Trawling the net, I came across this chapter from the forthcoming 2nd edition of the Dendrites monograph: Phylogeny and Evolution of Dendrites by Gayle M. Wittenberg and Samuel S.-H. Wang (pdf). I was hoping to find something about the origin of dendrites and maybe something about differences in primates or humans. Unfortunately, there isn't much to report. From reading this review, you'd get a definite impression that dendritic architecture is not the target of selection. The dendrites either don't scale at all with brain size or they scale in such a manner as to preserve the "isoelectric" distance from the soma. In other words, even when dendrites grow, they preserve computation and communication to the soma. We get three leads to follow if we want to know about novel primate dendritic architectures:

Another place in which unusual dendritic specializations may occur is the neocortex of great apes, which show unusual social and cognitive complexity. These animals have several types of giant neocortical neurons, including Betz cells (Sherwood et al., 2003), Meynert cells (Sherwood et al., 2003), and spindle cells (Nimchinsky et al., 1999). These giant cells may have arisen in great apes as extreme adaptations of pyramidal neurons. Among primates their somata show distinct scaling relationships relative to brain and body size. Like other neocortical pyramidal neurons (Elston et al., 2001), they may vary in dendritic extent and synapse number across species as well. At present, however, little is known about their dendrites.

We know a reasonable amount about molecular determinants of dendritic branching:

The commonly held view of dendritic morphogenesis is that general structural features result from genetic instructions, whereas fine connectivity details rely mostly on substrate interactions and functional activity. During early dendritic maturation, dendritic growth cone formation produces new branches at all dendritic roots. The second phase is growth cone independent and afferent input dependent, during which branching is limited to high order distal dendrites. During the third phase, activity-dependent synaptic maturation occurs with limited or subtle remodeling of branching.

So it seems like maybe some molecular phylogeny could point us in the direction of good hypotheses. It would sure be nice to know when dendrites first arrived though. Does anyone know? Were cells excitable first or polar first (polar meaning having separate extremities with distinct function)? I assume it happened back in flatworms or something but I haven't got a clue.