on very early Earth from afar.

Yasuhiro Oba of the University of Hokkaido, and lead author of the new study in Nature Communications, said that  extraterrestrial material arrived in much greater quantities on the early Earth — during what is called the period of “late heavy bombardment” — and so the discovery “of all five primary nucleobases in DNA/RNA indicates that these components should have been provided to the early Earth with such extraterrestrial materials.”

This certainly does not mean that fully formed DNA or RNA was delivered to Earth.  Oba said the process of making those nucleic acids from components parts, including nucleobases, is under active study but is not particularly well understood.  But it does mean that essential building blocks for the genetic backbone of life clearly did arrive from space for possible use in the life-forming process.

“We don’t know how life first started on the Earth, but the discovery of extraterrestrial nucleobases in meteorites provides additional support for the theory that meteorite delivery could have seeded the early Earth with the fundamental units of the genetic code found in DNA and RNA in all life today,” said co-author Daniel Glavin of NASA’s Goddard Spaceflight Center.

“These nucleobases are highly soluble in liquid water, so over time, any meteorite fragments exposed to water on the early Earth would be extracted from the meteorites into the water and could therefore contribute to the chemical inventory of the prebiotic soup from which life emerged.”… Read more