LIFE may have begun not in the heat, but in the cold. A famous biologist
argues that the basic components of life could not have survived long in the
near-boiling temperatures of deep ocean vents, thought to be home to the first
organisms on Earth.
Many researchers believe the first microbes were similar to the modern
bacteria that thrive in temperatures near the boiling point of water.
Consequently, they say, life probably began in hot spots where water was warmed
by volcanic activity.
But Stanley Miller of the University of California at San Diego disagrees. In
1953 he showed that the four building blocks, or bases, of RNA could have formed
spontaneously on the early Earth. Many believe RNA carried the first genetic
information. But this week, Miller and his colleague Matthew Levy report in the
Proceedings of the National Academy of Sciences (vol 95, p 7933) that
RNA bases quickly break down in hot water.
Advertisement
Levy and Miller stored sterilised solutions of the bases鈥攁denine,
guanine, cytosine and uracil鈥攁t 0 掳C and at 100 掳C for several
months. At the boiling point of water, the uracil broke down slowly, with a
half-life of 19 years. But the adenine and guanine had a half-life of only one
year, meaning that all traces would be lost from the solution in a few decades,
while cytosine鈥檚 half-life was just 19 days.
Miller concludes that the bases could not have accumulated in hot parts of
the early Earth. 鈥淚f these building blocks weren鈥檛 there, organisms couldn鈥檛 use
迟丑别尘.鈥�
Near 0 掳C, however, all the bases had half-lives of thousands or millions
of years. While this is a relatively short time on the geological time scale,
such periods are far more likely to permit the building blocks to accumulate,
Miller says.
But Gerald Joyce, a biochemist at the Scripps Research Institute in La Jolla,
California, says he would not rule out a high-temperature start just because
RNA鈥檚 bases are sensitive to heat. More likely, he says, the first life forms
didn鈥檛 use RNA: 鈥淭his is a strike against an RNA world.鈥�
Miller agrees he can鈥檛 rule out a hot start to life. But he argues that ice
on the ocean鈥檚 surface would have been a more hospitable spot for the building
blocks of life. 鈥淚ce is a great place to do prebiotic chemistry.鈥�