Biology Reference
In-Depth Information
Symbiosis provides a good example of all three aspects of the process, and
perhaps especially of the ways in which the net effect is to bring into being
entirely new kinds of entities.
Such an account would seem to be especially pertinent to the evolution
of cellularity: biological cells are replete with devices for ensuring survival,
stability, robustness. Consider, for example., the devices that have arisen to
regulate cell division, ensuring that cell division is not triggered too early (when
the cell is too small) or does not wait too long (when the cell has gotten
too big). Or consider the vastly complex kinds of machinery for guaranteeing
fidelity in DNA replication, the accuracy of translation, or the proper folding
of proteins. Each of these mechanisms, taken by itself, can be considered to
embody a function. Much as the function of a thermostat is to maintain the
temperature of a room, so too the function of a proofreading mechanism in DNA
replication is to maintain the identity of the nucleic acid molecule. The complex
of DNA cum proofreading device constitutes a composite with significantly
greater stability (or persistence) than either would have by itself. Furthermore,
each of these mechanisms has a long evolutionary history, much, if not most, of
which preceded the existence of the modern cell. Originally appearing as simple
devices, arising out of the fortuitous combination of already existing molecular
complexes, it is not hard to imagine their subsequent refinement, elaboration,
and integration into every more complex structure, all by virtue of the enhanced
stability they would provide. As Simon wrote, 'this is nothing more than survival
of the fittest - that is, of the stable.'
Inevitably, different mechanisms for ensuring robustness marked off differ-
ent evolutionary epochs. Nucleic acid molecules, for example, appearing on the
scene long before the advent of anything like a primitive cell, introduced a
significant advance over mechanisms of autocatalysis for making more because
it made possible the replication of molecules with arbitrary sequences. The
subsequent arrival of a translation mechanism between nucleic acid sequences
and peptide chains required the combination of already existing nucleic acid
molecules and already existing protein structures, but the innovation of a trans-
lation mechanism - in effect, the advent of genes - ushered in an entirely new
order of evolutionary dynamics. During the subsequent period, the few hundred
million years over which cellularity evolved, change seems to have depended
primarily on the horizontal flow of genetic bits between porous entities not
yet sufficiently sealed off to qualify as candidates for natural selection. Carl
Woese argues that cellular evolution, precisely because it needed so much com-
ponentry, 'can occur only in a context wherein a variety of other cell designs
are simultaneously evolving [and] globally disseminated'. He writes, 'The
componentry of primitive cells needs to be cosmopolitan in nature, for only by
passing through a number of diverse cellular environments can it be significantly
