Vents would scald a primordial soup.

Vents would scald a primordial soup

Experiments testing how quickly organic molecules decompose at high temperatures are quenching a popular idea among earth scientists that life may have originated in vents of superheated water on the ocean floor.

"This is probably the most unlikely area for the origin of life to occur," says Jeffrey L. Bada from the University of California, San Diego, who reports on the experiments along with colleage Stanley L. Miller in the Aug. 18 NATURE.

In submersible dives during the late 1970s, researchers discovered vents near ocean ridge crests that spew out jets of water at temperatures greater than 350 [deg.]C. The heat comes from chambers of molten rock below the ocean floor. Some oceanographers have suggested that in the oceans of the early Earth, heat from similar hydrothermal vents might have driven the chemical reactions that converted simple molecules into the first living organisms.

The reported discovery of bacteria cultured from 250[deg.]C vent water bolstered the idea that organisms can survive at such high temperatures; however, other researchers found that experimental error produced the appearance of bacterial growth and that no organisms live directly in the hottest vent water.

Biologists and chemists have objected to the vent theory for the origin of life because organic molecules are notoriously unstable at high temperatures, says Miller. Proponents of the vent theory have countered that high pressures at the ocean bottom may protect organic compounds from decomposition.

To test that argument, Miller and Bada measured how amino acids--the building blocks of protein subunits called peptides -- survived at 350[deg.]C and 265 atmospheres, which is within the range of the 200 to 400 atmospheres of pressure at the ocean floor vents. The researchers found the amino acids decomposed quite rapidly, some within seconds. "The amoung of stability you gain from [high pressure] is absolutely insignificant," says Bada. He adds that even if amino acids could survive in the vent outflow, they could not have joined together spontaneously to form peptides because this reaction requires dry conditions.

In an accompanying editorial Gerald Joyce of the Salk Institute for Biological Studies in La Jolla, Calif., says, "The problems of conducting useful chemical synthesis in the high-temperature outflow tract seem to be insurmountable."

Where, then, did life start? Miller speculates that some synthesis of important molecules probably happened in the atmosphere and then in lagoons, lakes or cool parts of the ocean.