“For every complex problem, there is a simple, easy to understand, incorrect answer” -Albert von Szent-Györgyi
Albert von Szent-Györgyi won the Nobel Prize in Physiology or Medicine in 1937 “for his discoveries in connection with the biological combustion process with special reference to vitamin C and the catalysis of fumaric acid” and in the view of many he was unfortunate not to have won a second Nobel Prize for his subsequent work on muscle physiology. Szent-Györgyi made the observation cited above because he was aware that in vitro experiments were unlikely to capture the complexity of the in vivoreality. This is because all of the metabolic pathways that are illustrated as separate entities in textbooks almost certainly interact with each other. Thus the resultant of a combination of pathways is not readily captured by experiments that are designed to isolate a single pathway.
I have often thought about Szent-Györgyi’s observation in connection with human evolution and I am convinced that we have unreasonable expectations about what can be achieved by paleoanthropologists. For example, a lot of intellectual effort goes into developing hypotheses that are meant to explain what we infer to be important “events” in human evolution (e.g., the evolution of the upright posture and bipedalism, the evolution of a large brain, the evolution of language, etc, etc.). These efforts make many assumptions, but I want to focus on three of them.
The first assumption is that we have a good enough fossil record, plus good contextual information about that fossil record, to “know” where these “events” occurred and the circumstances that surrounded their occurrence. It seems to me inconceivable that the meager fossil record we have of the first several million years of hominin evolution captures all there is to know about the tempo and mode of evolutionary change in the hominin clade. There is a widespread assumption, especially among those who see simplicity in the hominin fossil record, that an upright posture and bipedalism only evolved once. Sure, this assumption certainly simplifies the problem, but we do not yet know whether it over-simplifies it.
The second assumption is that the types of change I am referring to really are “events” in the sense that they took place in a defined place and their time course was relatively short. They look like that to us, but I suspect that they look that way because of the paucity of evidence, not because they really are actual events.
The third assumption is that these events are the result of selection pressures that have a single, primary, cause (e.g., endurance running, the substitution of one expensive tissue for another, meat eating, etc. etc.).
My instinct is that the principle Szent-Györgyi alluded to in relation to explanations for physiological phenomena applies in spades to attempts to “explain” morphological changes significant enough to be interpreted as “events” in hominin evolution. Consider that the researchers who study living populations of modern humans find it hard enough to “explain” why contemporary hunter-gatherers hunt. So why on earth should we expect it to be easier to determine the nature of selection pressures that operated several million years ago? Especially, when we are doing it with one hand tied behind our back (a metaphor for the sparse fossil record) and in the absence of a timepiece, a calendar or a map (metaphors for poor temporal control and thousands of square miles of missing data).
PS. I had started thinking about and writing this blog when I was asked to comment on a manuscript by Matt Cartmill that makes some of the same points. Depending on your point of view this could either be the result of canalization, or an example of homoplasy, or possibly both.