The History of Life on Earth

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The History of Life on Earth

• Evidence of the Past

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Fossils

• Fossils traces or imprints of once-living things
• Dead organism is covered by layer of sediment,
  which presses together to form sedimentary rock

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The Ages of Fossils

• Sedimentary rock has layers, with the oldest
  layers usually on the bottom and newest on the
  top
• Layers where the fossils are found tells a
  scientist the relative age of fossil
• Relative dating Estimating age of fossil by its
  position in the rock layers

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Relative Dating
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Absolute Dating

• When scientists want to determine the age of a
  fossil more precisely, they use absolute dating
  to get an exact age
• Absolute dating method of measuring age of
  object in years

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• Scientists examine atoms, which over time, decay
  by releasing energy
• The time it takes for half a sample of atoms to
  decay is its half-life
• Scientists measure the ratio of stable to
  unstable atoms to determine the age of a sample
  of rock

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The Geologic Time Scale

• Scientists use a type of calendar to divide the
  Earths long history
• Calendar is divided into very long units of time
  since the Earth formed so long ago
• Geologic Time Scale the standard method used to
  divide the Earths long natural history into
  manageable parts

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Divisions in Geologic Time Scale

• Divided into eras, which are characterized by the
  type of organisms that dominated the Earth at
  that time
• Precambrian Time from formation of Earth 4.6
  billion years ago to about 543 million years ago
• Volcanic eruptions, meteorites, intense radiation
  from the sun

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Precambrian Time
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• Early atmosphere had no oxygenfirst organisms
  were prokaryotes (no nucleus)
• Cyanobacteria appeared? produced own food and
  released oxygen
• Ozone layer forms in upper atmosphere and absorbs
  radiation from the sun

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• Paleozoic Era rocks rich in fossils of animals
  such as sponges, corals, clams, squids, and
  trilobites
• Fishes appeared, sharks more abundant, forests of
  giant ferns covered earth

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Paleozoic Era
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• Mesozoic Era dominated by dinosaurs and other
  reptiles, referred to as Age of Reptiles
• First birds appeared flowering plants appeared
• By end of era, dinosaurs and many other plants
  and animal species became extinct

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• Cenozoic Era sometimes called Age of Mammals
• Mammals included mastodons, saber-toothed cats,
  camels, and giant ground sloths
• Included some periods known as ice ages

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Mass Extinctions

• Some of the important divisions in geologic time
  scale mark times when rapid changes happened on
  Earth
• During these times, many species died out
  completely, or became extinct
• When a species is extinct, is does not reappear

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• Periods when many species suddenly become extinct
  are called mass extinctions
• Most scientists think that the extinction of the
  dinosaurs happened because of extreme changes in
  the climate on Earth

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• These changes could have resulted from a giant
  meteorite hitting the earth, or forces within
  causing major earthquakes and volcanic eruptions

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The Changing Earth

• Pangaea
• -German scientist Alfred Wegener noticed that
  the continents of Earth look like pieces of a
  puzzle
• -proposed that long ago the continents formed
  one landmass surrounded by gigantic ocean

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• Wegener called the single landmass Pangaea
  which means all Earth

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Do the Continents Move?

• In mid-1960s, J. Tuzo Wilson came up with idea
  that continents were not moving by themselves
• Wilson thought that huge pieces of Earths crust
  were pushed by forces within the planet

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• Each piece of crust is called a tectonic plate
• Wilsons theory of how the huge pieces of crust
  move is called plate tectonics

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• According to Wilson, outer crust of Earth is
  broken into seven large plates and several
  smaller ones
• Motion of the plates causes continents to move

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Adaptations to Slow Changes

• When conditions on Earth change, organisms may
  become extinct
• A rapid change, such as a meteorite impact, may
  cause mass extinction

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• Slow changes, such as moving continents, allow
  time for adaptation
• Everywhere on Earth, living things are well
  adapted to location where they live
• Yet in that location, there is evidence that
  organisms that lived there in the past were very
  different

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Precambrian Time
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Paleozoic Era
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Mesozoic Era
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Cenozoic Era