Life: Its Nature and Origin 57 



Origin of universe about 13-20,000,000,000 years ago 



Origin of oldest known stars 6,500,000,000 years ago 



Origin of solar system 5,000,000,000 years ago 



Origin of earth 4,500,000,000 years ago 



Origin of continents 



and final cool stage 3,500,000,000 years ago 



Origin of life 2,000,000,000 years ago 



Origin of oxidizing atmosphere 1,000,000,000 years ago 



First well-marked fossil beds 



of Paleozoic era over 600,000,000 years ago 



The last 600 million years have been divided into three long 

 eras called the Paleozoic, Mesozoic, and Cenozoic. Each of these 

 three eras is divided into periods and these into epochs. The follow- 

 ing list gives the periods and epochs of the Cenozoic and the 

 periods of the Mesozoic and Paleozoic. The number to the right 

 of the time divisions is the approximate number of million years 

 ago that each subdivision began. At the extreme right are Hsted 

 phenomena for which the various time divisions are best known. 

 The dates are from Kulp (1961). 



Cenozoic— Modern life 

 Quaternary-Pleistocene 1 The ice ages, to the present 



Tertiary— Pliocene 13 



Miocene 25 Dominance of flowering 



Oligocene 36 plants; diversification of 



Eocene 58 mammals 



Paleocene 63 



Mesozoic— Medieval life 



Cretaceous 135 Dominance of cycads, tree 



Jurassic 181 ferns, and conifers; era of 



Triassic 230 dinosaurs 



Paleozoic— Ancient life 



Permian 280 Evolution of modern insect 



orders 



Pennsylvanian 310 The Carboniferous; first great 



Mississippian 345 tropical forests 



Devonian 405 Beginnings of land animals 



Silurian 425 Predominance of marine 



Ordovician 500 invertebrates; rise of land 



Cambrian ? 600 plants 



