The diversity of life forms throughout the history of life on earth is so engaging and impressive that it is easy to overlook the other side of the coin: the continuity that connects all organisms to an array of common ancestors. In fact, any evolutionary model that used only data on divergence and none on conserved traits would fail to make any sense of the emergence of new species from ancestral ones. In The Plausibility of Life, Kirschner and Gerhardt focus on a number of conserved “core cellular processes” shared by all living things. Their thesis is that these core processes represent successful innovations that are inherited by evolutionary descendants. However, they argue that the success of these processes lies not in their highly specified functions, but in their abilities to produce quite variable outcomes under different environmental conditions.
In essence, this is the negation of the “irreducible complexity” argument of “intelligent design” proponents. The authors show how a single molecule with a highly specified function can perform a different one under different environmental conditions. In other words, the molecular imperative for the cell is flexibility, not specificity. The apparent specificity that we observe is so reliably produced, they argue, because the genome is selected for adaptability. How else could such complex organisms so full of complex biochemical and developmental pathways be produced with so few genes?
Kirschner and Gerhardt explore several specific examples in the text that illustrate their points quite effectively. They give examples of metabolic processes, body-plan evolution, developmental and regulatory change, and morphological specialization (for example, adaptations for flight). Two of the key concepts are weak linkage and exploratory behavior.
The first of these is based on the observation that there are many steps between the DNA sequence for a particular protein and the outcome of the process in which that protein will participate. In a number of well-documented cases, a protein produces a weak signal that produces a particular effect only under specific conditions. The “linkages” between the form and function are “weak” or “easily forged and broken” without any significant genetic change in the organism (p 110–1). This allows new pathways and new linkages to be formed to produce new pathways and products while retaining substantially the same DNA sequence.
Exploratory behavior is viewed from both organismal and cellular perspectives as the basis for the appearance of complex organization from simple actions. In the case of ant foraging, it is clear that the brains of ants do not encode territorial or resource “maps” but build a successful complex foraging strategy based on the accumulation of the results of random foraging behaviors. In the case of the development of blood vessels and nerves, the authors show how these structures emerge in response to signals generated by the target tissues so that they grow in the “right” directions and connect to the “right” cells. This exploratory behavior — whether cellular or organismal — produces complex outcomes from simple conditions, and, as the authors point out using the examples of the pattern of blood vessels that we all can see in the skin of our arms and hands, highly variable ones even within the same individual.
These two examples capture only a bit of the flavor of this book, which extracts the results from contemporary research and presents them in a format for nonspecialists. The authors succeed in illustrating their points from the biochemical to the behavioral levels of the organismal hierarchy with examples from each of the levels in between. They are frank about what is known and what is still to be learned, but they present a strong case for the conservation of core processes that allow for the evolution of complex, highly specific functions, but that also allow organisms to adapt these structure-function complexes to a variety of conditions with a variety of outcomes depending on the environment in which the organisms operates. Indeed, in their view of the evolution of complex structures, what is now a mousetrap could easily have started out as a starting gate or a spring latch. The conserved core process is geared to producing components, but the assembly and final configuration are anything but foreordained.