As far as we know, animals first evolved in the Ediacaran period, about 610 million years ago. By this point, unicellular organisms had already split into plants, animals, fungi, and other divisions. The first known animals resembled cnidarians (jellyfish), and included a number of mysterious carpet, oval, and bag-like forms that ranged up to a meter in diameter (the Ediacaran biota). It is difficult to place the way Ediacaran animals related to later forms, and this is currently under debate. By the dawn of the Cambrian, 542 million years ago, most of these animals had died out and were replaced by numerous phyla generated during what is known as the Cambrian explosion.
By the end of the Cambrian explosion, about 500 million years ago, representatives of practically all 38 modern animal phyla had emerged. Even the vertebrates were represented by primitive jawless fish. At this point, there were still no land animals, however. The Cambrian period was an era dominated by invertebrates, which occupied all major ecological niches in the sea, including that of apex predator (Anomalocaris). Many organisms evolved eyes, armor, and complex nervous systems for the first time during this period. Predation kicked off an evolutionary arms race between predators and prey. Animals related to each other in that most were arthropods.
Throughout the next 100 million years, fish evolved and became more numerous, solidifying the place of vertebrates alongside the invertebrates. By the Silurian, about 425 million years ago, plants began colonizing the land, followed quickly by the first insects, including millipedes, wingless insects, and an assortment of other arthropods. These insects evolved from aquatic arthropods. Meanwhile, the shallow seas were ruled by eurypterids, the sea scorpions, some of which exceeded 10 ft (3 m) in length. The movement of some arthropods on land meant that animals related to each other somewhat less, as the differing groups evolving in a variety of directions to deal with their respective environments.
About 330 million years ago, lobe-finned fish began to evolve primitive legs and started to hop around on land. This quickly gave rise to amphibians, which dominated the terrestrial world until the appearance of amniotes about 300 million years ago. The "mother of all mass extinctions" hit 251 million years ago, wiping out many fledgling amniotes, the largest amphibians, and constraining genetic diversity. Animals related to each other more closely than ever before. Eventually, the amniotes diversified and a dominant line, the dinosaurs, arose. These ruled the Earth for almost 150 million years, getting mostly wiped out at the end of the Cretaceous period 65 million years ago. Modern animals related to dinosaurs are the birds.
A primitive line of mammals that survived the extinction of the dinosaurs diversified and became the dominant form of life on Earth. This line of mammals eventually gave rise to us, humans.
Michael is a longtime WiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.