Cannibalism strikes most as the height of depravity, an act of derangement or desperation. In “Cannibalism: A Perfectly Natural History’’ (Algonquin), Bill Schutt, a professor of biology and author of “Dark Banquet,’’ a book about creatures who feed on blood, examines cannibalism up and down the food chain, from tadpoles and spiders to polar bears, primates, and people. He balances the inevitable gore with a breezy tone as he writes about entrails, encephalitis, and blue clots in human placenta meat. Schutt’s goal is to understand whether the taboos are hardwired or because of culture and ponders whether circumstances, particularly brought about by climate change, could popularize people eating. He finds, with humor and clarity, that cannibalism is an evolutionary act, one based on stress, reproduction, and survival.