We have really humble beginnings. ~ Italian evolutionary biologist David Pisani
Sponges were the forerunner of all complex metazoa. Sponges appear as incomplex animals, without nervous, digestive, or circulatory systems. Their bodies are jelly-like connective tissue (mesohyl) encased in 2 thin layers of cells.
At the cellular level, sponges are not so simple. Unlike other animals, sponges have differentiated cells that can transform into other types. Another novelty arises in the way sponges build their skeletons, employing specialized cells in a complex, coordinated process.
Division of labor by manufacturer, transporter, and cementer cells, and iteration of the sequential mechanical reactions, allows construction of the skeleton as a self-organized biological structure, with the great plasticity in size and shape required for indeterminate growth. ~ Japanese molecular biologist Sohei Nakayama et al
Sponges host a thriving microbiome that helps them digest the microorganisms they ingest, as well as other functions. Over 40% of sponge cells are bacterial.
Despite simple morphology, early sponges had latent genic sophistication. They carried the gene for sex determination (long thought to have evolved much later) and had the genetic bases for nerve conduction that developed in later animals.