The History of Life on Earth As We Know It. The History of Earth Earth is ~ 4.5 billion years old Earth’s history is divided into four eons –Hadean Eon:

The History of Life on Earth

As We Know It

The History of Earth• Earth is ~ 4.5 billion years old

• Earth’s history is divided into four eons

– Hadean Eon: (Gk: Hades - the netherworld)

– Archean Eon: (Gk: Arche - ancient)

– Proterozoic Eon: (Gk: Protero - former)

– Phanaerozoic Eon (Gk: Phanero - visible, apparent)

The History of Earth• The most recent eon is divided into three eras

– Paleozoic Era (Gk: Paleo - ancient)

– Mesozoic Era (Gk: Meso - middle)

– Cenozoic Era (Gk: Ceno - recent)

The History of Earth• eras of the modern eon are divided into

periods– Paleozoic Era

• Cambrian Period• Ordovician Period• Silurian Period• Devonian Period• Carboniferous Period• Permian Period

The History of Earth• eras of the modern eon are divided into

periods– Mesozoic Era

• Triassic Period• Jurassic Period• Cretaceous Period

– Cenozoic Era• Tertiary Period• Quaternary Period

Earth’s History: Changing ConditionsTable 22.1

Earth’s History: Changing Conditions

• atmospheric oxygen concentration has risen from near 0 to ~21%

• See Figure 22.3



• mean temperature has fluctuated significantly• sea level has fluctuated significantly• continents have entirely changed positions• relatively gradual changes

rapid, climate-changing events• “catastrophic” events have modified the

evolution of life

Log-log Oxygen levelsFigure 22.4

Changes in

Earth’s Mean

Temperature in the

Modern EonFigure 22.5

Changes in

Sea Level in the

Modern EonFigure 22.2



• mean temperature has fluctuated significantly

• sea level has fluctuated significantly

• continents have entirely changed positions

mid-Cambrian

Continental Drift

(~510 mya)

(forming Gondwana)Figure 22.9

Devonian Drift (400-350 mya)Figure 22.11

Permian Drift (formation of Pangaea)[~290-250 mya]

Figure 22.13

Cretaceous Continental Positions(~100 mya)Figure 22.15

“catastrophic” events have modified the evolution of life

• Dramatic climate change

– End of Ordovician, Devonian, Permian, Triassic, Cretaceous

• Volcanism

– End of Permian

– During Triassic, Cretaceous

• Meteorite collisions

– End of Triassic, End of Cretaceous

Figure 22.6

“catastrophic” events have modified the evolution of life

• several mass extinctions have occurred

– repeatedly, up to 75% of species became extinct

– afterward, new life forms proliferated and became dominant - evolutionary radiation

Dating events in Earth’s history• absolute dates are estimated from the decay of

radioactive elements

• relative ages are determined by the position in a series of rock layers

– remains of dead organisms are mineralized (fossilized) under the right conditions

• anaerobic

• undisturbed

• proper geochemistry

Dating events in Earth’s history• absolute dates are estimated from the decay of

radioactive elements

half-lives of some radioactive isotopes14C - 5,700 years40K - 1.3 x 106 years238U - 4.5 x 109 years

Figure 22.1

Figure 22.8

Figure 22.7

changing faunasFigure 22.17


• Earth’s past biotas are represented by fossilized remains

~300,000 described species

• Earth’s different ages are characterized by different types of fossils

• boundaries defining the eras and periods of the modern eon represent changes in fossil assemblages

The History of Life on Earth• Fossils from successive ages reveal patterns

– particular fossil types are found in rocks of the same age

– new types of organisms appear sequentially in younger rock layers

– types of organisms in shallower (younger) layers more closely resemble extant organisms

– the appearance of new types of organisms occurred at different rates through time

three dominant world faunasFigure 22.17


• Fossils from successive ages reveal patterns

– three faunas dominated animal life over time

– size and complexity of organisms increased with time

– predators became more efficient; defenses became more effective

– extinction happened• a typical species exists for ~10,000,000 years

• ~99% of species are extinct



– few innovations

• novel structures are rare

• most diversity is variation on existing body plans



– microevolutionary change:

• change that modifies species

Sticklebacks - no predatory fish, short spines

Figure 22.20



– macroevolutionary change:

• change that produces new types of organisms


• The history of Earth is characterized by dramatic changes

– gradual structural and climatic changes punctuated by catastrophic events

• The history of life on Earth is characterized by increases in size, complexity, competition & predation

Geologic History from a

Biologist’s Point of View

Precambrian ~87% of Geological Time Scale

EonEra Period "Age"

Phanaerozoic

Paleozoic

Mesozoic

Cenozoic

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Cambrian

Ordovician

SilurianDevonian

CarboniferousPermian

Age of the

Mammals

Age of the

Reptiles

Age of the Amphibians

Age of the Fishes

Age of the Invertebrates

Documents

The History of Life on Earth As We Know It. The History of Earth Earth is ~ 4.5 billion years old Earth’s history is divided into four eons –Hadean Eon: