Over 13 billion years ago at least one of the domains of life may have begun in nebular clouds. If restricted to the Milky Way, which is 13.6 billion years old, the first chemical combinations would have had billions of years to become a self-replicating organism with a DNA genome long before the existence of Earth.
Above and left of the Bubble's center is the hot, O-type star, several 100,000 times more luminous and approximately 45 times more massive than our Sun. Fierce stellar winds and intense radiation from that star has blasted out the structure of glowing gas against denser material in a surrounding molecular cloud.
Nebular clouds are thought to be most likely environment for synthesizing and promoting the evolution of molecules needed for the origin of life. The building blocks for DNA could have been generated or combined within interstellar clouds and DNA would become part of the molecular-protein-amino acid complex. Hydrogen, oxygen, carbon, calcium, sulfur, nitrogen and phosphorus for example are continually irradiated by ions, which can generate small organic molecules which evolve into larger complex organic molecules that result in the formation of amino acids and other compounds.
Phosphorus, for example, is rare in our solar system and may have been non-existent on the early Earth; phosphorus is essential for the manufacture of DNA.
Polarized radiation in the nebula cloud leads to the formation of proteins, nucleobases and then DNA. The combination of hydrogen, carbon, oxygen, nitrogen, cyanide and several other elements, could create adenine, which is a DNA base, whereas oxygen and phosphorus could ladder DNA base pairs. Glycine has also been identified in the interstellar clouds.
Fast forward 4.6 billion years, on Earth the steps leading from the random mixing of chemicals to the first nano-particle would likely require hundreds of millions and even billions of years before the first self-replicating molecular compound was fashioned. Even after billions of years, the first replicon may not have possessed DNA.
The Daily Galaxy via NASA
Image credit: NASA, ESA and the Hubble Heritage Team [STScI/AURA])