Armen Mulkidjanian, a biophysicist at Osnabrück University in Germany, is rethinking the standard story of how life appeared on Earth. Many of his colleagues have assumed life originated in water; he places it on a dry mudflat. And whereas researchers generally regard ultraviolet rays from the sun as a threat to early biochemical reactions, Mulkidjanian argues that those rays gave life an essential jump start.
Despite his divergent conclusions, Mulkidjanian starts with a standard scenario that simple molecules on the young Earth developed into sugar phosphates and nitrogen bases, which in turn combined to form RNAthe first self-replicating molecule. In a series of computer models, he and his colleagues find that RNA-like molecules withstand harsh ultraviolet exposure better than sugar phosphates alone because attached nitrogen bases protect the molecules from UV damage. Extended RNA chains, known as polymers, are more stable still. The results suggest that ultraviolet rays would have selected for larger, more complex RNA molecules. "People have never been able to explain how the first polymers could build up on the primordial Earth. This work suggests a way out," Mulkidjanian says.
The reactions that piece together long RNA chains do not work well in the presence of water, Mulkidjanian realized. If he is correct, the steps that set the stage for life must have occurred on a parched or semidry landscape. "Once you get something more complex, like the first cell, you need water. Cells need water to survive," he says. Tidal flats might have offered the ideal conditions, alternately soaking and baking organic molecules until life emerged.