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鈥檚 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鈥檚 atmosphere is organic matter.
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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. 鈥淭hey look like protocells, with a layer
of organic material on the outside,鈥� 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. 鈥淎erosols in the stratosphere can
last up to a year,鈥� says Murphy. 鈥淭hey have lots of time to pick up different
迟丑颈苍驳蝉.鈥�
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.
鈥淭here isn鈥檛 really any question that the building blocks would be there,鈥�
says Chris Dobson of Oxford University, a protein chemist and another member of
the team. 鈥淭he question is how polymerisation came about.鈥�
What鈥檚 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).
鈥淥n 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,鈥�
says Tuck. 鈥淭his is a characteristic of bacteria that has been hard to
别虫辫濒补颈苍.鈥�
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 鈥渟tartling and fertile鈥�, says Michael Russell of the Scottish
Universities Research and Reactor Centre in Glasgow. But he still has
reservations. 鈥淭hey have something that looks like a cell,鈥� he says. 鈥淏ut it
could be a coincidence.鈥� He also points out that the organic material in today鈥檚
aerosols comes from dead organisms. 鈥淏efore life emerged, would these organics
have been around in the ocean?鈥�
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.
鈥淭his is extremely imaginative and innovative work,鈥� 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鈥檚 one other way to test the theory. The size of aerosol particles
depends on gravity and atmospheric pressure, so it鈥檚 possible to calculate how
big they鈥檇 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鈥檚 careful not to suggest that this means the
structures are signs of life. 鈥淟et鈥檚 just say it鈥檚 a nice coincidence,鈥� Tuck
says.
