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The History of Life
To understand the history and evolution of creatures on our planet, scientists have divided time into various categories, called "Eras". These eras can then broken down into smaller units, or "periods". Below is a brief history of how life evolved from the dawn of time to the current day.
PRECAMBRIAN ERA: 4,500 - 600 million years ago. This era accounts for more than 90% of the total age of the earth. During this time, the earth, seas and the atmosphere formed. About three and a half billion years ago, life began at its simplest level, as single celled organisms. These creatures than began to evolve into ever more complex organisms.
PALEOZOIC ERA: 600-225 million years ago.
Cambrian Period 600-500 million years ago. Invertebrate life flourished, including trilobites, sponges, snails, and brachiopods. All life at this time was in the sea.
Ordovician Period 500-430 million years ago. The first corals and primitive fish which were the first vertebrates (or creatures with backbones ) appeared.
Silurian Period 430-395 million years ago. First land plants appeared. Sea life continued to flourish, and included algae, brachiopods, crinoids, trilobites, and corals.
Devonian Period 395-345 million years ago. First forests developed. Sharks and bony fishes now appeared. First spiders, millipedes and insects appeared. First primitive amphibians.
Mississippian Period 345-310 million years ago. Amphibians continued to develop on the land, and land plants spread in moist areas.
Pennsylvanian Period 310-280 million years ago. Trees grew to be 150 feet tall. Dragonflies with 30 inch wingspans developed, and there were many kinds of amphibians. The first reptiles appeared, developing from their amphibian ancestors.
Permian Period 280-225 million years ago. Ferns and conifers develop. Mammal - like reptiles developed from earlier reptiles. Large reptiles and amphibians lived in swampy areas.
MESOZOIC ERA: THE AGE OF THE DINOSAURS 225-65 million years ago.
Triassic Period 225-190 million years ago. After a great extinction that Wiped out over 90% of the creatures that lived in the Permian Period, other creatures evolved to fill the "niches" left vacant. The first dinosaurs appeared, as did the first mammals. Typical dinosaurs from this period included Coelophysis, a small meat eating creature that walked on two hind Iegs.
Jurassic Period 190-136 million years ago. Aquatic reptiles stalked the seas.
Dinosaurs continued to evolve into more advanced forms, including the armored
Stegosaurs, and the huge meat eating Allosaurs. It was during this time that dinosaurs
started to grow to truly enormous sizes, especially the Sauropods, the group that included
Apatosaurus and Brachiosaurus.
Cretaceous Period 136-65 million years ago. Dinosaurs continued to evolve into more specialized creatures. The armored dinosaurs now included the Ankylosaurs, who had bony armor over their bodies, and a huge club at the end of their tails. Another family of armored dinosaurs from this period were the Ceratopsians, or the horned dinosaurs". These creatures had horns and bony frills, and included such famous creatures at Triceratops and Styracosaurus. Meat eating dinosaurs included the famous Tyrannosaurus rex.
CENOZOIC ERA: THE AGE OF THE MAMMALS 65 million years ago to present
Tertiary Period 65-2 million years ago
At the end of the Cretaceous Period, another huge extinction occurred, wiping out every dinosaur. Many scientists feel this was caused by a huge meteor that smashed into the Earth, but some other experts feel there were other reasons, such as diseases or volcanic activity.
With the dinosaurs no longer around, the mammals could now evolve into larger size creatures to fill the empty ecological niches. Primitive horses, camels, pigs, and animals that would later evolve into our modern cats appeared.
Quaternary Period 2 million years ago - present
Mammals grow to huge sizes, and include the wooly mammoth', the mastodon, and the saber tooth "tiger". The very large mammals go extinct, while early man appears, and evolved into modern Homo sapiens.
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