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