Tar as Sunscreen for Early Life?

Gooey polymers may have protected the Earth's first life-forms from damaging ultraviolet rays. The finding, published in tomorrow's Proceedings of the National Academy of Sciences, argues that a skein of these shielding molecules covered Earth's primordial soup--a scenario that would boost the odds that life gained a foothold on other planets.

When life first arose on Earth some 3.5 billion to 4 billion years ago, up to 1000 times more ultraviolet light was reaching Earth's surface than it does today. The absence of an ozone layer would have allowed the light to pass through the atmosphere. Unless it was absorbed or reflected, the UV radiation would have fried any organic material on the surface, says Stanley Miller of the University of California, San Diego. Scientists have suggested that ice sheets covering the ocean, or a hydrogen-sulfide haze, might have protected nascent life, but attempts to model these conditions have given ambiguous results.

Now Miller and his colleagues have proposed another potential UV shield: by-products of the chemical reactions that synthesize the building blocks of RNA, a molecule considered to be crucial to life's origins. "Whenever you run [experiments to simulate conditions of early life] they produce yellow and brown tars," Miller says. He estimates that a 2 millimeter thick layer of these tar constituents, polymers called tholins, would absorb more than 99% of the UV energy at the most-damaging wavelengths.

At least one expert agrees that these polymers could have screened out UV light. "But it's not as clear cut as they make it look," says James Kasting, a geoscientist at Penn State University, University Park. To make the tarry polymers, he says, most of the atmospheric carbon would have to be in the form of methane, the basic subunit of the polymers. There's no proof that was the case, Kasting says, and some preliminary modeling experiments have suggested that carbon dioxide dominated the early atmosphere. Still, if the polymer scenario were valid, says Miller, it could have played out on planets with a chemical milieu similar to early Earth's. "Suitable havens for life are [probably] more common than previously thought," he says.