Bridging morphological transitions to the metazoa

Our inability to answer many questions regarding the developmentof metazoan complexity may be due in part to the prevailingidea that most eukaryote "phyla" originated within a short periodof geologic time from simple unicellular ancestors. This view,however, is contradicted by evidence that larger groups of eukaryotesshare characters, suggesting that these assemblages inheritedcharacters from a common ancestor. Because molecular analyseshave had limited success in resolving the relationships of highereukaryote taxa, we have undertaken a phylogenetic analysis basedprimarily on morphological characters. The analysis emphasizescharacters considered to have a high probability of having evolvedonly once. Transitions between taxa are evaluated for the likelihoodof character-state transformations. The analysis indicates thatthe evolutionary history of the clade containing the Metazoahas been complex, encompassing the gain and loss of a secondaryand perhaps a primary photosynthetic endosymbiont with accompanyingchanges in trophic level. The history also appears to have includeda hetero-autotrophic ancestor that possessed a "conoid" feedingapparatus and may have involved a transformation from a flagellateto an amoeboid body form, a trend toward increased intracellularcompartmentation, and the development of complex social behavior.Such changes could have been critical for establishing the underlyingcomplexity required for a rapid diversification of cell andtissue types in the early stages of metazoan evolution.