Geoscience Reference
In-Depth Information
Perhaps its most important quality was an easier mechanism of
assembly that was not possible in such complex structures as RNA.
Once formed, proto-RNA could undergo Darwinian evolution by
coding information that could be passed on to the next generation.
That information was selected for stability and versatility, and could
ultimately lead to the RNA that is found in the cells of all living
things.
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Whichever of these routes life took—via proto-RNA or by
the autocatalytic route, or through some complex interaction of the
two—one thing is certain: these processes must have occurred, and
so life was born, in water.
Bags of Water
To a silicon-based life form, all life on planet Earth might look like a
bag of water. In the Star Trek episode 'Home Soil', Captain Picard and
the crew of the starship
Enterprise
are accused of possessing just such
a physique by a crystal-based life form on the planet Velara III. How
much water there is in those bags depends on the organism, but in
humans about 60 per cent of the body by weight is water. That figure
hides greater complexity, because of that 60 per cent some two-thirds
is within the body's cells, and about one-third is extracellular in fluids
such as blood and cerebrospinal fluid. Even between different types of
cells in one body, and between different organisms, the amount of
water varies considerably,
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so that a human red blood cell is about 64
per cent water, whereas there can be much less water in cells of organ-
isms adapted to arid environments. One such desiccation-resistant
organism is the colloquially known 'sea monkey', a small shrimp
(
Artemia
) that is sold desiccated in kit-form to children, the shrimps
rapidly reanimating when placed in water. Nonetheless, all cells—
including those of desiccated sea monkeys—need some water.
Water is an essential medium in which the chemical reactions of
life take place. Water dissolves oxygen, salts, and sugars that provide
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