History of Life
Evolution Through Time
Eons of Evolution
Major milestones in Earth's biological history
Formation of Earth
Planet formation from solar nebula dust and planetesimals.
First Life
Simple unicellular organisms (prokaryotes) in oceans. Evidence from stromatolites and chemical biomarkers.
Great Oxidation Event
Cyanobacteria produce oxygen, fundamentally changing atmosphere composition and enabling aerobic life.
First Eukaryotes
Complex cells with nucleus and organelles. Major breakthrough enabling larger, more complex organisms.
First Multicellular Animals
Ediacaran fauna: strange soft-bodied organisms in shallow seas, representing earliest animal evolution.
Cambrian Explosion
Rapid diversification of animal phyla. Many modern animal groups appear in the fossil record with remarkable complexity.
Colonization of Land
Plants move onto land, followed by arthropods. Establish soil ecosystems and food webs on continents.
First Vertebrates on Land
Amphibians evolve from fish with limbs. Major transition enabling vertebrate dominance on land.
Amniotes Appear
Evolution of amniotic egg allows complete independence from water. Includes ancestors of reptiles, mammals, and birds.
Permian-Triassic Extinction
Deadliest extinction event—90% of marine species lost. Clears ecological niches for future diversification.
Dinosaurs Emerge
First dinosaurs appear in Triassic. Begin modest diversification before becoming dominant in Mesozoic.
First Mammals
Mammals evolve from mammal-like reptiles. Remain small and inconspicuous during dinosaur dominance.
First Birds
Archaeopteryx shows link between dinosaurs and birds. True birds diversify through Cretaceous.
Cretaceous-Paleogene Extinction
Asteroid impact kills non-avian dinosaurs. Mammals radiate to fill vacated ecological roles.
Human Evolution
Bipedal hominins in Africa. Modern humans appear ~300,000 years ago, spreading globally 70,000 years ago.
Major Evolutionary Themes
Patterns and processes shaping life's history
Adaptive Radiation
After major extinctions or colonization of new environments, organisms rapidly diversify to fill available ecological niches. Examples: Darwin's finches, Hawaiian honeycreepers, Cambrian animals.
Convergent Evolution
Unrelated organisms evolve similar traits when faced with similar environmental pressures. Wings in birds, pterosaurs, and insects; streamlined bodies in ichthyosaurs and dolphins.
Ecological Succession
Predictable sequence of community changes following environmental disruption. Seen in recovery from mass extinctions and colonization of new habitats over geological time.
Increasing Complexity
General trend toward larger body size, complex nervous systems, and sophisticated behaviors. Not universal—many organisms remain simple and highly successful.