Biology Reference
In-Depth Information
The beginning of a process of evolution based on the transmission of modular
templates probably inaugurated a stage of both vertical and horizontal exchanges
of hereditary components (as suggested by Woese
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). As a consequence, indi-
vidual organisms crucially depend on the (long-term) selection of the functional
hereditary components; and in turn this global process of selection of func-
tional hereditary components from a random process of variation will depend
on the performances of the individual organisms they instruct. From now on
the maintenance of the autonomous systems will depend on the maintenance
of this global historical-collective structure of relations. Furthermore, even the
structure of the autonomous systems will progressively depend on these larger
and wider relations that we know with the term “biological evolution” (recall
Dobzhansky famous dictum: “nothing in biology makes sense except in the light
of evolution”).
5.2. The origin of an informational organization
Hereditary autonomous systems must have been the immediate precursors of
present-day living organization. As has been mentioned in the previous section,
the organization of HAS was likely based on one single type of polymer to sup-
port at the same time template and catalytic functions.
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Now, this organization
cannot yield to an unlimited increase in complexity, and therefore the evolu-
tionary possibilities of HAS are blocked. The reason is the following: there is
a trade-off between the realization of catalytic and storage/replicative functions;
the better a given type of polymer is suited for template tasks, the worse it is
for exploring the catalytic space, and vice versa.
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Accordingly, the only way
for an unlimited increase in complexity is by introducing two different types
of polymers, devoted, respectively, to template and catalytic tasks (Ruiz-Mirazo
et al., 2004). This way, the systems that start to produce two different - and
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Such modular organization should have helped the exploration of new, more efficient forms of organization.
As Woese (2002) has pointed out, the beginning of cellular evolution was necessarily a collective process,
where different cellular designs evolved simultaneously, systematically exchanging genetic material (what
he calls 'horizontal gene transfer'). So, this early (pre) Darwinian evolution would allow an exploration of
different forms of organization, until a 'modern design' was reached.
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The current view of the origin of life postulates a stage of prebiotic systems based on certain type of
bi-functional polymers (like RNAs) capable of performing both template and catalytic functions, although in
a much less suitable way than DNA and proteins. Hence, despite its evident limitation in the exploitation of
both template and catalytic functions, this solution is organizationally much more simple (because it allows
the direct conversion of a specific sequence into a specific catalytic task) and therefore more likely to have
occurred. This is in fact the hypothesis of the so-called RNA world.
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This problem has a rather simple chemical interpretation. Template activity requires a stable, uniform
morphology, suitable to be linearly copied (i.e., a monotonous spatial arrangement that favors low reactivity
and is not altered by sequence changes), whereas catalytic diversity requires precisely the opposite: a very
wide range of 3D shapes (configuration of catalytic sites), which are highly sensitive to variations in the
sequence (Moreno & Fernández, 1990, Benner, 1999).
