Thanks to P.Z. Myers’s annual lampoon of me, I’ve thought more deeply about the relationship between development and evolution than I ever expected to do.
Moreover, "Paul Nelson Day" provides me with an annual teaching opportunity, because I can link from the comments thread at Pharyngula to the discussion here.
So — today, kids, I want to introduce you to a great puzzle, one that is central to the unsolved problem of the macroevolution of the animals.
Here’s the context you need to know to understand the significance of the puzzle, which we can dub "The Target Problem," for reasons that will become obvious.
Three key points:
(1) A hundred and fifty years after the Origin of Species, the evolutionary pathways from unicellular eukaryotes to any of the animal body plans remain entirely unknown. For details, see Steve Meyer’s book Darwin’s Doubt.
(2) These missing pathways are often blamed on the absence of "transitional" fossils. While having more fossils might help, the evolutionary mystery — at its core — would still not be solved. The Target Problem remains.
(3) The necessary evolutionary pathways will remain hidden as long as one seeks to explain their origin via undirected processes.
Now, (1) is simply the case. If you don’t agree that (1) is a fact, choose any of the phyla — or (Nick M., I’m thinking of you), if you don’t like the taxonomic category "phylum," choose any animal species present in the Cambrian — and send me the evolutionary pathway leading to that group from its unicellular eukaryotic ancestors. (Email me here.)
What’s the easiest way to grasp the Target Problem? Start by watching a short YouTube clip, of course, to ground the discussion in a concrete and well-understand example — namely, the development of the nematode Caenorhabditis elegans.
This clip shows, in four minutes, the twelve-hour developmental trajectory from fertilized egg to hatching worm. As you watch, keep in mind that all these cell divisions, with their asymmetric daughter cells (and their daughters, and theirs, and so forth, right down to the terminally differentiated cells in the hatching worm), must have been constructed by the undirected processes of evolution, if that’s what happened at the origin of C. elegans, anyway. John Sulston won the 2002 Nobel Prize in Physiology or Medicine for working out the details of the cell lineages in this species, from fertilized egg to hatching. You can see the famous diagram of the lineage here.
All right — so what’s the Target Problem? It arises when we consider the relationship between "developmental time" and "evolutionary time" (see the figure below). Developmental time we can actually observe and track in C. elegans. That’s what you just watched in the YouTube clip, and what Sulston’s branching diagram depicts schematically.
Evolutionary time, by contrast, we can’t observe or track, but we know what must have happened during evolutionary time, if an undirected process constructed what we can see unfolding in C. elegans. Several things must have occurred, among them:
- an increase in cell number, from one (i.e., in the unknown unicellular eukaryotic ancestors) to approximately one thousand cells in an adult C. elegans.
- the origins of cellular differentiation, namely, specialized cell types, such as cuticle, gut, nerve, etc., cells, with corresponding genetic controls
- the origin of the three-dimensional architecture (body plan) of C. elegans
- the origin of C. elegans sexual reproduction and gametes (eggs and sperm).
And, never to be neglected, the small difficulty of staying alive during the whole of evolutionary time, as required by the Principle of Continuity.
The Target Problem? How to describe functional (and reproductively capable) organisms, which must have existed during evolutionary time, as cell number count goes up, and cellular differentiation increases. We’re using C. elegans as our example because we have a complete cell lineage, genome, and lots of other details to ground the discussion, but the Target Problem exists for any other animal species thought to have evolved from a non-animal ancestor.
The Target Problem has never been solved. That’s why no one can tell you how the animals evolved from single-celled ancestors.