1. KEY CONCEPT Multicellular life evolved in distinct phases.

2. Paleozoic
Years: 544 mya-248 mya
Major events: Multicellular organisms first appeared. Members of every major animal group evolved. A huge mass extinction occurred at the end of the era
Cambrian explosion: Cambrian period; a huge diversity of animal species evolved

3. Mesozoic
Years: 248 mya-65 mya
Known as: Age of Reptiles
Major events: Mammals first appeared, crocodiles & dinosaurs arose. A mass extinction occurred & dinosaurs became extinct

4. Cenozoic
Years: 65 mya-Present
Major events: Placental mammals & monotremes evolved & diversified, primates evolved