4.5 Billion Years Ago
Earth forms — molten rock, no atmosphere, constant meteorite bombardment
3.8 Billion Years Ago
First Life — single-celled microbes (Archean Eon); Earth's oceans form
2.5 Billion Years Ago
Great Oxidation Event — cyanobacteria flood the atmosphere with oxygen; most anaerobic life goes extinct
541 Million Years Ago
Cambrian Explosion — sudden diversification of complex animal life; eyes, skeletons, and predators appear for the first time
443 Million Years Ago
Ordovician-Silurian Extinction — first mass extinction; ~85% of marine species lost due to glaciation
419 Million Years Ago
Devonian Period — Age of Fish; first forests, first amphibians crawl onto land
359 Million Years Ago
Late Devonian Extinction — ~75% of species lost; reef ecosystems collapse
299 Million Years Ago
Permian Period — first reptiles dominate; vast coal forests; Pangaea supercontinent assembles
252 Million Years Ago
"The Great Dying" — Permian-Triassic Extinction; 96% of all species wiped out — largest mass extinction in Earth's history
230 Million Years Ago
Triassic Period — first dinosaurs appear; Pangaea begins to split; first mammals emerge
201 Million Years Ago
Jurassic Period — dinosaurs dominate the planet; first birds evolve; Atlantic Ocean begins to open
145 Million Years Ago
Cretaceous Period — T-Rex, Triceratops; flowering plants appear; modern continents take shape
66 Million Years Ago
K-Pg Mass Extinction — asteroid impact ends the age of dinosaurs; ~75% of species lost; mammals rise to dominance
56 Million Years Ago
Eocene Epoch — early primates diversify; warm global climate; ancestors of modern mammals proliferate
23 Million Years Ago
Miocene Epoch — great apes emerge and diversify in Africa; grasslands spread
6 Million Years Ago
First Hominids — split from chimpanzees; emerge in Africa's Great Rift Valley
2.5 Million Years Ago
Paleolithic begins — first stone tools crafted in Africa; Homo habilis
300,000 Years Ago
Homo Sapiens — Born in Africa
70,000 Years Ago
Humans migrate out of Africa — all modern humans descend from this exodus
10,000 BC
Neolithic Revolution — agriculture begins; humans transition from hunter-gatherers to settled communities