Life's First Scalding Steps

Science News, Jan 9, 1999 by Sarah Simpson

Biology might have begun in cauldrons at the bottom of the sea

In a back room at the Carnegie Institution of Washington (D.C.), Jay A. Brandes packs water and powdered rock into a 24-carat-gold capsule not much bigger than a daily vitamin. He injects gases like those spewed by a volcano and then seals the receptacle. Finally, he places the shiny amulet inside "the bomb," a device isolated from the rest of the world by steel panels salvaged from a scrapped battleship. Within, the mixture is besieged with scorching temperatures and bone-crushing pressures, forces like those at work at seafloor geysers. Brandes and his colleagues then look inside the tortured capsule for evidence of the chemical steps that sparked the beginnings of life.

Using no more than the cocktail of chemicals discharged at undersea hydrothermal vents, the researchers are trying to mimic reactions that occur within living cells.

Oceanographer Jack Corliss first discovered these vents in 1977, while exploring a volcanic ridge at the bottom of the Pacific Ocean. Before then, oceanographers had considered the ocean floor cold and mostly barren. Corliss' view outside the tiny research submersible revealed a different world.

Peering out a porthole, Corliss became the first person to witness the biological wonderland of shoe-sized clams, 6-foot tube worms, and blizzards of strange microbes thriving at the vents, which spew out a hellish mix of shimmering brines. If these creatures can subsist in a bath of scorching chemicals and heat from the planet's interior, he reasoned, then perhaps this is where life got its start.

Corliss' proposal that life sprung from water, gas, and rocks far out of the sun's reach inspired grandiose theories but remained virtually untested for 20 years. Most origins-of-life researchers were still busy pondering the long-held notion that life's precursor chemicals linked up at the surface of a sun-drenched pond or ocean. In this more traditional scenario, the sun simmered a prebiotic soup for millions of years to cook up the first cellular organisms.

Only in the past few years have scientists such as those at the Carnegie Institution begun to roll up their shirtsleeves and get down to the business of determining what biochemical reactions are possible at hydrothermal vents. In a series of recent experiments, researchers have found that the harsh vent conditions can foster some of the chemical steps thought necessary for early life. Their results are capturing the attention of a growing group of scientists--and raising belief in the chance of finding life elsewhere in the universe.

The most detailed step-by-step blueprint for how Earth's oldest raw materials could have given rise to the stuff of life came out of the imagination of Gunter Wachtershauser, an organic chemist at the University of Regensberg in Germany. Ten years ago, Wachtershauser conceived of an assembly-line process at the ocean floor that transforms basic inorganic chemicals into organic chains, the biological molecules that are the building blocks of life.

Wachtershauser's factory enlists the elements of modern industry--all readily available at vents. The conveyor belt is the flat surface of metal sulfide minerals, such as iron pyrite, abundant in seafloor rocks. The raw materials are carbon- and hydrogen-rich gases from volcanic belches dissolved in the seawater. The workers that drive the assembly line--the keys to the whole process--are metallic ions in the sulfides.

In living cells, complex proteins called enzymes play the role of factory laborers, bringing certain molecules together and splitting others apart. Before enzymes appeared on the planet, Wachtershauser says that metallic ions filled that catalytic role. Without these mediators, reactions might take months or years, or never happen at all, he adds. New components would never get added to the molecules passing by on the conveyor.

In Wachtershauser's theory, the first organic molecule put together on the conveyor belt was acetic acid, a simple combination of carbon, hydrogen, and oxygen that is best known for giving vinegar its pungent odor. Formation of acetic acid is a primary step in metabolism, the series of chemical reactions that provides the energy that cells use to manufacture all the biological ingredients an organism needs.

According to the theory, metabolism came before all else. Once a primitive metabolism evolved, it began to run on its own, and only later were cells' other basic elements, such as a genetic code, invented.

Wachtershauser focuses on the heart of modern metabolism, the citric acid cycle. All living cells use this series of reactions to extract energy from food. The cycle makes changes in several chemical compounds, but it always begins with acetic acid. Inside a cell, the two carbon atoms in each acetic acid molecule are eventually expelled as carbon dioxide in a reaction that gives off a packet of energy.

Because the citric acid cycle is intrinsic to all modern life, Wachtershauser guesses that its basic reactions are close to the chemistry with which life began--with one significant variation. In the oxygen-deficient world at hydrothermal vents, heat-loving bacteria operate the cycle backward (SN: 3/29/97, p. 192). Instead of giving off carbon dioxide to make energy, they incorporate carbon atoms to build a succession of more complex organic molecules. Wachtershauser says life's first chemicals were built the same way.