The Tertiary period begins with the catastrophe that killed off the dinosaurs, about 65 million years ago, and it lasts down to 1.8 million years ago. With nearly all of the dinosaurs gone, there was plenty of room on Earth for the mammals to have lots of babies, and soon there were all different kinds of mammals all over the Earth.
As the continents continued to drift apart, plate tectonics caused a lot of volcanoes. The carbon dioxide released by these volcanoes cooled down the Earth gradually throughout the Tertiary period. Some continents ran into each other: about 60 million years ago California ran into North America, causing the Rocky Mountains.
About six million years ago, the first people evolved out of the early primates in Africa. By 2.5 million years ago, these people were using stone tools and entering the Paleolithic period of human history.
The Tertiary period ended with the planet getting cool enough to cause the last big Ice Age, about 1.8 million years ago. The next age is the age we're still in now; we call it the Quaternary period.
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