Blute Blog

The primary structure of a protein molecule is the sequence of amino acids in its chain and its secondary structure is the three-dimensional structure into which the former subsequently folds. On March 10, Nature (here) published a news feature titled “Breaking the protein rules” about the fact that many proteins, in part or in whole, do not assume a unique and fixed three-dimensional secondary structure. The terminology which seems to be emerging among physical chemists/structural biologists to describe the various phenomena involved include “intrinsically disordered”, “flexibility”, “multi-structural states” and “dynamic equilibrium”. Evolutionists too should be very much interested in these phenomena but would likely use different terms – e.g. roughly (secondary) phenotypic plasticity, adaptive plasticity, condition-dependent adaptive plasticity and development respectively.

That single protein molecules can possess such physiological/developmental/behavioural plastic properties (whatever one prefers to call them) normally associated with organisms is nothing less than astonishing. It has implications among other things it seems to me for understanding the origin and early evolution of life. In particular (leaving aside the question of membranes), it makes a protein-first rather than an RNA-first origin of life much more likely.

Early evolution by natural selection could have been based on something in between the very simple morphological (viability selection based on different static structures) and the very complex replicative (which includes heredity and an increase in numbers). In between is a simpler form of ‘repetition’ which could be called “competitive development” (see “The evolution of replication” here gated). The minimal conditions for such a type of evolution is a population of varying individuals, created by the direct action of the physio-chemical environment, in which two complementary alternative states each constructs the conditions that induce and favour the other. For example, polymers which were induced to grow when monomers were plentiful and to assume a more stable configuration when they were scarce would be favoured by selection over those which only grew, only maintained, or attempted to do both but under the reverse conditions because they would repeat their life cycle. Many other possibilities exist beyond growth versus maintenance e.g. growth versus motility with growth at one end and loss at the other resulting in the “tread-milling” form of motility utilized today by cytoskeltal elements for example.

Such a form of evolution could even result in increases in complexity because as many new “births” and deaths took place through time, the range of variation would increase. In any event, the increasing evidence for many “intrinsically disordered” i.e. plastic proteins deserve attention from evolutionists, including those interested in the origin and early evolution of life.