A new model of evolution's origins traces the possible outlines of a critical and mysterious stage in Earth's infancy, when a few odd chemicals developed into the molecular ancestors of life as we know it.

"This was the basis of all life on Earth," said Harvard University evolutionary dynamicist Martin Nowak, "and it's a good starting point to ask, how does life begin? How does evolution begin?"

That some molecules crossed a critical threshold from inanimate to animate is widely assumed by evolutionary biologists, but four billion intervening years have erased the details of this passage.

Nowak and fellow Harvard biologist Hisashi Ohtsuki posit a transitional stage: molecules subject to the forces of selection and mutation, incapable of replication – the final condition necessary for life – but drawn inexorably towards it.

"Evolutionary dynamics are a universal principle. They can operate with whatever is at hand," said Nowak.

Key to Nowak's model, published yesterday in the Proceedings of the

National Academy of Sciences, is the basic concept of selection and mutation in the absence of replication. He called this period prelife, and characterized it as a "generative system that can produce information."

"All I need is some chemical system that produces all sorts of chemicals, and some have the property of forming strings," said Nowak. "

In model units of one and zero, Nowak replicated the bonding of simple molecules in the primordial soup.

In this context, mutation occurred at the level of chemical reactions available to the newly-formed strings – or, in their real-world prelife analogue, in the precursors of RNA, the single-stranded relative of DNA.

Some strings grew faster than others, and were prone to different types of reactions. "Now I can ask which strings are more abundant than others, and this is a form of selection," said Nowak. "This is prelife.

All that's missing is replication."

The replication may have taken a form known as template-directed polymerization, in which molecules attract chemicals that assemble in mirror copies beside them.

"Many DNA-like molecules have this property," said Nowak. "Now I can ask, under what condition can replication emerge?"

That condition remains unknown, but synthetic biologists hope to replicate it in a laboratory.

"The transition between chemistry and biology is extremely murky," said Irene Chen, a Harvard University systems biologist and specialist in early biomolecules. She was not involved in the research.

"This helps an experimentalist like myself narrow in on the interesting questions. It's a clear framework for starting to tackle these problems experimentally," she said.

Prevolutionary dynamics and the origin of evolution [PNAS]

Images: Above, the abundance of various model polymers; below, their development under conditions of replication. Courtesy of Martin Nowak.

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