Let there be light. (Biochemistry).

Many scientists suspect that ribonucleic acid, or RNA, preceded DNA and served as life's first genetic material. Yet it's never been clear how long strands of RNA, or DNA for that matter, could form in the harsh conditions of the primordial Earth, especially under the intense ultraviolet (UV) light that flooded the planet. Instead of being a barrier to RNA creation, however, UV light might actually have been a help, a new study suggests.

When the planet formed, there was no ozone layer, so the amount of UV light hitting the surface was about 100 times what it is today. Most origin-of-life researchers argue that such a bombardment would destroy fledgling organic molecules unless they were hidden underwater or otherwise protected.

"The existing theories consider the high UV level as a major obstacle and offer several different strategies for hiding the first life forms from it," say Michael Y. Galperin of the National Center for Biotechnology Information in Bethesda, Md., and two of his colleagues. They challenge that notion in a paper published online May 28 in BMC Evolutionary Biology. "Here, we invoke the alternative possibility that UV irradiation played a positive role in the origin of life," they say.

The researchers note that the nitrogen-containing bases that are a part of RNA and DNA molecules are "powerful quenchers" of UV light. That is, these bases can absorb the radiation and quickly dissipate its energy, thus protecting the more vulnerable sugar-based backbone of RNA and DNA molecules. In computer simulations, UV light favors the creation of strands of nitrogen-containing bases over other organic molecules, Galperin and his colleagues found.--J.T.