Light, time and micro-organisms
-9 > pts > -15
Gigaseconds to kiloteraseconds. The fifth time domain
The major transitions in evolution.
Living organisms evolve, and their evolution has clearly been accompanied, in many cases, by increasing complexity. It is nevertheless a simple fallacy to conclude that a more complex organism is in some sense more highly evolved, or "higher" than a simpler one. Parasites are usually simpler that the free-living species from which they evolved. Parasites aside, many people still cling, intuitively, a crude, anthropocentric idea that all other living things are failed attempts along the path to the eventual emergence of human beings. The terms "lower eukaryotes" and "higher eukaryotes" are nevertheless still in general currency. A cynical view is that these are just fancy ways of saying "yeast" and "myself", respectively. A good way of exposing the contradictions inherent in these terms is to ask their user where on the scale one should place an oak tree, or an octopus. One then often finds that the progressivist fallacy attaches great significance to such characteristics as being a chemoheterotroph, living on the land, and to having a nervous system or an immune response. An articulate cyanobacterium might regard each of these steps as retrograde, limiting future evolutionary possibilities to narrow and highly specialised physical environments. Microbiologists are mostly free of progressivist, anthropocentric tendencies, and will recognise that their favourite species, far from being "primitive", has probably had longer to evolve in something like its present form. One might then wonder why vertebrates, for example, are thought to be more highly evolved than pseudomonads or purple photosynthetic bacteria. The latter, in particular, have retained the ability to do almost anything.
Maynard Smith and Szathmary (1995) have considered the major transitions in evolution, and propose a solution to the apparent paradox of increasing complexity without there being any rational basis for the idea of evolutionary progress. The solution is the existence of transitions in levels of organisation, particularly where these involve information transfer. Thus the largest step, second to the origin of life itself, was probably the separation of information coding from its translation - the emergence of the specialised roles of DNA, RNA and protein from the "RNA world" in which chemically similar macromolecules performed both catalysis and replication. The general theme of such major transitions seems to be division of labour, in which each new level of organisation incorporates specialised components which, on their own, become less versatile, but which, acting in a way that complements other specialised components, create new possibilities for the environments that can successfully be exploited by the whole.
|John Maynard Smith and Eors Szathmary: The major transitions in evolution (W. H. Freeman)|
|Richard Dawkins: The blind watchmaker (Norton)|