THE cherished assumption that life emerged in the oceans has been thrown into doubt. New research shows that primitive cellular membranes assemble more easily in freshwater than in salt water. So although the oldest known fossil organisms were ocean dwellers, life may actually have developed in freshwater ponds.
Most theories on the origin of cellular life presume that the first step was the formation of a spherical membrane called a vesicle that could enclose self-replicating chemical chains鈥攖he ancestors of modern DNA. The idea is that the ingredients for simple membranes were all present on the early Earth, and at some point formed vesicles spontaneously in water. It seemed most likely that this took place in the sea rather than freshwater, largely because of the sheer size of the oceans. With their unique chemistry, deep-sea thermal vents and tidal pools are generally believed to be the most likely sites.
Now research by graduate student Charles Apel of the University of California, Santa Cruz, suggests that this is wrong. Apel and his colleagues were able to create stable vesicles using freshwater solutions of ingredients found on the early Earth, but not salty solutions, they will report in a future issue of Astrobiology. 鈥淲hen sodium chloride or ions of magnesium or calcium were added the membranes fell apart,鈥� Apel says. This happened in water that was less salty than the oceans are today.
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Geologist L. Paul Knauth of Arizona State University points out that Earth鈥檚 early oceans were 1.5 to 2 times as salty as they are today, making it even more unlikely that viable cells could have arisen there. Giant salt deposits called evaporites that formed on the continents have actually made the seas less salty over time.
鈥淣o one in their right mind would use hot seawater for laboratory studies of early cellular evolution,鈥� says biochemist David Deamer of the University of California, Santa Cruz, who is reporting the work with Apel. 鈥淵et for years we have all accepted without question that life began in a marine environment. We were just the first to ask if we were really sure of that.鈥�
鈥淭his is a wake-up call,鈥� says mineralogist Robert Hazen at the Carnegie Institution of Washington. 鈥淲e鈥檝e assumed that life formed in the ocean, but encapsulation in freshwater bodies on land is appearing more likely.鈥�
The finding wouldn鈥檛 have surprised Darwin. Over a century ago he speculated in his personal letters that the origin of life was 鈥渋n some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present鈥�.