The history of life on Earth is still a mystery: Bacteria have been around for about 3 billion years, but for most of this time they had had the Earth to themselves. Seaweed, jellyfish-like creatures, sponges and worms arrived a few million years before the Cambrian period began, over 500 million years ago.
But 200 million years ago, higher forms of life suddenly arrived: arthropods, brachiopods, coelenterates, echinoderms, mollusks and even chordates, the animal group from which vertebrates like us developed. Each of these evolutionary changes can be coordinated with a period in which the amount of oxygen in the ocean rose. Each of these oxidation events corresponds with an increase in the size, complexity and diversity of life, both plant and animal.