The "strong" RNA world hypothesis: fifty years old.

This year marks the 50(th) anniversary of a proposal by Alex Rich that RNA, as a single biopolymer acting in two capacities, might have supported both genetics and catalysis at the origin of life. We review here both published and previously unreported experimental data that provide new perspectives on this old proposal. The new data include evidence that, in the presence of borate, small amounts of carbohydrates can fix large amounts of formaldehyde that are expected in an environment rich in carbon dioxide. Further, we consider other species, including arsenate, arsenite, phosphite, and germanate, that might replace phosphate as linkers in genetic biopolymers. While linkages involving these oxyanions are judged to be too unstable to support genetics on Earth, we consider the possibility that they might do so in colder semi-aqueous environments more exotic than those found on Earth, where cosolvents such as ammonia might prevent freezing at temperatures well below 273 K. These include the ammonia-water environments that are possibly present at low temperatures beneath the surface of Titan, Saturn's largest moon.