Dramatizing life's chemical prelude.

Dramatizing life's chemical prelude

The complex biochemical dance that continually unfolds in living cells must originated somewhere. According to the standard evolutionary picture, that somewhere was a primordial soup stocked with the kinds of small molecular building blocks, or monomers, that linked into nucleic acids (RNA and DNA) and proteins -- the long polymeric molecules central to contemporary life forms. But it's a long way from a brew of unconnected monomers to an interdependent set of intra-cellular polymers that pull off feats such as replication, metabolism and differentiation into specific cell types.

In cells, proteins assist in the replication of nucleic acids, which themselves orchestrate the assembly of proteins. "Neither can be produced without the other," notes J. Doyne Farmer, a complex-systems theorist affiliated with Los Alamos National Laboratory and the Santa Fe Institute, both in New Mexico. Chemists have known for decades that amino acids -- the monomers that link into proteins -- spontaneously form under laboratory simulations of the primordial soup. But these monomers fail to reliably link into long polymers. So a question emerges: How might a set of mutually dependent polymers have emerged from the primordial soup?

To probe this question, Farmer and co-workers model a primordial soup of monomers with a neural network computer program. The computer program feeds monomers into the model soup and "artificial chemistry" governs how they link into varying chains. The programs also bestows components of the soup with catalytic powers for either linking or breaking other components of the soup. As the program runs, different polymers become more abundant than others and certain sets of polymers develop so-called autocatalytic relationships. That means that each member of the set forms from one or more catalytic reactions involving other members of the set. In other words, sets of polymers develop the type of chemical interdependencies found inside cells.

"We would like to say that these [autocatalytic sets] have life-like properties," Farmer says. He and co-workers intend to use the simulations to study how autocatalytic sets evolve and how they might have provided a chemical scaffolding for life's emergence from the primordial soup.