The miscellaneous transitions in evolution

McShea, D. W. & Simpson, C. 2011. The Miscellaneous Transitions in Evolution. In, The Major Transitions Revisited, Brett Calcott & Kim Sterelny eds. MIT Press. pdf

In Mark Twain’s Letters from the Earth, his sardonic alter ego Puddin’head Wilson reflects that if the Eiffel Tower represented the history of the world, and the skin of paint atop the knob at the pinnacle were the portion of that history in which humans have existed, “anybody would perceive that that skin was what the tower was built for. I reckon they would, I dunno” (Twain 1962, 226).

Twain was being facetious, of course, but taken at face value the remark does reflect a common intuition that people are special, that their existence reflects something profound about the evolutionary process, that evolution somehow culminates in them, that they are what evolution at the largest scale — all life over its entire 3.5 billion-year history, the whole Eiffel Tower — is all about. In the spirit of Puddin’head’s remark, a modern analyst could choose to scorn this intuition as an expression of our natural human infatuation with ourselves. Alternatively, but equally skeptically, a hard-headed contemporary might treat this intuition as a cultural residue of our two-thousand-year-old obsession with the Great Chain of Being, the notion — dating back to Aristotle — that there is an ordering among organisms, from lower to higher, from monad to man.

But another avenue is open to us. We could, instead, take the widely shared impression of an ordering seriously. We could conjecture that the Great Chain represents a direct but difficult-to-articulate insight — perhaps partly confused, or perhaps only partly correct — into a true natural ordering of some kind. Of course, a modern Great Chain would include a time component, identifying not just a set of organisms but a set of transitions: bacterium to protist to multicellular animal, or more recently, within chordates, fish to reptile to mam- mal to human. In other words, we could take seriously the idea that an updated Great Chain reflects an actual ascent of some kind, an increase in some objective and important property or quantity over the history of life. And we could then investigate to discover what that property is. Our project would be to discover what we will call the “theoretical unity” that links the transitions leading to people. So, for example, we would ask what is the variable that increases in the transitions from fish to human? On what scale do reptiles score higher than fish and lower than mammals? Is it intelligence, energy intensiveness, or maybe fitness? The search would be open-ended. The goal would be to determine what it is that our intuition has grasped, to figure out how to say, in scientific language, what we already think we know prescientifically. If the Great Chain is real, even in part, what is it that is increasing as we move up it?