Heylighen, Francis. “Evolutionary transitions: how do levels of complexity emerge?.” COMPLEXITY-NEW YORK- 6, no. 1 (2000): 53-57.
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It is a common observation that complex systems have a nested or hierarchical structure:
they consist of subsystems, which themselves consist of subsystems, and so on, until
the simplest components we know, elementary particles. It is also generally accepted
that the simpler, smaller components appeared before the more complex, composite
systems. Thus, evolution tends to produce more complex systems, gradually
adding more levels to the hierarchy. For example, elementary particles evolved
subsequently into atoms, molecules, cells, multicellular organisms, and societies of
organisms. These discrete steps, characterized by the emergence of a higher level of
complexity, may be called “evolutionary transitions”. The logic behind this sequential
complexification appears obvious: you can only build a higher order system from
simpler systems after these building blocks have evolved themselves. The issue becomes
more complicated when you start looking for the precise mechanisms behind these
evolutionary transitions, and try to understand which levels have appeared at what
moment, and why.