A few years ago, P.Z. Myers — with his Mencken-like genius for the memorable putdown — devised “Paul Nelson Day,” aka April 7, to record my annual failure to follow up on a promise to elucidate “ontogenetic depth,” a notion I floated in 2003. Much as I enjoy having my own day and all, I figured it was time to explain ontogenetic depth (OD). OD is just not that hard an idea to grasp, in one sense. In fact, OD is downright pedestrian, not much more than a fancy way of saying…
Hey, wait a minute. Today is April 6. I still have a few hours to sort it all out. To warm up the audience, here’s an exercise. This bears on OD — really it does — so I’m not just tweaking your nose.
The figure shows a toy “adult organism” (in the box), with four cells of three types. The ovals and arrows to the left display a starting cell, which divides and differentiates in an ontogeny, or developmental trajectory, giving rise to two daughter cells, which then themselves divide into the four cells of the “adult.”
Here’s the exercise. How many instructions, or commands, must be present in the starting cell for (a) this ontogeny to unfold once, and (b) for it to happen reliably again, in reproduction?
That is, one or more of the four cells of the “adult” toy organism must give rise to an offspring organism, which repeats the same developmental trajectory: one cell dividing to two cells to four cells, of three differentiated types.
We need to know the minimal instruction (or command) set, which must be present in the starting cell, for this to happen reliably.
For those who want to see the same problem in a real biological system, watch this lecture by Eric Davidson.