From The Urantia Book
(59:1.18) The marine life was much alike the world over and consisted of the seaweeds, one-celled organisms, simple sponges, trilobites, and other crustaceans—shrimps, crabs, and lobsters. Three thousand varieties of brachiopods appeared at the close of this period, only two hundred of which have survived. These animals represent a variety of early life which has come down to the present time practically unchanged.
(59:2.11) Lime-secreting algae were widespread. There existed thousands of species of the early ancestors of the corals. Sea worms were abundant, and there were many varieties of jellyfish which have since become extinct. Corals and the later types of sponges evolved. The cephalopods were well developed, and they have survived as the modern pearly nautilus, octopus, cuttlefish, and squid.
(65:2.2) Very few species of the early types of marine vegetation that participated in those epochal changes which resulted in the animallike borderland organisms are in existence today. The sponges are the survivors of one of these early midway types, those organisms through which the gradual transition from the vegetable to the animal took place. These early transition forms, while not identical with modern sponges, were much like them; they were true borderline organisms—neither vegetable nor animal—but they eventually led to the development of the true animal forms of life.
Related research
See 2021 Science Daily article on research related to sponges as one of humanity’s most ancient ancestors.
See easy-to-read 2016 article that pushes back the date for the development of sponges. See the related scientific report on PNAS.