RNA folding argues against a hot-start origin of life

Opinion is strongly divided on whether life arose on earth under hot or col d conditions, the hot-start and cold-start Scenarios, respectively. The ori gin of life close to deep thermal vents appears as the majority opinion amo ng biologists, but there is considerable biochemical evidence that high tem peratures are incompatible with an RNA world. To be functional, RNA has to fold into a three-dimensional structure. We report both theoretical and exp erimental results on RNA folding and show that las expected) hot conditions strongly reduce RNA folding. The theoretical results come from energy-mini mization calculations of the average extent of folding of RNA, mainly from 0-90 degreesC, for both random sequences and tRNA sequences. The experiment al results are from circular-dichroism measurements of tRNA over a similar range of temperatures. The quantitative agreement between calculations and experiment is remarkable, even to the shape of the curves indicating the co operative nature of RNA folding and unfolding. These results provide additi onal evidence for a lower temperature stage being necessary in the origin o f life.