| Eon | Era | Period | Onset | Major physical changes on Earth | Major events in the history of life |
|---|
| Cenozoic | Quaternary (Q) | 2.6 mya | Cold/dry climate; repeated glaciations | Humans evolve; many large mammals become extinct |
| Tertiary (T) | 65.5 mya | Continents near current positions; climate cools | Diversification of birds, mammals, flowering plants, and insects |
| Mesozoic | Cretaceous (K) | 145.5 mya | Laurasian continents attached to one another; Gondwana begins to drift apart; meteorite strikes near current Yucatán Peninsula at end of period | Dinosaurs continue to diversify; mass extinction at end of period (~76% of species lost) |
| Jurassic (J) | 201.6 mya | Two large continents form: Laurasia (north) and Gondwana (south); climate warm | Diverse dinosaurs; radiation of ray-finned fishes; first fossils of flowering plants |
| Triassic (Tr) | 251.0 mya | Pangaea begins to drift apart; hot/humid climate | Early dinosaurs; first mammals; marine invertebrates diversify; mass extinction at end of period (~65% of species lost) |
| Phanerozoic (~0.5 billion years long) | Paleozoic | Permian (P) | 299 mya | Extensive lowland swamps; O2 levels 50% higher than present; by end of period continents aggregate to form Pangaea, and O2 levels drop rapidly | Reptiles diversify; giant amphibians and flying insects present; mass extinction at end of period (~96% of species lost) |
| Carboniferous (C) | 359 mya | Climate cools; marked latitudinal climate gradients | Extensive fern/horsetail/giant club moss forests; first reptiles; insects diversify |
| Devonian (D) | 416 mya | Continents collide at end of period; giant meteorite probably strikes Earth | Jawed fishes diversify; first insects and amphibians; mass extinction at end of period (~75% of marine species lost) |
| Silurian (S) | 444 mya | Sea levels rise; two large land masses emerge; hot/humid climate | Jawless fishes diversify; first ray-finned fishes; plants and animals colonize land |
| Ordovician (O) | 488 mya | Massive glaciation; sea level drops 50 meters | Mass extinction at end of period (~75% of species lost) |
| Cambrian (C) | 542 mya | Atmospheric O2 levels approach current levels | Rapid diversification of multicellular animals; diverse photosynthetic protists |
| Proterozoic | Collectively called the Precambrian (~4 billionyears long) | 2.5 bya | Atmospheric O2 levels increase from negligible to about 18%; “snowball Earth” from about 750 to 580 mya | Origin of photosynthesis, multicellular organisms, and eukaryotes |
| Archean | 3.8 bya | Earth accumulates more atmosphere (still almost no O2); meteorite impacts greatly reduced | Origin of life; prokaryotes flourish |
| Hadean | 4.5–4.6 bya | Formation of Earth; cooling of Earth’s surface; atmosphere contains almost no free O2; oceans form; Earth under almost continuous bombardment from meteorites | Life not yet present |
| Note: mya, million years ago; bya, billion years ago. |