Geologic Time Scale

• The tale of life on Earth has been unfolding for about 4 billion years
• Geologic Time Scale


  The system used to bind all the chapters of life's history together
• Danish scientist Nicolas Steno published the first laws of stratigraphy


  1669
• Steno's laws of stratigraphy


  The layers closer to the surface must be younger than the layers below them
• Italian geologist Giovanni Arduino began naming the layers of rock


  1760s
• Primary layer


  Lowest layers of metamorphic and volcanic rocks
• Secondary layer


  Hard sedimentary rocks above the Primary layer
• Tertiary and Quaternary layers


  Softer alluvial deposits at the top
• English geologist William Smith figured out the solution to the problem of comparing rock layers: fossils


  1819
• Fossils


  Remains of ancient organisms that can be used to match the ages of different rock formations
• Modern scientists have used the work of early geologists to create the Geologic Time Scale (GTS)
• Geologic Time Scale (GTS)

  Organized into five subgroups: Eons, Eras, Periods, Epochs and Ages
• Eons are the largest slices of time, ranging from a half-billion to nearly 2 billion years long
• Hadean Eon


  Begins with the formation of the Earth around 4.6 billion years ago and ends 4 billion years ago
• The Hadean Eon had no fossils because the world was a searing wasteland
• Small amounts of organic carbon found in Hadean rocks may be evidence of the earliest life
• Archean Eon


  Ran from 4 billion to 2.5 billion years ago, when life flourished and formed mats of microbes in the primordial seas
• Proterozoic Eon


  Began 2.5 billion years ago, when photosynthetic bacteria and some multicellular forms of life spewed tons of oxygen into the atmosphere
• Phanerozoic Eon


  The current eon, which began 541 million years ago and is home to life as we know it
• Paleozoic Era


  The first era of the Phanerozoic Eon, defined by the diversification of visible life and the Cambrian explosion
• Trilobites were so common all over the world that they've been used as index fossils for the Paleozoic Era
• The first era of our current eon is the Paleozoic Era, which began 541 million years ago
• The Paleozoic Era was defined by the diversification of visible life, starting with the Cambrian explosion
• The Hadean, Archean, and the Proterozoic eons are collectively known as the Precambrian
• At the start of the Paleozoic, over about 25 million years, the fossil record suddenly reveals the appearance of complex animals with mineralized remains
• Trilobites


  They were so common all over the world that they've been used as index fossils for the Palaeozoic Era
• Fish developed teeth and jaws, and came to dominate the seas, including the first sharks and armored giants known as placoderms
• The land was finally being populated - first by plants and then by arthropods
• By 370 million years ago entire ecosystems had developed on the primeval continents
• The earliest amphibians evolved and hauled themselves out of the water, leaving the first vertebrate footprints in the mud
• 299 million years ago, the supercontinent Pangea had formed, with an enormous desert at its center
• The desert was quickly populated by the ancestors of what would eventually become reptiles and mammals, which could thrive in dry conditions, unlike amphibians
• 252 million years ago, 70% of land vertebrates and 96% of marine species disappeared from the fossil record, including survivors of previous extinctions, like trilobites
• The event, known as the Great Dying, was the most severe extinction in our planet's history
• A possible meteorite impact site off the coast of South AmericaIslands, and massive volcanic eruptions in Siberia, coincided with the end of the Palaeozoic
• The Palaeozoic began with an explosion of life, but ended in nearly absolute death
• It took millions of years for life to recover, but when it did, a new world, The Mesozoic Era, had arrived
• The Mesozoic Era is often called the Age of Reptiles, as reptiles were incredibly successful, including dinosaurs, pterosaurs, and marine species
• All of the non-avian dinosaurs lived only in the Mesozoic, so they remain one of the best index fossils of this era
• The Mesozoic Era came to an end 66 million years ago, with the Cretaceous-Paleogene, or K-Pg, Extinction Event