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LIFE may have begun not in the sea but in tiny water droplets drifting high in the sky. Thrown up by ocean waves, these droplets could have provided just the conditions needed for complex molecules to form.

This radical theory, proposed by an international team of researchers at the Royal Meteorological Society’s millennium conference in Cambridge this week, could explain long-standing mysteries about the origin of life, such as how cells got their membranes and how simple organic molecules became concentrated enough to join together to form large, complex ones.

The theory arose when Adrian Tuck of the National Oceanic and Atmospheric Administration in Boulder, Colorado, noticed the work of Daniel Murphy, also of NOAA. Murphy had discovered that instead of being just seawater, up to half the material in the droplets in today’s atmosphere is organic matter.

Tuck and his colleagues Veronica Vaida and Barney Ellison of the University of Colorado realised that the droplets, or aerosol particles, contain so much organic material because they pick up a lipid coating from the film of oily molecules on the surface of the ocean. “They look like protocells, with a layer of organic material on the outside,” he says.

While the droplets are floating in the upper atmosphere, they often fuse with other particles, which might contain substances such as iron and nickel derived from meteorites burning up in the atmosphere. “Aerosols in the stratosphere can last up to a year,” says Murphy. “They have lots of time to pick up different things.”

As the water in the droplets evaporates, the diverse substances within them become concentrated. This, combined with the energy provided by the strong sunlight, encourages chemical reactions. That could explain how the simple organic molecules on the primordial Earth came to form complex chemicals such as DNA and proteins.

“There isn’t really any question that the building blocks would be there,” says Chris Dobson of Oxford University, a protein chemist and another member of the team. “The question is how polymerisation came about.”

What’s more, when the droplets eventually fall back into the ocean, they can acquire another coating, ending up with a lipid bilayer just like the membrane around all living cells (see Diagram, below, and Inside Science, this issue).

“On re-entry, the aerosol with its monolayer of surfactants comes down to another part of the ocean and picks up a second layer that would be different,” says Tuck. “This is a characteristic of bacteria that has been hard to explain.”

The droplets are also the same size as bacterial cells, as only particles between 0.1 and 5 micrometres across make it to the upper atmosphere. If they are any smaller they fuse together; any larger and they fall back to the ocean.

The theory is “startling and fertile”, says Michael Russell of the Scottish Universities Research and Reactor Centre in Glasgow. But he still has reservations. “They have something that looks like a cell,” he says. “But it could be a coincidence.” He also points out that the organic material in today’s aerosols comes from dead organisms. “Before life emerged, would these organics have been around in the ocean?”

Tuck, however, suggests that the organic molecules in the primal sea could have become gradually more concentrated over tens of millions of years. He envisages building huge simulators to see what really happens inside the droplets.

“This is extremely imaginative and innovative work,” says biochemist Tom Cech, head of the Howard Hughes Institute in Maryland, although he points out that we can never know for certain how life began.

There’s one other way to test the theory. The size of aerosol particles depends on gravity and atmospheric pressure, so it’s possible to calculate how big they’d be on other planets. Tuck has already worked out that aerosol particles on Mars would be smaller than those on Earth. In fact, he says, they are more the size of the bacterium-like structures found in the Martian meteorite ALH84001. But he’s careful not to suggest that this means the structures are signs of life. “Let’s just say it’s a nice coincidence,” Tuck says.


Author: Joanna Marchant

New Scientist issue: 15 July 2000