Francois Jacob

from Tinkering and Evolution [note 1]

Nature functions by integration. Whatever the level, the objects analyzed by natural sciences are always organizations, or systems. Each system at a given level uses as ingredients some systems of the simpler level, but some only. The hierarchy of complexity of objects is thus accompanied by a series of restrictions and limitations. At each level, new properties may appear which impose new constraints on the system. But these are merely additional constraints. Those that operate at any given level are still valid at the more complex level. Every proposition that is true for physics is also true for chemistry, biology, or sociology. Similarly, every proposition that is valid for biology holds true for sociology. But as a general rule, the statements of greatest importance at one level are of no interest at the more complex ones. The law of perfect gases is no less true for the objects of biology or sociology than for those of physics. It is simply irrelevant in the context of the problems with which biologists, and even more so sociologists, are concerned.

This hierarchy of successive integrations, characterized by restrictions and by the appearance of new properties at each level, has several consequences…. (One) concerns the nature of the restrictions and limitations found at every step of increasing complexity. Can one explain why, among all the possible interactions at one level, only certain are observed at the more complex one ?… There is no general answer to such questions, and it seems doubtful that there will ever be a specific answer for any one particular level of complexity. Complex objects are produced by evolutionary processes in which two factors are paramount: the constraints at every level control the systems involved, and the historical circumstances that control the actual interactions between the systems. The combination of constraints and history exists at every level, although in different proportions. Simpler objects are more dependent on constraints than on history. As complexity increases, history plays a greater part.

How do constraints and history interact ? Jacob develops this theme by applying to the question the Levi-Strauss metaphor of bricolage (for which word he, a Frenchman, prefers the English “tinkering”), in his vision of evolution.

This mode of operation has several aspects in common with the process of evolution. Often, without any well-defined long-term project, the tinkerer (sic) gives his materials unexpected functions to produce a new object…. Similarly evolution makes a wing from a leg or a part of an ear from a piece of jaw. Naturally, this takes a long time. Evolution behaves like a tinkerer who, during eons upon eons, would slowly modify his work, unceasingly retouching it, cutting here, lengthening there, seizing the opportunities to adapt it progressively to its new use…. The lung of vertebrates was formed in the following way. Its development started in certain fresh water fishes living in stagnant pools with insufficient oxygen. They adopted the habit of swallowing air and absorbing oxygen through the walls of the esophagus. Under these conditions, enlargment of the surface area of the esophagus provided a selective advantage. Diverticula of the esophagus appeared and, under continuous selective pressure, enlarged into lungs…. To make a lung with a piece of esophagus sounds very much like tinkering.

Unlike engineers, tinkerers who tackle the same problem are likely to end up with different solutions. This also applies to evolution, as exemplified by the variety of eyes found in the living world. It is obvious a great advantage under many conditions to possess light receptors, and the variety of photoreceptors in the living world is amazing….Eyes appeared a great many times in the course of evolution, based on at least three principles — pinhole, lens, and multiple tubes. Lens eyes, like ours, appeared both in mollusks and vertebrates. Nothing looks so much like our eye as the octopus eye. Both work in almost exactly the same way. Yet they did not evolve in the same way. Whereas in vertebrates photoreceptor cells of the retina point away from light, in mollusks they point toward light. Among all solutions found to the problem of photoreceptors, these two are similar but not identical. In each case, natural selection did what it could with the materials at its disposal.

At The Molecular Level
It is at the molecular level that the tinkering aspect of natural selection is perhaps most apparent…. It is not biochemical novelties that generated diversification of organisms…. What distinguishes a butterfly from a lion, a hen from a fly, or a worm from a whale, is much less a difference of chemical constituents than in the organization and the distribution of these constituents…. The genetic program is executed through complex regulatory circuits that switch the different biochemical activities of the organism on or off. Very little is known as yet about the regulatory circuits that operate in the development of complex organisms. It is known, however, that among related organisms such as mammals, the first steps of embryonic development are remarkably similar, with divergences showing up only progressively as development proceeds. These divergences concern much less the actual structure of cellular or molecular types than their number and position…. Small changes modifying the distribution in time and space of the same structures are sufficient to affect deeply the form, the functioning, and the behavior of the final product — the adult animal. It is always a matter of using the same elements, and of adjusting them, of altering here or there, of arranging various combinations to produce new objects on increasing complexity. It is always a matter of tinkering.

A Final Example: The Human Brain
Although our brain represents the main adaptive feature of our species, what it is adapted to is not clear at all. What is clear, however, is that, like the rest of our body, our brain is a product of natural selection, that is of differential reproductions accumulated over millions of years under the pressure of various environmental conditions…. But curiously enough, brain development in mammals was not an integrated a process as, for example, the transformation of a leg into a wing. The human brain was formed by the superposition of new structures on old ones….. The formation of a dominating neocortex coupled with the persistence of a nervous and hormonal system partially, but not totally, under the rule of the neocortex — strongly resembles the tinkerer’s procedure. It is somewhat like adding a jet engine to an old horse cart. It is not surprising, in either case, that accidents, difficulties, and conflicts can occur.

It is hard to realize that the living world as we know it is just one among many possibilities; that its actual structure results from the history of the earth. Yet living organisms are historical structures: literally, creations of history. They represent, not a perfect product of engineering, but a patchwork of odd sets pieced together when and where opportunities arose. For the opportunism of natural selection is not simply a matter of indifference to the structure and operation of its products. It represents the nature of a historical process full of contingency.

The interplay of local opportunities — physical, ecological, and constitutional — produces a net historical opportunity which in turn determines how genetic opportunities will be exploited. It is this net historical opportunity that mainly controls the direction and pace of adaptive evolution.

Jacob’s use of the Levi-Strauss metaphor, presented crisply with a profound application, is primary evidence of the fecundity of the point of view. If we ask more precisely what are the interrelation of constraints and history in human cognitive development, we must look to a kind and level of exploration represented by my attempt in the four analytical chapters of this work.

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