542-488 million years ago
Humans and all other vertebrates are classified in the animal phylum Chordata. Although primitive chordates are invertebrates (as are the first fish), they share a number of important anatomical and molecular features with vertebrates. Urochordates are the most primitive chordates alive today. The Early Cambrian oceans were filled with countless numbers of ancestral urochordates (typically under an inch in length) for more than five million years prior to the evolution of the cephalochordates, which were the ancestors of fish.
Modern urochordates known as tunicates (adult and larva) are pictured below.
Fossil tunicates are known from the Early Cambrian.