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Biology LibreTexts

History of Life

Continental Drift

The earth’s crust is divided into 7 major plates that are "floating" on a less rigid material, the mantle.

Movement within the mantle and volcanic activity at the boundary of the plates pushes the plates apart. The volcanic activity adds new material to the crusts from deeper within the earth.

Ocean trenches are formed as one plate rides on top of another, pushing it down into the earth.

The pressure of plates pushing against each other at their boundaries is often sufficient to melt, deform, or uplift rock, producing volcanic activity and mountains. Earthquakes are also common in these regions.

Time

Earth's history can be divided into five major time units called eras. The first two eras (the archean and the proterozoic) will be grouped together and called the: Precambrian. The next three eras are the Paleozoic, Mesozoic, and Cenozoic. These three eras are further divided into units called periods. For example, the Mesozoic era contains the Triassic, Jurassic, and Cretaceous periods.

Major events in the history of life are shown on the timeline diagram below.

Many of the geological time periods end with mass extinction. Five major mass-extinctions are shown in the diagram below.

Precambrian

The Precambrian includes all of the eons of time prior to the Phanerozoic. The Phanerozoic eon includes the Paleozoic, Mesozoic, and Cenozoic eras.

The most recent eon of time, the Phanerozoic, is divided into the Paleozoic, Mesozoic, and Cenozoic eras. The eons prior to the Phanerozoic collectively form the Precambrian.

Relatively few fossils from the Precambrian have been found because hard bones and shells did not evolve until the end of the Precambrian.

Organisms:

Prokaryotes became abundant; protists, fungi, and invertebrate animals evolved.

 

Paleozoic Era

The Paleozoic begins with the appearance of organisms with hard parts (see diagram above).

Organisms:

Plants evolved, became common, and expanded into terrestrial environments. Our fossil fuels (coal, oil, natural gas) came from the remains of dense forests of club mosses, horsetails, and ferns (seedless plants).

Marine invertebrates were common at the beginning of this era. Next came the first vertebrates (fish) followed by amphibians, then reptiles. Insects also evolved during this period.

Extinction at the end of the Paleozoic

Approximately 251 mya, near the end of the Paleozoic, all of the continents on earth drifted together to form a large continent called Pangaea.

The fossil record for marine organisms indicates that approxivmately 96% of marine species went extinct.

A high degree of volcanic activity in the region that is now Siberia covered 1.6 million km2 with lava. Volcanic activity is thought to have triggered a number of ecological events that led to mass extinction. Hot lava may have ignited thick deposits of coal, which released large amounts of CO2 causing global warming and ocean acidification. The effects of volcanic activity may have also led to a decline in oxygen availability in the oceans and the production of toxic H2S gas.

Mesozoic Era

Pangaea began to break up during the early Mesozoic and many new species evolved as a result of the formation of new continents.

The Mesozoic ends with mass extinctions including the dinosaurs. It may have been caused by an asteroid colliding with the earth.

Organisms:

Gymnosperms (plants with seeds but no fruits, example: evergreen trees with needles) became abundant by the beginning of the Mesozoic. Angiosperms first appeared by the middle of the mesozoic but became dominant during the the next (Cenozoic) era.

Dinosaurs evolved, then became extinct. The first birds and mammals appeared. Mammals remained small and insignificant while the dinosaurs were dominant. The extinction of the dinosaurs at the end of the Mesozoic led to the diversification of mammals in the Cenozoic.

Cenozoic Era

The Cenozoic is the era of modern life.

Organisms:

animals: abundance of birds, mammals, and insects

plants: Angiosperms (flowering plants) became the dominant plants.

Important Dates

Event

MYA (million years ago)

Human recorded history (5,000 yrs)

0.005

Earliest humans (Cro-Magnon)

0.2

Hominids (ancestors of humans)

6.5

Beginning of the Cenozoic
Extinction of the Dinosaurs
Angiosperms became dominant

66

First Angiosperms (flowering plants)

144

First dinosaurs and mammals

245

Gymnosperms became abundant (This group includes evergreen trees.)

245

Beginning of the Mesozoic
Pangaea formed

Mass extinctions occurred

251

Forests that formed fossil fuels (coal and oil)

300

First tetrapods

365

First vertebrates (fish)

400

Arthropods colonized the land (This group includes insects.)

420

Seedless land plants become common

450

The colonization of land

500

Beginning of the Paleozoic
Evolution of hard parts; many current animal phyla appeared; the Cambrian Explosion

542

Proliferation of multicellular organisms

575

First animals

635

First multicellular organisms

1200

First eukaryotes

2100

Oldest fossils

3500

Formation of the earth

4600