The three possible topologies for a tree describing the evolutionary relationships between nematodes, arthropods, and humans. Tree A (blue) reflects the conventional interpretation of metazoan phylogeny with nematodes as a “protocoelomate” group basal to arthropods and humans. Tree A was supported by neighbor-joining analysis of 24 protein quartets as described in the text. Tree B (red) represents the “Ecdysozoa” phylogeny derived from 18S rRNA gene sequences of a variety of nematodes and arthropods (Aguinaldo et al. 1997), and is supported by 11 protein quartets. Tree C (green) is not expected from any metazoan phylogenetic hypothesis and is supported by a single protein quartet. Average bootstrap values and their standard deviations are shown for each tree.
