Today, demosponges are by far the most abundant and diverse group of sponges on Earth. With more than 4,500 species occupying a wide range of habitats, including both freshwater and marine, they are the dominant group of sponges. However, this may not always have been the case. While demosponges are amongst the earliest sponges (along with glass sponges), they make up less than half of the fossil record of sponges. They may be under-represented in the fossil record because they have few hard parts that fossilize. Some demosponges have tiny, hard skeletal parts (spicules), but others are just made of spongy protein fibers (spongin). The first definite demosponges are Cambrian fossils (from more than 500 million years ago), although they may have evolved much earlier. They radiated into a variety of habitat during the millions of years that followed, reaching their peak of diversity about 100 million years ago during the Cretaceous.

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.