In Cells to Civilizations: The Principles of Change that Shape Life, author and plant biologist (and corecipient of the Darwin Medal) Enrico Coen is one of the latest scientists to carry the banner claiming the existence of a pattern to the transformation of all living things—an evolutionary transformation that embraces the processes (or the four domains) of biological evolution, development, learning, and culture. Coen is in distinguished company, both past and present. Donald T. Campbell pressed for a structural parallel between evolutionary biology and evolutionary epistemology based on random variation and selective retention. Gerald M. Edelman's “neural Darwinism” asserted a pattern between learning and other complex adaptive systems using the concept of feedback. Richard Dawkins and Daniel Dennett each strive to prove the existence of similarities between biological and cultural evolution. Stuart Kauffman ambitiously asserts that a single set of processes guides both evolution and development, as well as the dynamics of other complex systems. Peter J. Richerson and Robert Boyd together have developed an impressive multilevel account of evolution. All good company aside, Coen does not simply reiterate what has gone before; he develops and compares models of evolutionary transformation within four distinct domains, stating the presence of a formally similar set of mechanisms in each case.
In Cells to Civilizations, the author identifies a total of seven principles that are involved in evolutionary transformation, and the core of his approach is rooted in Alan Turing's (1952) classic account of morphogenesis using the model of a reaction—diffusion system, in which Turing showed us that it is possible to generate interesting patterns of growth. Coen depicts two feedback loops, one positive and one negative, regulating a specific domain. The positive loop is described as reinforcement and the negative loop as competition. Using this dual-feedback system, Coen explains the developmental patterning within Escherichia coli, and specifically, how E. coli reliably divides in its midsection. When the organism prepares to reproduce, two crucial proteins, MinD and MinE, are involved. As MinD attaches to a membrane, it improves the chances that other MinD will attach to the same region. This is the chemical reaction, or reinforcement stage. When MinE binds to MinD, MinD detaches from the cell membrane. Diffusion distributes the Min proteins throughout the cell, but with the right affinities and diffusion rates, the MinD proteins oscillate and begin to concentrate at opposite ends of the cell. This reaction—diffusion process continues until the partition of the cell forms in the middle, where the concentrations of proteins are lowest. According to Coen's dual-feedback loop, the binding of proteins is positively reinforced, but as more MinD become present, a decrease in binding occurs; instead of uniformity, there is oscillation.
These sorts of dual-feedback loops are common in natural systems. Coen uses the term transformation to capture changes in living systems over time. Cells to Civilizations presents the various components of transformation and demonstrates how they regulate limited-density growth (competition) and patterning. Describing these seven principles, along with how they work and interact, is to offer what Coen calls “life's creative recipe” (p. 60). These principles form the basic structure on which Coen builds his transformations in each of his favored domains.
According to the principle of variation, variation in a population is essential for change, and it can have a variety of sources, including both mutation and recombination. The point is familiar in evolutionary biology: In the absence of substantial variation, there would be no evolutionary change.
In the principle of persistence, the necessity that change accumulates over time is emphasized. Organisms are relatively stable entities, as is the DNA that ensures their persistence. Coen uses persistence to cover both replication and simple continuity, noticing that, for evolutionary change to take place, there must be a trade-off between persistence and variation.
Some variations influence reproductive capacity, and this gives rise to the principle of reinforcement. Simple growth is a matter of reinforcement. If a population grows, it may not lead to a change in the relative numbers of variations; it would still count as reinforcement but not as a transformation.
Following the principle of competition, reinforcement is not sufficient for natural selection (or sexual selection) to operate. Competition, as Darwin saw it, emerges when limitations on resources are imposed by limitations on growth. Coen recognizes that, with competition, change can occur even in the absence of differences in fitness. When competition and reinforcement are both present, along with variation and persistence, the result is evolution by natural selection.
Coen also recognizes the principle of cooperation, which affirms that cooperation, as well as competition, can influence evolutionary outcomes. He uses the term cooperation in a broad sense to include, for example, a series of bases cooperating in order to produce a protein; likewise, it is cooperation when genes in the same organism form a phenotype.
Simpler elements can combine to form elaborate complexes, increasing what Coen calls the “richness of the world.” The principle of combinatorial richness acknowledges that linkage creates a greater variation. The genetic code is a simple illustration of this principle. If there are 25,000 genes in the human genome, each with thousands of base pairs, the number of possible genotypes is astronomical.
Evolution is constrained by history; organisms modify their own environments, creating a context for further evolution. This understanding that competition and cooperation occur in a context that the organism creates is what Coen calls the principle of recurrence. Bat wings and bird wings are very different structures, but the basic forelimb structure is preserved in both. The result is a historical trajectory that is “convoluted and idiosyncratic” (p. 51).
These principles were initially developed from the perspective of biological evolution, but the book aims to extend them to three other domains: development, learning, and culture. Development is not just growth but transformation. Whereas evolution results in diverse life forms arising over time, development is “the recipe [that] involves populations of molecules and cells within the same individual and leads to the emergence of an adult within a single generation” (p. 109). Within development there is selective reinforcement for some cell types and repression for others, with the result being not just growth but a change in conformation.
Likewise, learning is not a simple matter of conditioning, although conditioning does, of course, change expectations. Coen uses a model from Montague and colleagues (1996) called temporal difference learning to explain the change in expectations. The details are interesting, but the core idea is that conditioning incrementally alters synaptic strengths through feedback (reinforcement). This is matched with a decline in the response to rewards (inhibition): “At the heart of learning, we have a double feedback loop of reinforcement and competition, fueled by a balance of variation and persistence” (p. 167).
The same set of principles, Coen says, are at play in cultural transformations, and he uses the artistic trends of fifteenth-century Florence as an example. Artistic innovation fueled artistic innovation, and as innovations spread, so did competition. Once again, Coen sees the familiar dual-feedback loop, fueled by individual variation and persistence. In this domain, however, cooperation also plays a crucial role: “Cooperation and competition are partners in cultural change” (p. 257).
Although the author is clear to make distinctions among the domains of evolutionary change, development, learning, and cultural change, what is emphasized in Cells to Civilizations is that, at the appropriate level of abstraction, their similarities can be enlightening. There is much more in this book that I have not mentioned. It is replete with biological examples— from the stripes of zebras to plant genetics—that illustrate the author's claims, all accomplished with clarity and grace. Cells to Civilizations is an intelligent and entertaining book by a distinguished biologist.
