What was life's first sunblock?

Since the early Earth lacked a protective layer of atmospheric ozone, researchers have wondered what shielded the primordial building blocks of life from destruction by ultraviolet (UV) light. Two chemists now argue that tarlike organic polymers in the oceans could have provided an ancient sunscreen for the amino acids and nucleic acid bases forming there.

Biochemists Stanley L. Miller and H. James Cleaves at the University of California, San Diego used an experimental approach that Miller had pioneered 45 years ago. In that 1953 study, Miller showed how the organic building blocks of life could have developed in the primordial ocean.

In the new study, published in the June 23 issue of Proceedings of the National Academy of Science, Miller and Cleaves focused on a class of UV-screening polymers called tholins, which resemble kerogen, a gunky material in oil shale that yields petroleum. As in the 1953 experiment, they simulated organic reactions in the early oceans by heating a mixture of nitrogen, methane, and water. Tholins and other organic compounds resulted. Tholins, which absorb UV light well, may have given those seas a yellowish hue, Miller says.

Tholins in the top 2 millimeters of ocean water would have blocked all but 1 percent of a damaging wavelength of UV light, Miller says. He suggests that this screening would have been adequate for protecting newly formed organic building blocks.

Furthermore, the biochemists argue, a variety of ocean-based compounds, including dissolved salts, sulfur or iron, an ice cap, or a worldwide slick of naturally produced oil, could have supplemented this UV protection.

"This shows that there could have been a whole variety of shielding candidates in the early oceans, and that's good to know," said Christopher E Chyba, a planetary scientist at the University of Arizona in Tucson.