Evidence of the Past
The History of Life on Earth
Fossils
Fossils: traces or imprints of once-living things
Dead organism is covered by layer of sediment, which presses together to form sedimentary rock
The Ages of FossilsSedimentary 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
Relative Dating
Absolute DatingWhen 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
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
The Geologic Time Scale
Scientists use a type of calendar to divide the Earth’s 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 Earth’s long natural history into manageable parts
Divisions in Geologic Time ScaleDivided 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 agoVolcanic eruptions, meteorites, intense radiation from the sun
Early atmosphere had no oxygen—first 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
Paleozoic Era: rocks rich in fossils of animals such as sponges, corals, clams, squids, and trilobitesFish appeared, sharks more abundant, forests of giant ferns covered earth
Paleozoic Era
Mesozoic Era: dominated by dinosaurs and other reptiles, referred to as Age of ReptilesFirst birds
appeared; flowering plants appeared
By end of era, dinosaurs and many other plants and animal species became extinct
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
Mass ExtinctionsSome 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
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
These changes could have resulted from a giant meteorite hitting the earth, or forces within causing major earthquakes and volcanic eruptions
The Changing EarthPangaea “all
Earth”-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
Wegener called the single landmass “Pangaea” which means “all Earth”
Do the Continents Move?In mid-1960’s, J.
Tuzo Wilson came up with idea that continents were not moving by themselves
Wilson thought that huge pieces of Earth’s crust were pushed by forces within the planet
Each piece of crust is called a tectonic plate
Wilson’s theory of how the huge pieces of crust move is called plate tectonics
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
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
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
Example: Animals currently living in Antarctica are able to survive very cold temperatures. But under the frozen surface are the remains of tropical forests
Conditions have changed many times during Earth’s history
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