Complexity of Life
Critical steps in forming the complexity of modern life
Diagram from Farnsworth et al. 2012; Creatures drawn by John Nelson.
This diagram above is used to explain a sequence of steps needed to build up the complexity of life on earth as we find it today. It is important to realise, though, that the kind of life produced at each stage is not replaced by the previous stage, but coexists with it (typically outnumbering it in terms of individuals). Proto-cells are the only exception to this: it seems they have been thoroughly out-competed by the far more efficient true-cells of life as we know it. As well as that, some of the attributes of ecological networks have existed as long as the true cells - especially competition and probably 'conveyors' of material processing, whereby the products of one cell-type become the resources for another. Indeed, such conveyers are likely to be an important pre-cursor to the forming of colonies and eventual development of multi-cellular organisms. Similar diagrams are sometimes used to illustrate major developments in evolution, but the intention here is rather different. The diagram catalogues major steps in emergence: the information-dependent property of complex systems that creates new layers of organisation.
For example, as the first true cells diversified in ecological function (for example the metabolites they produced), networks of interaction among them gave each context for the other: an emergence of functional information creating a new level of organisation. To this, a cell signalling network may have been added and these networks made colonies of co-operating cells possible. The colony was thus 'born' as a new, higher, level of organisation, having its own properties and functions. The formation of Eukaryotic cells represents a very special case of this process, as bacteria joined in a single cell to become obligatory symbiotes (the best known example being that of the mitochondria).