Topic
Earth History & Geology
Deep time, changing continents, mass extinctions, dating evidence, and the chemical history of Earth.
Start Here
- Geological Time OverviewEarth history is 4.54 billion years long, but the part most familiar from fossils occupies only a thin recent slice. The Phanerozoic Eon — abundant visible animal life — begins only 538.8 million years ago, roughly the last 12% of Earth's history.
Go Deeper
- How We Know the Age of EarthEarth is about 4.54 billion years old. The important part is not memorizing the number; it is understanding why we trust it.
- Radiometric DatingRadiometric dating turns radioactive decay into a clock. If a parent isotope decays with constant decay constant λ, then
- Early Earth: What the Rocks Actually Tell UsA wet planet is not yet a living planet. Before asking how life began, ask a narrower question: what environments were actually available, and how do we know? This is where origin-of-life research meets geology rather than a cinematic reconstruction of a steaming ocean.
- Prebiotic Chemistry: Ingredients Are Not Yet a SystemLife uses carbon compounds, but “organic” does not mean “made by an organism.” The deeper problem is how nonliving chemistry could repeatedly make useful products faster than dilution, decomposition and side reactions removed them.
- RNA and the Problem of Making CopiesA sequence can store information without being able to reproduce it. RNA is interesting because it can potentially join two roles that modern cells distribute among different molecules: a sequence that can be copied and a folded structure that can catalyze chemistry.
- Protocells, Metabolism and Competing Origin HypothesesA catalyst dispersed in an ocean may help every nearby competitor. A useful reaction inside a compartment can preferentially help the compartment containing it. A boundary changes who benefits from chemistry, making it a potential bridge from molecular reactions to reproducing lineages.
- Earliest Life: Fossils, Inference and the Road to OxygenThe oldest surviving evidence of life supplies a minimum antiquity, not a birthday. If a convincing fossil is 3.48 billion years old, life was present by then; it may have begun much earlier. Nor does finding life in one habitat prove that life originated there.
- Great Oxidation EventFor much of early Earth history, the atmosphere contained almost no free oxygen. Photosynthetic microbes were producing oxygen long before it accumulated in the air, because oxygen was rapidly consumed by sinks — reduced volcanic gases, minerals, and especially dissolved iron in the oceans.
- Cambrian ExplosionThe Cambrian explosion is the interval early in the Cambrian when the animal fossil record suddenly becomes much richer and many major body-plan lineages become recognizable. The current international boundary for the start of the Cambrian is 538.8 ± 0.6 million years ago.
- The Great DyingThe Permian–Triassic extinction, about 251.9 million years ago, was the most severe known mass extinction. Exact percentages depend on how the fossil record is analyzed, but estimates commonly put marine species loss above 80%, with catastrophic disruption on land as well.
- Pangaea and Continental DriftPangaea was the supercontinent that assembled by the late Paleozoic and contained most of Earth's continental land. It began breaking apart roughly 200 million years ago, eventually producing today's Atlantic Ocean and continental arrangement.
- End-Cretaceous ExtinctionAbout 66 million years ago, the Cretaceous ended in a mass extinction that eliminated all non-avian dinosaurs along with ammonites and many marine plankton groups. Roughly three-quarters of species are estimated to have disappeared.