Invertebrates—creatures without internal skeletons or spines—were the earliest animals to evolve.
They existed in the Proterozoic era, but it wasn’t until the next era, the Paleozoic (570 million to 245 million years ago) that more familiar life forms proliferated. The first stretch of time in the Paleozoic era is called the Cambrian period.
Sponges, coral, jellyfish, snails, clams, squids, worms, leeches, insects, spiders, lobsters, shrimp, sand dollars, and starfish have all been around since then.
Without internal skeletons, all of these creatures depended on the water around them or external skeletal structures, such as shells, for support.