Around 550 million years ago it was simple to be an animal. You might be a marine sponge, attached to rock, beating your tiny whip-like flagella to pass seawater through your body in order to obtain oxygen and filter out bacteria and other small food particles. You’d have specialized cells that allow parts of your body to slowly contract to regulate this flow of water, but you couldn’t move across the sea floor properly. Or you might be an odd, simple animal called a placozoan, a beast that looks like the world’s smallest crepe—a flattened disc of tissue about 2 millimeters in diameter with cilia sprouting from your underside like an upside-down shag carpet. These cilia propel you slowly across the sea floor, allowing you to seek out the clumps of bacteria growing on the sea floor that are your food. When you found a particularly delicious clump, you could fold your body around it and secrete digestive juices into this makeshift pouch to speed your absorption of nutrients. Once digestion was finished, you would then unfold yourself and resume your slow ciliated crawl. Remarkably, as either a sponge or a placozoan, you could accomplish all sorts of useful tasks—sensing and responding to your environment, finding food, moving slowly, and reproducing yourself—without a brain or even any of the specialized cells called neurons that are the main building blocks of brains and nerves.

Around 550 million years ago it was simple to be an animal. You might be a marine sponge, attached to rock, beating your tiny whip-like flagella to pass seawater through your body in order to obtain oxygen and filter out bacteria and other small food particles. You’d have specialized cells that allow parts of your body to slowly contract to regulate this flow of water, but you couldn’t move across the sea floor properly. Or you might be an odd, simple animal called a placozoan, a beast that looks like the world’s smallest crepe—a flattened disc of tissue about 2 millimeters in diameter with cilia sprouting from your underside like an upside-down shag carpet. These cilia propel you slowly across the sea floor, allowing you to seek out the clumps of bacteria growing on the sea floor that are your food. When you found a particularly delicious clump, you could fold your body around it and secrete digestive juices into this makeshift pouch to speed your absorption of nutrients. Once digestion was finished, you would then unfold yourself and resume your slow ciliated crawl. Remarkably, as either a sponge or a placozoan, you could accomplish all sorts of useful tasks—sensing and responding to your environment, finding food, moving slowly, and reproducing yourself—without a brain or even any of the specialized cells called neurons that are the main building blocks of brains and nerves.