The predator-prey relationship, in which some animals (predators) eat other animals (prey)—began when multi-celled animals appeared.
Jaws and teeth eventually became essential developments for survival in the ocean. The heavy protective plates of the early fish gave way to lighter scales.
This change, along with the evolution of fins, allowed fish to swim in the water rather than inch along the bottom of the sea.
These developments made it easier for them to search for food and to escape becoming food for another fish.