JTW's Evolutionary Origins - Author: Wood, John T.

The Importance of Historical Contingency in Biological Evolution

The Biological sciences deal with complex systems whose evolutionary solution space is vast and largely unexplored.

While the mechanisms driving the evolution of organismal structures follow deterministic laws, the resulting structures are the product of a historical sequence of events, or contingencies, in which environmental context and spatiotemporal contingencies determine the outcome of specific mechanistic events.

Evolutionary outcomes are path dependent and usually irreversible.

"Contingency is the affirmation of control by immediate events over destiny..."
(Stephen Jay Gould, 1989, p. 284)

"Charles Darwin recognized this central distinction between laws in the background and contingency in the details..."
(Stephen Jay Gould, 1989, p.290)

"Gould appropriately distinguishes between that is referred to as contingency and the notion of chance. Pure chance precludes any explanation of particulars, but contingency, while denying that predictions can be made with confidence at the outset, does assert the possibility of explanation after a particular history has unfolded. Contingency represents the historian's mode of knowability; pure chance denies that particulars can be explicated at all (Gould, 1993). However, chance, far from being the same as indeterminacy, obeys a type of lawfulness or determination; the laws of chance."
(Dubrovsky, 2002, pp. 2)

Many examples of the importance of historical contingency in understanding the evolution of complex organisms can be found throughout biology; two well known examples, for instance , are the endosymbiotic origins of eukaryotes through a multi-prokaryotic merger and, the emergence of modern mammals after the K-T extinction event.

The K-T Extinction Event

The emergence of mammals as dominant terrestrial vertebrates is a case in point, providing an excellent example of the importance of contingency in understanding the details of a specific evolutionary outcome.

The emergence of mammals after the K-T boundary is the culmination of a remarkable series of historical contingencies, that while entirely deterministic in nature, would have been quite unpredictable from first principles due to the low probability of predicting the exact historical sequence neccessary to bring it about.

Theraspid mammals split off the main amniote/reptilian line early on.

Initially theraspids and reptiles paralleled each other in overall body size trend and occupied similiar scale ecological niches.

As the phylogenetic line leading to large dinosaurs emerged, this parallel in body size trend came to an end.

The larger, more powerful dinosaurs came to dominate the large scale eco-space, forcing the miniturization of the early mammalian lineages thru selective predation of the larger theraspid mammals.

Theraspid mammals that took refuge in subterrean and arboreal econiches became subject to a new selection pressure for smaller body size.

This has several consequences of importance in understanding the architecture of mammals vis a vis other vertebrates.

First, this process initiates the great age of the dinosaurs, some 250 million years of earthly dominance.

Secondly, the forcing of early theraspid mammals into the small ecospaces, with it's concommitant miniturization of the body plan, results in the emergence of a tighter, denser, more integrated nervous system structure compared to that of other vertebrates.

All of this would have come to naught, had not a unsuspected and seemingly unpredictable event occured.

In 1980, Walter and Luis Alvarez and their collaborators discovered/postulated something spectacular.

About 65 million years ago, a massive 10 km wide impactor struck off the Yucatan Penninsula (Chicxulub Impactor Crater), effectively eliminating about 70 percent of the contempary life forms thereby resulting in the extinction of the remaining dinosaurs (except of course their most abundant descendents, the birds).

In the aftermath of the K-T event, a new evolutionary radiation occurred amongst the survivors of the disaster.

With the large eco-spaces available once again, the selective pressures for small mammalian morphologies was released and mammals underwent an overall increase in body size trend as they diversified into the modern mammalian orders.

In the process of doing so, they retained the advantageous features that they had acquired during the previous epochs (ie. compact, dense nervous system structures, warm-bloodedness, tightly integrated physiological structures).

This proves to be of extreme importance in the line leading to primates and hominids.

We all agree that humanity is a rare and precious thing indeed, but life is ubiquitous on earth and we are only one of many possible evolutionary solutions to life's unfolding development.

There are many other possible evolutionary solutions and we owe our good fortune to the historical contingencies that made the manifestation of the human solution possible.

Understanding our emergence from a physical universe can only be accomplished by considering both the physical and chemical principles of matter, in conjunction with the historical development that has shaped us into what they are today from what we are composed of in the past.

While the K-T Impactor Event appears at first to be a completely random event, it is nonetheless deterministic in that it is consistent with the principles of physics and chemistry.

Indeed the consequences of the event lead to biological insights concerning the nature of contingency that are informative and consistent with the underlying Darwinian framework of the biological sciences as well.

Evolutionary Bottlenecks in the Solution Space

Evolutionary transitions represent bottlenecks in the solution space.

These bottlenecks usually occur thru low-propensity events that become differentially amplified over time and space.

The evolutionary trajectory of organismal development can only be understood in terms of both physical principles and historical contingencies interacting with each other, under structural constraints, within a vast, but finite, solution space of potential outcomes.

Chief among OOLs challenges is the integration of the deterministic models of chemistry with the historically contingent processes of biochemical evolution.

It is here that the low-propensity events within a chemical reaction system can become differentially amplified and the effects of these events subsequently ripple through the system to both perpetuate and transform the system.

Occasionally the molecular products of such low-propensity events may serve as autocatalyts in their own formation, or that of the initial reaction network leading to their formation, thereby providing a mechanism for there differential amplification of the molecule and increased robustness of the reaction network.

The interface for chemistry and biology in OOL studies is the establisment of a selective system capable of replication (somewhat imperfect), with occasional variation, that operates on chemical principles.

Class reactions, associated with the possesion of specific functional groups, serve to provide a degenerate population of organic molecules in aqueous solution.

In the abscence of catalytic mechanisms, the population of molecules is kinetically inhibited by the solvent, water, which competes with the substrates for intermolecular interactions, effectively prohibiting the otherwise uninhibited reactants from easily attaining the active complexes/transition states that lead to products.

Catalysts serve as a means to differentially amplify selected reactions, from the kinetically inhibited population of organic molecules, by lowering the activation energy for that type of reaction or class of reactions.

Occassionally a product will be produced which is capable of being autocatalytic to itself or a network of reactions that generates itself.

Such autocatalytic products eventually serve to tranform the population of molecules and shape the evolution of the system.

"From the point of view of evolution, however, they [low-propensity branch reactions] constitute the wellspring of genuine evolutionary variations. For on occasion, one of these (low-propensity) branch constituents will show a positive feedback effect by turning into a catalyst for a rate-limiting reaction within the autocatalytic cycle(s). Such a catalytic effect is itself cyclic: the catalyst (e.g. TPP) combines with a reactant in the autocatalytic cycle; the catalyst-reactant adduct undergoes subsequent conversion into a catalyst-product (faster than conversion of the reactant into product); finally, the catalyst-product adduct is cleaved into the product and the catalyst."
(Wachtershauser, 1992)

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