Eons on this page are not to scale. Hadean Eon Archean Eon Proterozoic Eon The earth formed 4.6 billion The Archean Eon began with the During the Proterozoic Eon, years ago. During this period, first signs of life. Fossils from oxygen built up in the the earth cooled and water this time are so small, scientists atmosphere. The earth may also appeared. must use microscopes to see have experienced some them. “snowball earth” periods. The Hadean Eon lasted 600 The Archean Eon lasted 1.5 The Proterozoic Eon lasted 1.96 million years. If this scale billion years. If this scale showed billion years. If this scale showed showed it at actual size, it would it at actual size, it would be it at actual size, it would be over be nearly 7 pages long. nearly 17 pages long. 21 pages long. 4.6 billion 4 billion 4 billion 2.5 billion 2.5 billion 540 million years ago years ago years ago years ago years ago years ago 1 1 inch = 10 million years Paleozoic Era Cambrian Period Ordovician Period Silu Life spread into many The first corals appeared, species very quickly, an and more species of animals event that scientists call the developed shells. The earth Cambrian Explosion. experienced an ice age. Land formed into the continent Gondwana, but no life lived on land yet. 540 million 485 million 485 million 445 million 445 years ago years ago years ago years ago years 2 rian Period Devonian Period Carboni Fish developed Scientists call the Devonian Mississipp jawbones, which Period the “Age of Fishes.” they passed on Fish developed fins and to their began to dominate the seas. descendants. Most of the fish that did not have jawbones went extinct. Plants sprouted Some fish crept onto land, on land. where the first forests were growing. million 419 million 419 million 359 million 359 millio ago years ago years ago years ago years ago 3 1 inch = 10 million years ferous Period Permian Period ian Subperiod Pennsylvanian Subperiod All land masses merged into one continent, Pangaea. Scientists in the United Gondwana moved Mammal-like reptiles, such States divide the toward the south pole as Dimetrodon, dominated Carboniferous Period and glaciers formed on this continent. into two subperiods. the land. The air was During both high in oxygen, At the end of the Permian subperiods, rainforests leading to giant Period, a major catastrophe were buried and insects. killed 95% of all life on fossilized into coal. earth. Scientists do not yet The first reptiles understand what caused this Amphibians appeared. appeared. Amphibians disaster. laid eggs in the water, but reptiles laid eggs on land. n 323 million 323 million 299 million 299 million 252 mi years ago years ago years ago years ago years 4 Mesozoic Era—Age of Reptiles Triassic Period Jurassic Period Early Middle Late Early Middle Life slowly recovered from Reptiles like plesiosaurs The first the Permian extinction. The and ichthyosaurs true first dinosaurs appeared, but flourished in the oceans. mammals most were small. Non- Dinosaurs began to appeared. dinosaur reptiles dominated dominate the land. the land. The Triassic Period ended with an extinction event in the oceans. llion 252 million 201 million 201 million ago years ago years ago years ago 5 1 inch = 10 million years Cretaceous Period Late Early Late Pangaea split into New dinosaurs appeared, The continents two continents. such as spinosaurs. continued to spread. Crocodiles and birds Ichthyosaurs went extinct. Pliosaurs went extinct, first appeared, and The first flowering plants replaced by mosasaurs dinosaurs continued appeared. and sharks. Dinosaurs to grow larger. dominated the land. The Cretaceous Period ended abruptly when a meteor struck the earth, killing 75% of all life. 145 million 145 million years ago years ago 6 Cenozoic Era—Age of Mammals Quaternary Paleogene Period Neogene Period Period Paleocene Eocene Epoch Oligocene Miocene Epoch Epoch Epoch Mammals This epoch started The first Mammals and very out as one of the grasslands changed in large birds hottest periods earth appeared. response to grass take the experienced. Forests Mammals overtaking other place of covered most land. spread plant life. Some Pliocene Epoch Holocene Epoch dinosaurs. rapidly. mammals became Pleistocene Epoch The Eocene Epoch very large. ended with rapid cooling and ice caps forming at the earth’s poles. Continents neared their present present their neared Continents First hominids appeared. locations. Modern humans appeared. Many extinct. large mammals went Glaciers began to recede. We live in the Holocene Epoch of the Quaternary Period of the Cenozoic Era. The Holocene Epoch began 10,000 years ago. Written history began 6,000 years ago. 66 million 66 million 23 million 23 mil 2.6 mil years ago years ago years ago years ago years ago Copyright Waco Mammoth Site, 2.6 mil years ago to present December 2014. 7 .