Animals evolved in the Ediacaran Period, resembling jellyfish. The Cambrian explosion saw the emergence of almost all modern animal phyla, dominated by invertebrates. Fish evolved, followed by plants and insects on land. Lobe-finned fish developed legs, leading to amphibians and amniotes. Dinosaurs ruled for 150 million years, followed by mammals and eventually humans.
As far as we know, animals first evolved in the Ediacaran Period, about 610 million years ago. By this time, single-celled organisms had already split into plants, animals, fungi, and other divisions. The earliest known animals resembled cnidarians (jellyfish) and included a series of mysterious mat, oval and bag-like shapes that varied up to a meter in diameter (the Ediacara biota). It is difficult to place how Ediacaran animals related to later forms, and this is currently under debate. At the dawn of the Cambrian 542 million years ago, most of these animals were extinct and replaced by numerous phyla spawned during what is known as the Cambrian explosion.
By the end of the Cambrian explosion about 500 million years ago, representatives of almost all 38 modern animal phyla had emerged. Vertebrates were also represented by jawless primitive fish. At this point, however, there were still no land animals. The Cambrian period was an era dominated by invertebrates, which occupied all the main ecological niches of the sea, including that of the apex predator (Anomalocaris). Many organisms developed complex eyes, armor, and nervous systems for the first time during this period. Predation has kickstarted an evolutionary arms race between predators and prey. Animals related to each other in that most were arthropods.
Over the next 100 million years, fish evolved and became more numerous, solidifying the place of vertebrates alongside 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 feet (3 m) in length. The movement of some arthropods on land meant that the animals related a little less to one another, as different groups evolved in a variety of directions to deal with their respective environments.
About 330 million years ago, lobe-finned fish began to develop primitive legs and began hopping on land. This quickly gave rise to the 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 limiting genetic diversity. Animals bonded to each other more closely than ever. Eventually, the amniotes diversified and a dominant line arose, the dinosaurs. These ruled the Earth for nearly 150 million years, mostly being wiped out at the end of the Cretaceous period 65 million years ago. Modern animals related to dinosaurs are birds.
A primitive line of mammals that survived the extinction of the dinosaurs diversified and became the dominant life form on Earth. This line of mammals eventually gave rise to us humans.
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