2 Dispute Popular Theory on Life Origins

Two scientists from UC San Diego say they have debunked the widely accepted idea that primitive life originated in a chemical soup in volcanic hot-water vents on the ocean floor.

The researchers have examined the chemical reactions the theory would require and concluded that it just was not possible for life-giving molecules to have been synthesized in the hot temperatures and high pressure of the vent areas.

Life could not have originated at hot-water vents on the ocean floor because temperatures there are too hot for basic organic molecules to form or, even if they did, to persist, biochemists Stanley Miller and Jeffrey L. Bada report today in the journal Nature.

In an experiment born out of frustration with the popularity of the vent hypothesis, Bada and Miller said they found that amino acids, peptides, nucleotides, sugars, proteins and other molecules basic to life disintegrate in seconds to minutes at the 350-degree Celsius temperatures common in the vents.

In an interview Wednesday, Bada said he and Miller hope that their experiment will disprove once and for all the vent hypothesis, which was first proposed in 1981 and has been gaining steam ever since.

“When this came out, people in the field who had thought about the origin of life realized that this was crazy, that this was the absolute worst place for the origin of life because of the hot temperatures,” Bada said. “But unfortunately people who didn’t know better jumped on the bandwagon, and before we knew it this thing started to appear everywhere.”

The experiment was triggered this last spring when Bada received a new edition of the textbook he uses for teaching a course on the oceans.

“I finally hit the roof when the undergraduate textbook that I use here had a whole three pages on the idea that life originated at these vents,” he said. “Needless to say, that textbook is no longer used in my class.”

Likewise, Miller said he was concerned about an offhand reference in a Nature article to life having originated at hydrothermal vents.

To conduct their experiment, Miller and Bada placed organic chemicals in a specially constructed chamber at Scripps Institution of Oceanography. The chamber allows the contents to be subjected to intense heat and pressure, but also allows scientists to monitor the chemicals over time.

First discovered in 1977 near the Galapagos Islands in the Eastern Pacific, the vents occur where seawater sinks through cracks in the ocean floor, comes into contact with hot basalt rocks and then rises back into the ocean. It has been estimated that this process recycles the entire volume of the world’s oceans every 10 million years.

Indirect evidence for the vent origin-of-life hypothesis came in 1983, when John A. Baross, now at the University of Washington, reported that he had found vent bacteria that thrived at temperatures of 250 degrees Celsius.

A. Aristides Yayanos at Scripps later failed to replicate those results and wrote a paper in Nature labeling Baross’ findings an experimental mistake.

Baross will embark on another expedition to study vent microbiology Aug. 27, using the deep-diving submersible Alvin to explore vents 180 miles off the Washington state coast. He would not comment Wednesday on the Miller-Bada paper, except to say there is “not a direct relationship” between the paper’s focus and the research trip.

Despite their experiment, the San Diego biochemists may have trouble curbing what they view as runaway enthusiasm for the vent hypothesis.

The results “would just imply that those particular chemicals couldn’t have taken place. It wouldn’t exclude life originating in the vents using different chemicals,” Robert Shapiro, a New York University professor who specializes in the origin of life, said of the San Diegans’ findings. “The key question on the origin of life is, ‘What were the actual chemicals involved when life originated?’ ”

But Gerald Joyce of the Salk Institute in San Diego said in a companion piece to the Miller-Bada article that their experiment demonstrates serious flaws in the popularly embraced vent hypothesis.

“The problems of conducting useful chemical syntheses in the high-temperature outflow tract (of a vent) seem to be insurmountable,” Joyce wrote.

Shapiro said some of the other competing theories on how life on Earth originated include that it began in clays; that essential organic molecules were brought to Earth by comets; that the most essential precursor to life was the first formation of fatty membranes, and that small systems of interacting chemicals were the key.

Miller, who in 1952 conducted the first experiment indicating that electrical charges in the atmosphere could have synthesized the first organic molecules, still favors a hypothesis that involves the atmosphere. He suggests that simple organic chemicals began there, were transferred to the oceans and then synthesized into larger molecules on hot, dry beaches.

No matter who is right, he and Bada say, the latest experiment has some interesting implications for those who are considering the problem.

“The whole ocean goes through those vents in 10 million years, so all the organic material that was synthesized would get cooked and destroyed in the vents,” Miller said.

So, whatever mechanism was creating the compounds must have been a very efficient one for life to have arisen, he said.