1. Early Earth Chapter 15

2. Earth Forms Scientists hypothesize that Earth formed about 4.6 billion years ago. They also believe that Earth started as a ball of dust, rock, and ice in space, pulled together by gravity.

3. Earth’s Surface Forms We call the first 4 billion years the Precambrian Time, when the atmosphere, oceans, and continents began to form. Precambrian time consisted of the Hadean, Archean, and the Proterozoic Eons. About 88% of Earth’s total history.

4. Atmosphere Earth’s first atmosphere was hydrogen, helium, ammonia, and methane. It left after a huge collision with Earth and another object. Our second atmosphere consisted of carbon dioxide, water vapor and nitrogen. Oxygen formed later (photosynthesis)

5. The Oceans Too hot at first for liquid oceans, most water evaporated into water vapor. Then would condense to form rain, which would help to cool the Earth.

6. The Continents Earth’s rock started to cool and harden to form the continents. The first rock was extrusive igneous rock formed from cooled lava.

7. Life Develops Scientists can not pinpoint when or where life began. They have found fossils of single-celled organisms in rocks that formed about 3.5 billion years ago. These earliest life forms were probably similar to present- day bacteria (cyanobacteria).

8. Phanerozoic Eon The Phanerozoic means “visible life”. The Phanerozoic Eon is made up of three eras; Paleozoic Era- means “old life”. Mesozoic Era- means “Age of Reptiles”. Cenozoic Era- means “Age of Mammals”.

9. Paleozoic Era The Paleozoic Era consisted of 7 periods; Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian.

10. Paleozoic Era At the beginning of the Paleozoic Era, a great number of different kinds of organisms evolved. Scientists call this the Cambrian Explosion. Most of these animals were invertebrates, lacking a backbone.

11. Paleozoic Era During this time, jawless fishes evolved. Jawless fishes were the first vertebrates, animals with a backbone. Land plants show up, amphibians and reptiles appear, and insects were abundant. The greatest mass extinction of all time ends the Paleozoic Era, killing plants, animals, and trees on land and in the ocean (96% of all life).

12. Pangaea About 260-250 million years ago, Earth’s continents moved together to form a great landmass, or supercontinent. Theories on why the mass extinction occurred is the climate change of Pangaea, volcanic activity, or asteroid collision.

13. Panthalassa Panthalassa was a single large ocean body that surrounded Pangaea. It actually means “all seas”.

14. Mesozoic Era Reptiles were so successful during the Mesozoic Era that this time is often called the “Age of the Reptiles”. Mammals first appeared (Eozostrodon). Dinosaurs ruled the earth (Jurassic Period is the Golden Age of Dinosaurs). Another mass extinction about 65 million years ago ends the Cretaceous Period.

15. Mesozoic Era Scientists believe that this extinction was caused by an asteroid hitting the Earth in the Gulf of Mexico (Yucatan Peninsula).

16. Cenozoic Era The extinction of dinosaurs created an opportunity for mammals. During this era, mammals evolved to live in many different environments- on land, in water, and even in the air. Ice age (glaciation) occurs when Earth starts to cool and ice layers begin to form over large areas of the Earth.

17. Cenozoic Era Modern day humans show up about 10,000 years ago.