Week of 2/23/15
Lesson 4: Unit 1 Notes – Diversity of Living Things Textbook
The History of Life on Earth
Objectives:
 Understand fossil formation and the geological timescale
Fossils are preserved by geological processes and help us understand what organisms
lived during different time periods on Earth
Fossil Formation:
1. An organism dies and becomes buried in sediment
2. Organism decomposes and leaves a hollow impression, or mold in the sediment
3. Over time, the mold fills with sediment and forms a cast of the organism
Fossil Dating:
 Sedimentary rock has many layers where the oldest layer is usually at the
bottom
 The layers can give a relative age of a fossil depending on where the fossil is
found
Relative Dating – estimating the age of rocks and fossils based on its position in
sedimentary rock
Absolute Dating – a method that measures the age of fossils or rocks in years; this is a
more precise measurement
 Scientists use what knowledge they understand about isotopes and atoms to
determine the age of a fossil sample
 Atoms (particles that make up all matter) release energy when they naturally
decay and we call the time it takes for half of the unstable atoms in a sample to
decay its half-life
 Scientists measure the ratio of unstable decaying atoms to stable atoms in a
sample and this ratio gives an approximate age of a sample of rock
Geologic Time Scale: a calendar of events that scientists use to outline the history of
life on Earth
 Eras are characterized by the type of organism that dominated Earth at the time
 Eras are further divided into periods which are further divided into epochs
Mass Extinctions – periods in which many species suddenly become extinct; scientists
are unsure what happened exactly, but many think extreme changes in climate on Earth
result in these extinctions
Pangaea
 Alfred Wegener proposed that the continents were once a gigantic landmass and
moved apart to form our now 7 continents
Plate Tectonics is a theory about how the plates of the Earth’s crust move around
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Fossils have been used to divide the geologic history of Earth into four major
parts: the Precambrian, Paleozoic, Mesozoic, & Cenozoic Eras
Precambrian:
 Period from the formation of Earth to the beginning of Paleozoic era, from about
4.6 billion to 542 million years ago
 Early earth atmosphere was filled with gases and there were violent storms,
meteorite impacts and volcanic eruptions
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Theory on Life Formation:
Believed that prokaryotes (single-celled organisms) were first inhabitants since
they didn’t require oxygen, which wasn’t present in the atmosphere at first
Cyanobacteria appeared 3 billion years ago and may have released oxygen into
the oceans and air
Eventually the ozone layer was formed to absorb harmful radiation from the sun;
before the ozone layer life only existed in the oceans and underground
Multicellular Organisms
Appeared in fossil record about 1 billion years ago
Paleozic Era:
 Began 543 million years ago and ended about 251 million years ago
 Animals such as sponges, corals, snails, clams, squids, and trilobites were
abundant
 Plants, fungi and air-breathing animals were slowly colonizing the land
 Believed that mass extinctions wiped out 90% of marine species at that time
Mesozoic Era: means middle life
 Began 251 million years ago and lasted about 185 million years
 The age of the reptiles
 Dinosaurs became extinct about 65 million years ago
Cenozoic Era: means recent life
 Began about 65 million year ago & is present today
 This is the age of mammals, including saber-toothed cats, camels, small horses
and humans