Meeting Abstract
The concept of homology fuels many discussions (and frustrations) about the evolution of animal morphology. Little clarity comes out of these discussions due to a general lack of consensus about the general meaning of homology; however, we argue that a bigger problem lies in a lack of understanding about how the limited morphospace under which cells evolve can lead to erroneous conclusions that morphological similarity reflects meaningful homology. For example, the morphological similarity of the unicellular choanoflagellates to the choanocytes of sponges was the chief evidence used to place Choanoflagellata as the sister lineage to Animalia prior to the dawn of molecular phylogenetics. This placement was later reinforced by molecular phylogenies, artificially inflating the apparent value of morphological similarities. Given constraints on the biophysical capacity of membranes, cytoskeletal elements, and the surface area-to-volume ratio of the cell, we argue that eukaryotic cells really only do four things: grow/shrink, undergo membrane elaboration, adopt independent shapes, and gain/lose organelles. As such, cells with similar morphological features are expected to have arisen multiple times during the expansive diversification of animals. As a case study, we survey the evolutionary distribution of cells with apical cilia and microvilli across animals and summarize evidence for shared ancestry in a subset of these. We emphasize that cellular morphology alone is insufficient support for arguments of shared ancestry and propose a new standard for discussion of homology that requires evidence of embryological, genomic, and functional similarity as well.
