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.

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.

Many of the geological time periods end with mass extinction.

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

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.

 

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

Near the end of the Paleozoic (230 mya), all of the continents on earth drifted together to form a large continent called Pangaea.

When Pangaea was formed, many species became extinct due to the reduction in the amount of shallow water habitat.

Land species were no longer isolated. When they came together, similar species competed for the same resources resulting in extinction of many due to interspecific competition.

Explanation and Hypothetical Example of Interspecific Competition

Suppose that there are 2 different species living on 2 different islands in an ocean. The species are very similar in that they both live in the same kind of habitat, eat the same kinds of food and have similar requirements for other resources such as nesting sites. If a bridge were to form between the two islands so that individuals on one island were able to move to the other, the two species would then compete for the same resources. Because the two species are not identical, one is probably better at obtaining food and other resources than the other, and so the less efficient one will decline in numbers and eventually become extinct.

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.

The Cenozoic is the era of modern life.

Organisms:

animals: abundance of birds, mammals, and insects

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

Event	MYA (million years ago)
Formation of the earth	4600
Oldest fossils	3600
First eukaryotes	1500
First multicellular organisms	650
First animals	600
Beginning of the Paleozoic Evolution of hard parts	570
Seedless land plants become common	400
First vertebrates (fish)	400
Forests that formed fossil fuels (coal and oil)	300
Beginning of the Mesozoic Pangaea formed Mass extinctions occurred	245
Gymnosperms became abundant	245
First dinosaurs and mammals	245
First Angiosperms	144
Beginning of the Cenozoic Extinction of the Dinosaurs Angiosperms became dominant	66
Hominids (ancestors of humans)	3
Earliest humans (Cro-Magnon)	0.1
Human recorded history (5,000 yrs)	0.005