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Symbiosis -- the merger of two organisms into one -- was once thought to
occur only in isolated curiosities like lichens. After Lynn Margulis
postulated bacterial symbiosis as a central event in the formation of
the ancestral cell, biologists found symbiosis popping up frequently in
microbial life. Since microbial life is (and has always been) the bulk
of all life on Earth, and the primary Gaian workhorse, widespread
microbial symbiosis makes symbiosis fundamental, both in the past and in
the present.
In contrast to the traditional picture of a population seething with
tiny, random, incremental changes in their routine until they hit upon a
stable new configuration, Margulis would have us consider the accidental
merging of two working simple systems into one larger, more complex
system. As illustration, a proven system for oxygen transport inherited
by one cell line might be married to an existing system for air exchange
in another cell line. Combined in symbiosis, the two might form a
respiratory system unlikely to develop incrementally.
For a historical example, Margulis suggests her own studies on the
symbiotic nature of nucleated cells. These emerging cells did not have
to reinvent by trial and error over a billion years the clever processes
of photosynthesis and respiration worked out by several types of
bacteria. Instead, the membraned cells incorporated the bacteria and
their informational assets as wholly owned subsidiaries working for the
cells. They kidnapped the innovations.
In some cases the genetic strands of two symbiotic partners may fuse.
One proposed mechanism for the informational coordination needed for
this kind of symbiosis is the known intercell gene transfer, which
happens at a terrific rate among bacteria in the wild. The know-how of
one system can be shuttled back and forth between separate species. A
new bacteriology views all the bacteria of the world as a single
genetically interacting superorganism that rapidly absorbs and
broadcasts genetic innovations among its members. Interspecies gene
transfer also occurs (at an unknown rate) among more complex species,
including humans. Species of every sort are constantly swapping genes,
often with naked viruses as the messengers. Viruses themselves are
sometimes taken in symbiotically. A number of biologists believe that
large chunks of human DNA were inserted viruses. A few even think that
it's a loop -- that many human disease viruses are escaped hunks of human
DNA.
If true, the symbiotic nature of a cell provides a couple of lessons.
First, it gives an example of a significant evolutionary change that
lessens immediate benefits to the individual (since the individual
disappears), in contradiction to classical Darwinian dogma. Second, it
gives an example of evolutionary change that is not amassed by slight
incremental differences, also in contradiction to Darwinian dogma.
Routine symbiosis on a large scale could drive many of the complexities
in nature that seem to require multiple simultaneous innovations. It
would provide evolution with several other advantages; for instance, it
would exploit the power of cooperation, rather than competition,
exclusively. At the very least, cooperation nurtures a distinct set of
niches and a type of diversity that competition cannot produce -- such as
lichens. In other words, it unleashes another dimension in evolution by
enlarging its library of forms. Also, a small amount of symbiotic
coordination at the right time could replace an eon of minor
alterations. In one mutual relationship, evolution could jump past a
million years of individual trial and error.
Perhaps evolution could have discovered nucleated cells directly,
without symbiosis, but it might have taken another billion years, or
five, to do so. Lastly, symbiosis recombines widely diverse know-how
separated in life's divergent genealogy. The picture to keep in mind is
the diagrammatic tree of life, with ever dividing, ever spreading
branches. Symbiotic alliances, on the other hand, bring divergent
branches of the tree of life together again, to intersect. Evolution,
charted with symbiosis included, may resemble a briar patch more than a
tree -- the Thicket of Life. If the Thicket of Life is sufficiently
tangled, it may require a rethinking of our past and future.
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