Evolution of Multicellular Life

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1. Paleozoic Era

A. Cambrian Period

I. "Cambrian Explosion:" the diversification of life early on in this period

II. First time organisms had hard parts, shells and outer skeletons

III. Invertebrates: worms, jellyfishes, sponges; brachiopods and trilobites

B. Ordovician & Silurian Periods

I. Ancestors of modern squid and octopi, as well as aquatic arthopods

II. Jawless fishes

III. First insects appeared, and first plants evolved from aquatic ancestors

C. Devonian Period

I. Plants like ferns adapted to drier areas

II. Invertebrates and vertebrates thrived in the seas

III. "Age of Fishes"

D. Carboniferous and Permian Periods

I. Reptiles evolved from amphibians

II. Plants such as giant ferns formed vast swampy forests; the remains formed the deposits of sediment that changed into coal

III. At the end of this era, a mass extinction occurred, which affected both plants and animals; as much as 95% of the complex life in the oceans disappeared

2. Mesozoic Era, "Age of Reptiles" - 180 million years

A. Triassic Period

I. Important organisms: fishes, insects, reptiles, and cone-bearing plants

II. First dinosaurs and mammals appeared

B. Jurassic Period

I. Dinosaurs dominated land

II. First birds appeared

C. Cretaceous Period

I. Flying reptiles became extinct

II. New forms of life, including leafy trees, shrubs, and small flowering plants

III. Another mass extinction occurred, wiping out more than half of all plant and animals groups and killing all the dinosaurs

3. Cenozoic Era - "Age of Mammals"

A. Tertiary Period

I. Earth's climates were generally mild and warm

II. Whales, dolphins, insects, flowering plants, etc. flourished

III. Grasses evolved, which encouraged evolution of grazing mammals

B. Quaternary Period

I. Climate cooled, causing a series of ice ages

II. About 20,000 years ago, Earth's climate began to warm

III. Continental glaciers melted, causing sea levels to rise again

IIII. As early as 100,000 years ago, Homo sapiens may have evolved in Africa

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