Earth’s History

Origin of the Earth

• Nebular Hypothesis– Bodies of our solar system condensed from

an enormous cloud– Cloud began contracting, spinning, and

flattening

• Earth– Density sorted material and eventually, gases

escaped from the interior to form an atmosphere

Earth’s Atmosphere Evolves

• Outgassing– Gases that are in molten rock are gradually

released

• Water vapor condensed to form clouds– Rain cooled Earth’s crust until the rains slowly

filled low areas, forming the oceans– Reduced CO2 because it became trapped in

water

• Plants evolved and released oxygen

Precambrian Time

• 4.6 billion to 570 million years ago

• Precambrian Rocks– Shields

• Areas shaped like shields that have been extensively eroded to reveal Precambrian rock

• Mostly deformed metamorphic rock• Ore deposits

Precambrian Time

• Precambrian Fossils– Stromatolites

• Calcium carbonate deposited by algae• Indirect evidence of algae

– Tiny organisms• Life 3.5 billion years ago• Most preserved in chert, a hard, dense chemical

sedimentary rock

– Prokaryotes found (blue-green algae & bacteria)• Southern Africa—3.1 billion years• Lake Superior—1.7 billion years

Precambrian Time

– Eukaryotes found (green algae)• Bitter Springs, Australia—1 billion years

– Plant fossils date from the middle Precambrian

– Trace fossils of animals• Trails and worm holes• Fossil impressions

– Animal fossils date from the late Precambrian– Diverse and multicelled organisms exist by

the close of the Precambrian

Paleozoic Era—Life Explodes

• 570-245 million years ago

• Hard parts appeared on organisms

• Early Paleozoic History– Mountain building affected eastern North

America—long since eroded

Paleozoic Era—Life Explodes

• Early Paleozoic Life: “Age of Invertebrates”• Restricted to seas

– Cambrian• Trilobites

– Mud-burrowing scavengers– More than 600 genera

– Ordovician—shallow seas• Organisms with hard parts• Brachiopods

– Adults attached to the sea floor, larvae free-swimming

• Cephalopods– Mobile, highly developed mollusks; predators

Paleozoic Era—Life Explodes

• Late Paleozoic History– Devonian, Mississippian, Pennsylvanian, and

Permian periods– Pangaea

• Extreme seasons that caused one of the most dramatic biological declines in history

Paleozoic Era—Life Explodes

• Late Paleozoic Life– Land plants developed– Fishes continued to adapt– Devonian period: “Age of Fishes”– Lobe-finned fish evolved into amphibians– Insects– Extensive coal swamps develop

Paleozoic Extinction

• Climates changed because of the formation of Pangaea

• 75% of amphibians disappeared

• Up to 95% of marine life disappeared

• 1 of 5 mass extinctions

• Cause is uncertain

Mesozoic Era—Age of the Dinosaurs

• 245 to 66 million years ago

• Mesozoic History– Much of world’s land above sea level– Breakup of Pangaea– Mountains of western North America begin

forming

Mesozoic Era—Age of the Dinosaurs

• Mesozoic Life– Survivors of the great Paleozoic extinction– Gymnosperms became dominant– Reptiles adapt to dry climate– Reptiles have shell-covered eggs that can be

laid on the land– One group of reptiles led to the birds– Many reptile groups and other animal groups

became extinct at the close of the Mesozoic

Cenozoic Era—Age of Mammals

• 66 million years ago to present

• North America– Most was above sea level– Mountain building, volcanism, and

earthquakes in the West– In the East: stable with abundant marine

sedimentation; eroded Appalachians were raised by isostatic adjustments

Cenozoic Era—Age of Mammals

– Western North America• Rivers erode and form gorges (Grand Canyon)• Volcanic activity common• Coast ranges form• Sierra Nevada become fault-block mountains

Cenozoic Era—Age of Mammals

• Cenozoic life– Mammals replace reptiles as dominant land

animals– Angiosperms dominate the plant world– Mammals evolve after the reptilian extinctions

of the Mesozoic• Marsupials• Placentals

Cenozoic Era—Age of Mammals

– Mammals diversify rapidly and some groups become very large

• Hornless rhinoceros—nearly 16 feet high• Many large animals became extinct• Humans evolve