According to this view, early sponges sit at the base of the animal tree of life, which then forks into four other groups: comb jellies, jellyfish, primitive
multicellular animals called placazoa, and another group of early symmetrical animals that led to worms, insects, and ultimately us (bilaterians).
The evolution of a distinct germ line that is protected by the diverse somatic functions of the organism is thought to confer an evolutionary advantage — what we
call a fitness benefit — to
multicellular organisms, whether plant or
animal or slime mold.