SAN DIEGO--RNA, the genetic molecule that many researchers think was the basis of the first life, might not have been able to take the heat in the early world. A study described here today at the American Geophysical Union's biennial Ocean Sciences Meeting shows that RNA's chemical building blocks fall apart within days to years at temperatures near boiling--a finding that poses problems for some origin of life theories, especially ones picturing that life arose in scalding settings such as deep-sea hydrothermal vents. An RNA-driven origin of life at or below freezing is much more likely, researchers maintain, unless different and more heat-tolerant genetic components were involved.
The first molecule that could copy itself and forge chemical bonds critical to life, most biochemists believe, was RNA. This "RNA world" scenario assumes that chemical reactions in Earth's primitive atmosphere and oceans assembled the raw ingredients for RNA, which served as both the cell's genetic librarian and its molecular workhorse until DNA and proteins, respectively, evolved to assume these functions. However, scientists had not yet examined how long RNA's four units, or nucleobases--adenine (A), cytosine (C), guanine (G) and uracil (U)--survive over a wide range of temperatures.
At 100 degrees Celsius, the new study reveals, half of the nucleobases degrade within 19 days (for C) to 12 years (for U). That's far too fast for them to form the stuff of life, says chemist Matthew Levy, who works with origin of life specialist Stanley Miller at the University of California, San Diego. At 0 degrees, A, G, and U persist for a million years or more, but C's half-life is just 17,000 years. "We conclude that perhaps C wasn't part of the first genetic material," Levy says, because assembly into viable organisms probably took much longer than that.
A cold early Earth, say Levy and Miller, may be necessary to preserve the RNA world. However, atmospheric chemist James Kasting of Pennsylvania State University in University Park says most models still point to a toasty primitive Earth engulfed by thick greenhouse gases. "No one has ruled out a cold Earth, because there are no data," Kasting says. "However, we know so little about how life began that we should not use the stability of nucleobases to constrain our models of Earth's temperature."