A once-discarded idea about how life started on our planet has been given a new life of its own, thanks to a serendipitous find.
The story traces back to the early 1950s, when chemists Stanley Miller and Harold Urey of the University of Chicago in Illinois tried to recreate the building blocks of life under conditions they thought resembled those on the young Earth. The duo filled a closed loop of glass chambers and tubes with water and different mixes of hydrogen, ammonia, and methane--gases presumed at the time to be the main constituents of the atmosphere billions of years ago. Then, in an attempt to confirm a hypothesis that lightning may have triggered the origin of life, they zapped the mixture with an electrical current. The researchers then analyzed the gunk that began to collect after a few hours.
The residue contained traces of some of the amino acids that make up proteins. Their presence suggested that the molecular precursors of life could form through a simple electrochemical process. The problem was that theoretical models and analyses of ancient rocks eventually convinced scientists that Earth's earliest atmosphere was not rich in hydrogen.
Last year, after Miller's death, two of his former graduate students--geochemists Jim Cleaves of the Carnegie Institution of Washington (CIW) in Washington, D.C., and Jeffrey Bada of Indiana University, Bloomington--were examining samples left in their mentor's lab. They discovered the vials of products from the original experiment and decided to take a second look with updated technology. Using extremely sensitive mass spectrometers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, Cleaves, Bada, and colleagues found traces of 22 amino acids in the experimental residues. That is about double the number originally reported by Miller and Urey and includes all of the 20 amino acids found in living things, the scientists report tomorrow in Science.
So could lightning have helped jump-start life on Earth? Possibly, Cleaves says. Although Earth's primordial atmosphere was not hydrogen-rich, as were the chambers in the Miller-Urey experiment, gas clouds from volcanic eruptions did contain the right combination of molecules. It is possible that volcanoes, which were much more active early in Earth's history, seeded our planet with life's ingredients. The big question is what happened next--how did those molecules turn into self-replicating organic compounds? "That's the frontier," Cleaves says, "and we're sort of stuck there."
The new study "highlights how easy it is to make the building blocks of life in plausible prebiotic conditions," says geochemist Robert Hazen of CIW, who was not involved in the research. At the same time, he says, the findings reinforce "the pioneering insight and experiments of Stanley Miller and Harold Urey."