Lecture 32 Earth History Geologic Epochs Human History Chapter  15.18

Precambrian Time Nothing is known about the continental locations, but there is evidence of plate tectonics at that time. At least 2 glacial periods were identified in the Precambrian. Bacterial life structures found in Greenland were dated to live about 3.6 billion years ago. These are primitive cyanobacteria which formed colonies in the oceans and survived intense solar UV radiation.cyanobacteria The Earth was formed about 4.5 billion years ago. Already 4 billion years old rocks show a sedimentary component, which implies the presence of erosion.

The Paleozoic Era The history of the Paleozoic Era is well-studied because of a wealth of fossils available.fossils The oldest fossils of this era are marine invertebrates, creatures without internal skeletons but with external shells. Small corals, ancestors of starfish and sea urchins, a variety of fishes, and first amphibians (relatives of modern frogs and salamanders) were present in the Devonian Period.Period Fossils of reptiles were found in the Carboniferous Period.Period Coal and petroleum were formed in this period from remains of plants and marine creatures.plants

The Mesozoic Era This era began ~225 million years ago, when the modern continents were joined together in Pangaea. A bit later Pangaea split into Laurasia and Gondwana. North America parted from Europe ~120 million years ago. Reptiles and dinosaurs (descendants of primitive reptiles that had survived from the Paleozoic and became extinct ~65 million years ago) developed in the Mesozoic Era.extinct Flowering plants also existed in this period. The first birds with feathering wings arose from dinosaur ancestors in the Jurassic.birdsJurassic First (small) mammals appeared in the Triassic period.Triassic

The Cenozoic Era Main features of this era which began 65 million years ago:ago No shallow seas Widespread volcanic activity Continuous tectonic disturbance (the reason for dividing the Cenozoic from the Mesozoic era) In mid-Tertiary period the largest European (the Alps) and Asian (the Himalayas) mountains were folded and uplifted. Mountains in the North America were repeatedly uplifted during the Cenozoic, and erosion following the uplifts shaped the present day landscapes.

Mammals Some mammals survived the mass extinction that ended the Mesozoic era thanks to their great adaptability.survived They significantly evolved because of the dinosaur extinction.evolved Mammals dominated the Earth by mid-Tertiary period. Every ~100,000 years glaciers advanced from the mountains to the plains of Europe and North America. However, most mammals survived the ice ages. The last glacial period ended ~10,000 years ago and suggested another one to come in the near future.

Human History Humans seem to appear a few million years ago.few There is biological evidence that the descendants of a common ancestor split into 2 branches (humans and apes) about 6 million years ago. By 2.4 million years ago the line from which modern humans could come had large brains.brains Modern humans, Homo sapiens or wise man, appeared a few 100,000 years ago.sapiens They seem to spread from Africa into Europe and Asia. Humans came to North America ~15,000-20,000 years ago.

Summary Rocks and fossils provide a source of knowledge about the Earth’s history The Earth was formed about 4.5 billion years ago Stable life appeared on Earth in the form of simple bacteria about 3.8 billion years ago The first humans appeared a few million years ago and their population continues to growpopulationgrow