Several years ago, PZ Myers inaugurated what he called “Paul Nelson Day” (April 7), his annual lampooning of me for my failure to articulate the concept of “ontogenetic depth.” Actually, the concept itself is not that hard to understand, as I explained two years ago in a series at ENV (see Part 1 and Part 2). Since then, in lectures around the country — to both academic and lay audiences — I’ve explained the central difficulty (related to ontogenetic depth) facing theories for the origin of the animals via any undirected evolutionary process, and why the difficulty is likely to persist as unsolved within the “undirected” framework philosophically preferred by most evolutionary biologists. You can watch one such lecture here (the biology starts around 9:45) to see why.
But lately, I’ve run across something related to ontogenetic depth that is, well, mind-blowing.
Since 1859, the origin of not a single bilaterian phylum (animal body plan) has been explained in a step-by-step (neo-Darwinian) fashion, where random mutation and natural selection were, as textbooks assert, the primary causal mechanisms. Take your pick of the phyla: Mollusca, Brachiopoda, Chordata, Arthropoda, you name it — and go looking in the scientific literature for the incremental pathway, via mutation and selection, showing how that body plan was assembled from its putative bilaterian Last Common Ancestor.
You’ll be looking a long time.
When a scientific theory, namely neo-Darwinism (i.e., textbook evolutionary theory), claims to explain something like the origin of the animal phyla, and then signally fails to do so over a period of many decades, the failure may have more to do with nature resisting the attempts than with any lack of effort on the part of the relevant community (evolutionary biology). You can’t solve a problem you don’t understand.
When nature resists, it’s because the phenomena to be explained exist qualitatively beyond the reach of the mechanisms thought to be causally responsible. In this instance, random mutation and selection fail, not because they don’t occur (they do), but because developmental pathways in all animals are end-directed processes, where the cause needs to see the target to put the entire pathway in place. Undirected mechanisms such as mutation and selection lack the foresight required.
That’s the problem of ontogenetic depth, and evolutionary biology has yet to grasp it. There are hints, however — such as the discussion in Douglas Erwin and James Valentine’s new book The Cambrian Explosion (2013) — that the problem is swimming into view. See too, of course, Steve Meyer’s forthcoming Darwin’s Doubt: The Explosive Origin of Animal Life and the Case for Intelligent Design.
Image: Opabinia regalis, from the Cambrian Burgess Shale; Nobu Tamura/Wikipedia.