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The
question of the origin of cells was illuminated by the research of the
American biochemist Sidney Walter Fox. It seemed to him that early Earth
must have been quite hot, and that the energy of heat alone could be
sufficient to form complex compounds out of simple ones. In 1958, to test
this theory, Fox heated a mixture of amino acids and found that they
formed long chains that resembled those in protein molecules. These proteinoids
were digested by enzymes that digested ordinary proteins, and could be
used as food by bacteria.
Most
startling of all, when Fox dissolved the proteinoids in hot water and let
the solution cool, he found they would cling together in little
microspheres about the size of small bacteria. These microspheres were not
alive by the usual standards but behaved as cells do, in some respects at
least (they are surrounded by a kind of membrane, for instance). By adding
certain chemicals to the solution, Fox could make the microspheres swell
or shrink, much as ordinary cells do. They can produce buds, which
sometimes seem to grow larger and then break off. Microspheres can
separate, divide in two, or cling together in chains.
Perhaps
in primordial times, such tiny, not-quite-living aggregates of materials
formed in several varieties. Some were particularly rich in DNA and were
very good at replicating, though only moderately successful at storing
energy. Other aggregates could handle energy well but replicated only
limpingly. Eventually, collections of such aggregates might have
cooperated, each supplying the deficiencies of the other, to form the
modern cell, which was much more efficient than any of its parts alone.
The modern cell still has the nucleus - rich in DNA but unable of itself
to handle oxygen - and numerous mitochondria - which handle oxygen with
remarkable efficiency but cannot reproduce in the absence of nuclei. That
mitochondria may once have been independent entities is indicated by the
fact that they still possess small quantities of DNA.
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