Because sponges are soft bodied, the only fossilized parts are silica spicules. The oldest fossil sponge known appears to be a glass sponge, dating to the Ediacaran (about 550 million years ago). By about 50 million years later (mid Cambrian), glass sponges were widespread on Earth. They are distinct, with bodies that don’t contract, have unusual cell types, and can conduct electrical impulses. Odd as they are, glass sponges were the common form of sponge living in a variety of water depths. In contrast, modern glass sponges are mostly confined to deep ocean, 200 meters (650 feet) or below. Like modern ones, early glass sponges were associated with slow-moving waters. Their fossils have been found in fine-grained limestones and shales, which form in quiet waters as sediments gradually accumulate. Glass sponges were common during some periods of Earth's history, and even formed reefs in the Tethys Sea during the Jurassic about 150 million years ago.

Sponges are ancient. Scientists think they were the first multicellular (made of more than one cell) organisms on Earth. The earliest fossil sponges from China, Iran, and Australia have been dated to at least 555 million years ago. That is hundreds of times as long as humans have been around (about 2 million years). All that remains of a sponge in the fossil record are usually the mineralized rods (spicules) that make up its skeleton. Paleontologists use a microscope to analyze the shape of the spicules to determine the type of sponge. Two groups of sponges (Hexactinellida and Demospongiae) appear early in the fossil record, and scientists are still determining which one appeared first on Earth. The first sponges probably had simple, vase-like bodies.