MOUNTAIN VIEW, CALIFORNIA--Salt water is a comfortable home for life today, but it was too harsh for the first cells. That's the surprising conclusion of new laboratory studies, which have found that primitive membranes assemble far more easily in fresh water. The research suggests that life arose in ponds on the earliest continents, rather than in tide pools or the deep sea.
Most scenarios of the origin of life require an enclosed membrane, or vesicle, to protect the first chemical chains capable of copying themselves. The simplest vesicles are made of amphiphiles: long molecules with a head that binds to water and an oily, carbon-rich tail that repels water. Two layers of amphiphiles can combine to form the walls of a vesicle; in the outer layer, the water-loving heads point outward and the inner layer has these heads facing toward the inside of the enclosure. The oily chains of both layers are linked inside the membrane. Such vesicles can arise spontaneously from ingredients in space--hydrocarbons such as fatty acids, contained in comets and meteorites that seeded the early Earth.
To investigate where those hydrocarbons might have coalesced into the first cells, a group led by graduate student Charles Apel of the University of California, Santa Cruz (UCSC), placed them into different solutions. Stable vesicles formed in solutions of water and a dash of alcohol. However, when the team added sodium chloride or ions of magnesium or calcium--at levels less than the saltiness of today's ocean--the membranes fell apart. "It seems possible to me to concede now that life did not have a marine origin," says biochemist David Deamer of UCSC, who reported the work here with Apel on 9 April at the Second Astrobiology Science Conference. The team's analysis also will appear in an upcoming issue of the journal Astrobiology.
"This is a wake-up call," says mineralogist Robert Hazen of the Carnegie Institution of Washington in Washington, D.C. "We've assumed that life formed in the ocean, but encapsulation in freshwater bodies on land appears more likely." Geologist L. Paul Knauth of Arizona State University in Tempe adds that Earth's early oceans were up to twice as salty as they are today--making it even more difficult for viable cells to arise. Giant salt deposits that formed on the continents made the seas less salty as time went on, Knauth says.