What is the predominant hypothesis why animals only appeared in the fossil record about 3 billion years after life originated?

Match the following terms and definitions. the transformation of a larva to an adult; occurs only in animals having great differences in form between the larva and adult pupa 2. type of mammal that lays eggs monotreme nymph the part of certain insect life cycles in which the young animal has no wings or reproductive organs the developmental stage between larva and adult in insects undergoing metamorphosis metamorphosis 5. the male gonad which produces sperm testes