As such single-stranded RNA molecules could also combine into double strands, however, this gave rise to the theoretical possibility that the molecules could replicate themselves -- i.e. reproduce. Only two nucleotides fit together in each case, meaning that one strand is the exact counterpart of another and thus forms the template for another strand. In the course of evolution, this replication could have improved and at some stage yielded more complex life. "The RNA world idea has the big advantage that it sketches out a pathway whereby complex biomolecules such as nucleic acids with optimized catalytic and, at the same time, information-coding properties can emerge," says LMU chemist Thomas Carell. Genetic material, as we understand it today, is made up of double strands of DNA, a slightly modified, durable form of macromolecule composed of nucleotides.