Invertebrates

Invertebrates The first animals to evolve were invertebrates. Fossil evidence of invertebrates dates back to the late Precambrian, 600 million years ago. Invertebrates evolved from single-celled microorganisms. Since then, invertebrates have diversified into countless forms. An estimated 97 percent of all species are alive today are invertebrates. Invertebrates are united more by what they lack (a backbone) than by shared characteristics. Invertebrates include animal groups such as sponges, cnidarians, flatworms, molluscs, arthropods, insects, segmented worms, and echinoderms as well as many other lesser-known groups of animals.