(NOTE: All ages are approximate, especially the oldest ones)
- 4.5 billion years ago: age of the oldest rocks
- 4.1 billion years ago: age of the oldest fossils; atmosphere was hydrogen-rich ("reducing")
- 2.5 to 1.5 billion years ago: Age of blue-green Cyanobacteria;
gradual accumulation of oxygen in the atmosphere, making "oxidizing" conditions.
- 1 billion years: estimated origin of animal and plant kingdoms, but few fossil remains
- 530 million years (start of Cambrian period): Evolution of many organisms with hard parts capable of leaving a good fossil record.
Life's history is thus much better documented from this point on. Most major animal groups (phyla) now represented. Trilobites abundant.
- 450 million years (Ordovician period): first vertebrate (backboned) fossils
- 430 million years (Silurian period): first land plants; also first insects
- 410-360 million years (Devonian period): "Age of Fishes"
- 330 million years (Mississippian period): first amphibians
- 340-300 million years ago (Mississippian and Pennsylvanian, or "Carboniferous"): Age of great "coal swamps" of seed-ferns or Pteridosperms,
whose abundant fossilized remains formed most of the world's great coal deposits.
- 310 million years (late Pennsylvanian): first reptiles
- 250 million years (end of Permian): "Great Dying", including the extinction of an estimated 30% of all species (including the last trilobites)
- 250-65 million years (Mesozoic Era): "Age of Reptiles", including marine reptiles and dinosaurs
- 220 million years (Triassic period): first mammals
- 200-150 million years (Jurassic period): Jurassic dinosaurs (Brontosaurus, Stegosaurus, Allosaurus, etc.); first flowering plants.
- 150-65 million years (Cretaceous period): Cretaceous dinosaurs (Tyrannosaurus, Triceratops, etc.); flowering plants slowly replace cycads.
- 65 million years (end of Cretaceous): asteroid impact; extinction of many reptile groups including non-avian dinosaurs; explosive evolution of mammal diversity begins.
- 65-1.5 million years (Tertiary period): "Age of Mammals"
- 50 million years (late Eocene epoch): first Catarrhine (ape-like) primates
- 4.1 million years (early Pliocene epoch): first evidence of upright walking in humans
- 2.3 million years (late Pliocene epoch): South African Australopithecus fossils
- 1.75 million years (late Pliocene epoch): human fossils at Olduvai Gorge, Tanzania
- 800,000 years (lower Pleistocene): first appearance of Homo erectus
- 15,000 years (end of Pleistocene): retreat of Ice Age glaciers
- 8,000 years: origin of agriculture in Fertile Crescent of Middle East
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