The first life forms on Earth did not require oxygen and included single-celled organisms like cyanobacteria. Around 2.5 billion years ago, cyanobacteria that performed photosynthesis began producing oxygen as a byproduct, leading to its accumulation in the atmosphere. During the Cambrian period, the dominant forms of life were trilobites and brachiopods, while cephalopods were the first large predators. Various plants and animals evolved and diversified over the Paleozoic era, including fish, amphibians, and reptiles. By the late Paleozoic, the continents had collided to form the supercontinent Pangaea surrounded by a global ocean.