Flexi Says: Nearly all vertebrates have red blood cells. Although their structure varies somewhat among different groups, red blood cells all contain the protein hemoglobin which is used to transport oxygen. Even the most primitive vertebrates, the jawless fishes (Agnatha), possess red blood cells. This group dates back over 400 million years. Scientists recently discovered a marine invertebrate, Botryllus schlosseri, that produces blood cells using stem cells in a similar manner to humans and other mammals. This lineage dates back over 500 million years. However, the earliest blood cells may have evolved in sponges, some of the earliest multicellular animals. Sponge blood cells are very different from vertebrate blood cells in that they perform both functions of oxygen transport and immune defense. They are one form taken by archaeocytes, a class of sponge cells that can differentiate into a variety of specialized cell types.