It's a chicken-and-egg problem that's concerned biologists for decades: how did the basic biochemicals of life appear before the biological catalysts needed to form them had come into existence?
But a US team believes it's found the answer - another type of catalyst which could have jumpstarted metabolism and life itself, deep in hydrothermal ocean vents.
According to the scientists' model - which is experimentally testable, they say - molecular structures involving transition metal elements such as iron, copper or nickel and small organic molecules known as ligands could have catalyzed the synthesis of basic biochemicals. These 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," says Harold Morowitz of George Mason University.
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."
Gradually, he says, the basic molecular ingredients of metabolism accumulated and were able to self-organize into networks of chemical reactions that laid the foundation for life.
Morowitz and his colleagues are preparing experiments to test the catalytic properties of transition metal-ligand complexes built with different types of ligands.
"We think life probably began with the reductive citric acid cycle, and there is evidence that under hydrothermal vent conditions some of the cycle's intermediates form," Morowitz says. "We are going to start with these molecules and mix them with various transition metals, cook them at different temperatures for a while, and see what kinds of catalysts we've made."
Such experiments could reveal what kinds of catalytic reactions took place to lay the foundations for life. The hypothesis also allows for the possibility that life could have arisen more than once.
"Life could have originated multiple times, and, if we find life elsewhere in the universe, it could be very similar to the life we know here because it will be based on the same transition metals and ligands," Morowitz says. "It's a conjecture at the moment, but it could become a formal scientific core for the emergence of life."