The iconography of an expectation: Redux

"Scientific illustrations are not frills or summaries; they are foci for modes of thought." -Gould, 1991

Almost certainly, one of Stephen Jay Gould's pet peeves was the human tendency to assume that evolution proceeds like a ladder, from simple to complex species, often culminating in human kind. He spilled a lot of ink trying to dispel this common misunderstanding. His essay "Life's Little Joke", reprinted as Chapter 11 of Bully For Brontosaurus (1991), shows that the classic case of horse evolution is not a trend, as often portrayed. Rather, horses diversified, leaving only a few single-toed, large bodied species survivors.

My favorite example of Gouldian ladder-bashing is Chapter 1 of Wonderful Life (1989) entitled The Iconography of an Expectation. Here, Gould uses figure after figure from popular culture depicting evolution as linear series. Most are variations on the familiar parade of knuckle walking primate (brining up the rear) to modern human (leading the way). These quite commonly end with a joke, such as ending the parade with a modern human crouched over a computer.

A quick search with Google image, using the simple search term "evolution", confirms Gould's observation almost two decades later: The first seven hits were variations on the human walk of progress.





Here is one of the joke variety:




The metaphor even extends beyond living things, in this case to cellular phones:



Perceived evolutionary ladders abound, it's true. So where is the "redux" from the title? Well, I've been noticing that the fallacious ladder of progress not only applies to species, but also applies to traits, like eyes. Here is a recent example from a PNAS paper by Ayala:



Illustrated are the photoreceptive organs of five mollusk species. Like a 'primative' knuckle-walking proto-human, the 'lowly' pigment spot of a limpet begins the parade.
Ever so slightly more complex is a pigment cup, followed by the pin-hole eye of a Nautilus. At the top of this ladder of progress are the lensed eyes of a snail and an octopus. Cephalopod eyes, like that of Octopus, are widely heralded as strongly convergent with our own eyes, where a lens focuses light on a retina at the back of the eye. Thus completes the march of progress, from lowly pigment spot right on up to the most human-like eye of the invertebrate world. Congratulations, humans, we're at the top of the ladder again! Dr. Gould may be rolling in his grave.

There are a lot of extensions to this idea, which I will continue in future posts. I have several examples from eye evolution of iconographic ladders of progress. What are these ladders trying to accomplish, and what is wrong with such a view anyway? Is it really a model of how evolution works?

To be continued.