Classification

• Beginning of Earth


  ~4.6 billion years ago
• PRECAMBRIAN ERA


  542 million years ago
• Archaean period


  3.5 billion - 2.5 billion years ago
• Single-celled bacteria form in the seas
• Proterozoic period


  2.5 billion to 544 million years ago
• Photosynthetic bacteria cause oxygen levels to rise on earth
• Multicellular organisms such as algae develop
• The first sexual reproduction occurs
• Soft-bodied animals follow hundreds of millions of years later
• PALEOZOIC ERA


  542 million years ago
• Cambrian period


  542 million to 488 million years ago
• Marine invertebrates, such as trilobites, dominate
• "Cambrian explosion"—major diversification of life
• Ordovician period


  488 million to 444 million years ago
• The first vertebrates, such as primitive, jawless fish, form
• Silurian period


  444 million to 416 million years ago
• Fresh water fish and the first fish with jaws appear
• Animals and plants make the move to land
• Devonian period


  416 million to 359 million years ago
• The first fern forests and land-dwelling vertebrates form
• Carboniferous period


  359 million to 299 million years ago
• Coal deposits begin
• Sharks and large primitive trees develop
• The first reptiles appear
• Insects and fern forests spread
• Permian period


  299 million to 251 million years ago
• In the largest mass extinction ever recorded, most marine invertebrates die off
• MESOZOIC ERA


  251 million years ago
• Triassic period


  251 million to 200 million years ago
• Survivors spread and re-colonize
• Jurassic period


  200 million to 146 million years ago
• The age of the dinosaurs
• The first mammals appear around 200 million years ago, and the first birds take to the sky
• Cretaceous period


  146 million to 65 million years ago
• Flowering plants appear
• Mass dinosaur extinctions begin
• The earliest known placental mammal, the mouse-sized Eomaia, appears around 125 million years ago
• CENOZOIC ERA


  65 million years ago
• Tertiary period


  65 million to 1.8 million years ago
• The first primates evolve around 55 million years ago, followed by the first human-like creatures