Last month, in the journal Trends in Ecology & Evolution, a group of scientists published a tub-thumping defense of sponges and other supposedly simple animals. In their paper, Casey Dunn, Sally Leys, and Steve Haddock argue that humans have systematically underestimated these creatures, largely because of our innate bias against organisms outside our taxonomic clique. That clique, actually called a clade, includes all of the so-called bilaterians—animals with left-right symmetry that share a single ancestor. Tigers, hummingbirds, octopuses, scorpions, crocodiles, mantises, sharks, earthworms: all are bilaterians.
Dunn, Leys, and Haddock write that, as bilaterians ourselves, and rather narcissistic ones at that, we tend to look down on the other four animal clades: the placozoans (flat, creeping mats that are represented by just one known species); the cnidarians (jellyfish, sea anemones, corals, and their stinging kin); the ctenophores, or comb jellies; and the sponges. Even some professional biologists disregard sponges as lowly, primitive proto-animals, sitting at the bottom of an evolutionary ladder with us on the top rung. We treat their biology as an impoverished subset of our biology. We relegate their existence to a checklist of missing traits: no limbs, muscles, nerves, or organs, and none of the tiger’s fearful symmetry. But these creatures, according to Dunn, Leys, and Haddock, are not primitive relics; they are modern animals that excel at their own particular life styles. By ignoring them, we blind ourselves to a wondrous hidden biology and get a misleading view of evolution.