Time Scale of the Earth

EON ERA PERIOD EPOCH  BEGINNING  MAJOR EVENTS GLOBAL
EXTINCTION
EVENT
P
H
A
N
E
R
O
Z
O
I
C		 Cenozoic
 (age of  mammals)		   Quaternary Holocene 11,477±85 present climate; only modern humans  
Pleistocene 1,810,000 recent ice ages; various human species
Tertiary Pliocene 5,300,000 near-human species and other near-modern mammals
Miocene 23,000,000 apes flourish; savanna grazing animals evolve
Oligocene 33,900,000 monkeys, apes, and other mammal families evolve
Eocene 55,800,000 prosimians flourish; possible early monkeys
Paleocene     65,500,000 earliest primates (proto-prosimians)
Mesozoic
 (age of reptiles)		 Cretaceous   145,500,000 archaic mammals and birds begin to replace dinosaurs; flowering plants 65,500,000
 (76% of species lost)
Jurassic 199,600,000 dinosaurs dominant; primitive mammals spread; toothed birds
Triassic   251,000,000 first dinosaurs and first egg-laying mammals 200,000,000
 (80% of species lost)
Paleozoic
   (ancient life forms)		 Permian filler.gif (42 bytes) 299,000,000 spread of reptiles and insects; first mammal-like reptiles 251,000,000
 (95-96% of species lost)
Carboniferous   359,200,000 amphibians dominant; forests flourish; reptiles and modern insects appear
Devonian    416,000,000 fish dominant; amphibians appear; first forests 360,000,000
 (83% of species lost)
Silurian   443,700,000 first land plants; fish with jaws; air breathing animals
Ordovician   488,300,000 invertebrates dominant; first vertebrates (jawless fish) 444,000,000
 (85% of species lost)
Cambrian   542,000,000 invertebrates dominant (worms, jellyfish, trilobites, etc.) 488,000,000?
(% of species lost ?)
P
R
E
C
A
M
B
R
I
U
M		 Proterozoic
 (earliest life forms)		 filler.gif (42 bytes) filler.gif (42 bytes) 3,000,000,000 protozoa, sponges, and algae
3,500,000,000 first clear evidence of  life (one celled bacteria)
Azoic
 (no life forms)		 filler.gif (42 bytes) filler.gif (42 bytes) 4,540,000,000 origin of the earth

"BEGINNING" refers to the number of years before the present to the beginning of the Era, Period, or Epoch. In some cases, the dates differ somewhat from those in other geologic time scales.  Most notably, the origin of the earth is sometimes given as 4.5 or 6 billion years ago.

 [Source: John Relethford (2008), Physical Anthropology, 9th ed.; Robert Jurmain et al. (2008), Introduction to Physical Anthropology, 11th ed.; Philip Stein and Bruce Rowe (2006),The Human Species: an Introduction to Biological Anthropology, 7th ed.; Richard Harter (1998),Changing Views of the History of the Earth; F. M. Gradstein and J.G. Ogg, Geologic Time Scale 2004--Why, How, and Where Next!; and U.S. Geological Survey (2007), The Geologic Time Scale]
  

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This page was last updated on Thursday, May 08, 2008.
Copyright © 1999-2008 by Dennis O'Neil. All rights reserved.