Prevailing theories suggest that sponges are early animals which produced no subsequent evolutionary line. For example, bodily composition: the elastic skeletons of sponges are made from collagen, the same protein found in human tendons and skin. Zoologists imagined that sponges occupied an isolated position in the animal kingdom, but molecular testing has since proved that sponges and more complex animals (like humans) developed from a common ancestor sponges also possess many of the qualities biologists use to distinguish people from plants. In the eighteenth century, however, scientists began to notice animal characteristics of sponges, including the changes in diameter of their central cavity, and their creation of distinct water currents. The ever-sage Encyclopedia Britannica informs us that early naturalists classed sponges as plants because, you know, they lack organs, don’t move, and often have branches. Lue Barrel Sponge (Scientific American Creative Commons Chris Coccaro)
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