Ponds, not oceans, the cradle of life

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 - the 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.

"When sodium chloride or ions of magnesium or calcium were added the membranes fell apart," Apel says. This happened in water that was less salty than the oceans are today.

Wake-up call

Geologist L. Paul Knauth of Arizona State University points out that Earth's 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.

"No one in their right mind would use hot seawater for laboratory studies of early cellular evolution," says biochemist David Deamer of the University of California, Santa Cruz, who is reporting the work with Apel. "Yet 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."

"This is a wake-up call," says mineralogist Robert Hazen at the Carnegie Institution of Washington. "We've assumed that life formed in the ocean, but encapsulation in freshwater bodies on land is appearing more likely."

The finding would not have surprised Charles Darwin. Over a century ago he speculated in his personal letters that the origin of life was "in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present".

Matt Kaplan

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British Psychological Society