After 2 billion years of simple microbes, evolution produced a sudden, dazzling diversity of complex life. Mysterious beings started to appear 635 million years ago. The soft bodies of these first large multicellular animals left intriguing impressions on muddy seafloors—and left us with many questions about them. Answers remain elusive, though, since these enigmatic Ediacaran animals all went extinct. Later, some 540 million years ago, life blossomed anew during the Cambrian Period. Complex animals with body forms more familiar to us rapidly evolved and occupied marine realms. As animals began eating one another, they developed shells and other protective hard parts. This burst of biodiversity spawned most major animal groups alive today—including the ancestor of all vertebrates, or animals with backbones.

Trilobites—distinctive hard-shelled arthropods like this Mesonacis vermontanus—evolved in the Cambrian, the start of their long existence on Earth as marine scavengers and predators.

Pikaia gracilens had a distinct head and a flexible nerve cord running through its body. These traits made this Burgess Shale fossil a likely precursor to backboned animals.

Shells sheltered Cambrian sea snails such as Sinuopea eminensis.