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2. Earth's Crust Forms
The oldest rock was created when molten lava rose from cracks in the seafloor about 3.8 billion years ago in the Precambrian Period. The rock cooled and hardened creating Earth's early crust. These early rocks are thought to have had the first traces of bacterial life.
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3. A Usable Atmosphere Earth's oxygen level wasn't usable because it was almost nonexistent but around 2.5 billion years ago, the level began to rise. It was thought to have risen because of the Cyanobacteria, a bacteria that lived in the ocean and harnessed the sunlight through photosynthesis. | 4. Ocean's Form 3.8 billion years ago oceans were formed on Earth. The water came from two sources, outgassing from within Earth and comet bombardment. Out gassing is where molten rock releases gas by volcanic activity. Comets and meteoroids brought gases with them when they struck the Earth. The gases stayed in gas form until the Earth cooled to below 100 degrees Celsius where they then became rain and slowly collected on Earth creating the oceans. | 5. Vertebrates appear During the Cambrian Period, vertebrates came to life about 530 million years ago. The first vertebrate was a jawless fish called the agnathan. The agnathan was very common until it became extinct and was replaced by a jawed fish. |
3. A Usable Atmosphere
Earth's oxygen level wasn't usable because it was almost nonexistent but around 2.5 billion years ago, the level began to rise. It was thought to have risen because of the Cyanobacteria, a bacteria that lived in the ocean and harnessed the sunlight through photosynthesis.
4. Ocean's Form
3.8 billion years ago oceans were formed on Earth. The water came from two sources, outgassing from within Earth and comet bombardment. Out gassing is where molten rock releases gas by volcanic activity. Comets and meteoroids brought gases with them when they struck the Earth. The gases stayed in gas form until the Earth cooled to below 100 degrees Celsius where they then became rain and slowly collected on Earth creating the oceans.
5. Vertebrates appear
During the Cambrian Period, vertebrates came to life about 530 million years ago. The first vertebrate was a jawless fish called the agnathan. The agnathan was very common until it became extinct and was replaced by a jawed fish.
6. Plants Begin to Grow on Land
430 million years ago, during the Silurian Period, plants began to grow on land although it was very difficult. Because it was difficult, plants grew tube-like tissues to transport water and nutrients from the water to land. The first plants grew on mushy ground and were not very tall.
8. The Age of the Dinosaurs
240 million years ago a kangaroo-sized animal that was a plant eater and was known as the prosauropods was born. More commonly known dinosaurs such as the Tyrannosauruses Rex were not alive during the Triassic Period but during the Jurassic Period. During the Jurassic Period, dinosaurs dominated the Earth for about 150 million years. There was a wide variety of different dinosaurs, some were plant eaters and some meat eaters.
9. The Beginning of Ice Ages
Ice ages began to take over Earth starting 1.8 million years ago. The size and intensity of each varies, however, each was linked to variations of the Earth's orbit around the sun. These ice ages led to the rise of animals such as woolly mammoths, mastodons, and woolly rhinos, all of which are now extinct. Ice ages ceased to exist because the Earth fell into a normal orbit around the sun.