According to this paper: Andrew M. Steer, Nicolas Bia, David K. Smith, Paul A. Clarke. Prebiotic synthesis of 2-deoxy-d-ribose from interstellar building blocks promoted by amino esters or amino nitriles. Chem. Commun., 2017; DOI: 10.1039/C7CC06083A, the DNA building blocks for life may have come from outer space in meteor showers:
“Dr Paul Clarke, from the University of York’s Department of Chemistry, said: “The origin of important biological molecules is one of the key fundamental questions in science. The molecules that form the building blocks of DNA had to come from somewhere; either they were present on Earth when it formed or they came from space, hitting earth in a meteor shower.
“Scientists had already shown that there were particular molecules present in space that came to Earth in an ice comet; this made our team at York think about investigating whether they could be used to make one of the building blocks of DNA. If this was possible, then it could mean that a building block of DNA was present before amino acids.”
https://www.sciencedaily.com/releases/2017/09/170914152256.htm
