Most of the forms of animal organization that exist today first appeared during the Cambrian Period (about 500 million years ago). In paleobiology, the term “Cambrian Explosion” refers to this period when organisms with skeletons and distinct limbs first appeared. These strange creatures emerged in a whirlwind of underwater creativity, displaying a wide variety of shapes and sizes. Evolution then took a giant leap forward. The first predators appeared, and prey developed ways to escape, protect themselves, and see their surroundings. This arms race gave rise to the ancestors of nearly all modern animals. It is one of the most extraordinary chapters in the history of life.