1) Approximately 6 billion years ago, the Earth formed from a part of the sun that cooled and separated. While the interior remains hot, tectonic forces later caused hills, mountains, and changes to the surface. 2) Around 4 billion years ago, the cooled Earth developed water and an atmosphere, allowing the first living single-celled organisms to form in oceans about 3.5 billion years ago. 3) Multicellular life eventually emerged, followed by more complex organisms like fish, amphibians, reptiles, birds, and later humans. Periods like the ice age were challenging as many species went extinct while others survived.