The start of life on Earth presents a paradox, scientists say: How did amino acids arise before there were biological catalysts needed to build them?

It's a chicken-and-the-egg puzzle: How could the basic biochemicals like amino acids and nucleotides have come about when there were no catalysts, like proteins or ribosomes, around to create them?

Now scientists propose that a third type of catalyst could have jumpstarted metabolism and life itself, deep in hydrothermal ocean vents, an article in The Biological Bulletin says.

The scientists' theory says molecular structures involving transition metal elements -- iron, copper, nickel, etc. -- and ligands -- small organic molecules -- could have catalyzed the synthesis of basic biochemicals, monomers, that acted as building blocks for more complex molecules, leading ultimately to the origin of life.

"There has been a big problem in the origin of life (theory) for the last 50 years in that you need large protein molecules to be catalysts to make monomers, but you need monomers to make the catalysts," Harold Morowitz of George Mason University in Virginia says.

However, he suggests, "You can start out with these small metal-ligand catalysts, and they'll build up the monomers that can be used to make" the large protein catalysts.

Morowitz and his colleagues propose that simple transition metal-ligand complexes in hydrothermal ocean vents catalyzed reactions that gave rise to more complex molecules. These complex molecules then acted as ligands in increasingly efficient catalysts. Gradually, the basic molecular ingredients of metabolism accumulated and self-organized into chemical reactions laying the foundation for life.

"The idea has emerged from a study of the periodic table," Morowitz says. "We strongly feel that unless you're able to see how life comes about in some formal chemical way, you're never really going to solve the problem."