"The problem was insoluble: you cannot, we thought, have DNA without proteins, and you cannot have proteins without DNA. RNA, however, being a DNA equivalent (it can store and replicate genetic information) as well as a protein equivalent (it can catalyze critical chemical reactions) offers an answer. In fact, in the “RNA world” the chicken-and-egg problem simply disappears. RNA is both the chicken and the egg. RNA is an evolutionary heirloom. Once natural selection has solved a problem, it tends to stick with that solution, in effect following the maxim “if it ain’t broke, don’t fix it.” In other words, in the absence of selective pressure to change, cellular systems do not innovate and so bear many imprints of the evolutionary past. A process may be carried out in a certain way simply because it first evolved that way, not because that is absolutely the best and most efficient way."