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12 December 2013 1:00 pm ,
Vol. 342 ,
Stefan Behnisch has won awards for designing science labs and other buildings that are smart, sustainable, and...
The iconic 125-year-old Lick Observatory on Mount Hamilton near San Jose, California, is facing the threat of closure...
Recent results from the Curiosity Mars rover have helped scientists formulate a plan for the next phase of its mission...
A new, remarkably powerful drug that cripples the hepatitis C virus (HCV) came to market last week, but it sells for $...
In pretoothbrush populations, gumlines would often be marred by a thick, visible crust of calcium phosphate, food...
Evolutionary biologists have long studied how the Mexican tetra, a drab fish that lives in rivers and creeks but has...
Victorian astronomers spent countless hours laboriously charting the positions of stars in the sky. Such sky mapping,...
In an ambitious project to study 1000 years of sickness and health, researchers are excavating the graveyard of the now...
- 12 December 2013 1:00 pm , Vol. 342 , #6164
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Tar as Sunscreen for Early Life?
22 June 1998 7:00 pm
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.