INTENSE ultraviolet light from the young sun may have spurred on the evolution of life. Until now, scientists thought UV radiation was an obstacle to life emerging, but a new theoretical study turns that notion on its head.
Life on Earth is believed to have evolved about 3.7 billion years ago, when there was no protective ozone layer surrounding the planet. This meant that the solar UV radiation reaching the Earth鈥檚 surface would have been about 100 times more intense than it is today.
The earliest life is widely thought to have been based on RNA, the chemical cousin of DNA that now acts mainly as its messenger. Like DNA, RNA is made of subunits containing a sugar, a phosphate and a nitrogen-containing base, which link together to form long polymer chains.
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But scientists did not see how these RNA polymers could form in the intense solar UV light, which they believed would destroy any organic molecules exposed to it. 鈥淪imple RNA molecules can evolve under certain conditions,鈥� says Armen Mulkidjanian, a biophysicist at the University of Osnabr眉ck in Germany. 鈥淏ut the question left is: how do you get the first long-enough RNA polymer?鈥�
To find out more, his team constructed a computer model based on how various organic molecules interact with light, and simulated how they would behave in UV light. They found that, contrary to popular belief, RNA was actually much more likely than other molecules to form long chains in intense UV radiation.
The researchers think the secret of RNA鈥檚 success is the ability of its bases to absorb the energy of the UV light. This protects the sugar and phosphate components that form the spine of the chain, giving RNA a survival advantage.
The UV light could even encourage polymerisation, Mulkidjanian adds. When UV light hits an RNA base, there is a small probability that it can be excited into a chemically reactive state. That encourages it to react with another molecule, forming a new link in the chain (BMC Evolutionary Biology, vol 3, p 12).
Mulkidjanian鈥檚 work is important because it attacks the crucial question of how long RNA polymers formed, says Michael Yarus of the University of Colorado at Boulder, who studies the possibilities of early RNA-based life, the so-called 鈥淩NA world鈥�. 鈥淚f this problem could really be solved in a way that people agreed on, the RNA world would become a fact rather than a speculation,鈥� he says. He adds that although Mulkidjanian鈥檚 work is theoretical, it could help promote experimental work on the early evolution of life.
