Sponges were the first animals to evolve from protists, about 700 million years ago. They are the least complex of the animals, sharing the most in common with protists. Their evolutionary relationship comes from the feeding cells within sponges, which bear great resemblance to the choanoflagellates, which are still extant, that sponges supposedly evolved from.
Sponges, unlike choanoflagellates, are multicellular. Protists from which sponges split off were mostly unicellular, often living in large colonies. Sponges are categorized as animals because they represent the first shift to multicellular organization that characterizes the animal kingdom. Modern sponges and other animals share a common ancestor, but sponges have undergone relatively little change over the last 700 million years, while other animals have developed complex organizational systems, tissues, and organs.
Sponges, unlike choanoflagellates, are multicellular. Protists from which sponges split off were mostly unicellular, often living in large colonies. Sponges are categorized as animals because they represent the first shift to multicellular organization that characterizes the animal kingdom. Modern sponges and other animals share a common ancestor, but sponges have undergone relatively little change over the last 700 million years, while other animals have developed complex organizational systems, tissues, and organs.