The geologic time scale divides Earth's 4.6-billion-year history into eons, eras, periods, epochs, and ages. These divisions mark major shifts in life and environments, from the fiery Hadean to the diverse Phanerozoic eon we live in today.

Mass extinctions have shaped life's evolution, wiping out up to 95% of species in short spans. Five major extinctions reset Earth's ecosystems, creating opportunities for new dominant groups. Understanding these events helps us grasp the current biodiversity crisis.

Geologic Time Scale Structure

Divisions of geologic time scale

• Eons largest units span billions of years divide Earth's history into four major phases
  • Hadean Earth's fiery formation and early cooling 4.6 to 4.0 billion years ago
  • Archean first continents and primitive life forms 4.0 to 2.5 billion years ago
  • Proterozoic oxygen-rich atmosphere and complex cells 2.5 billion to 541 million years ago
  • Phanerozoic diverse multicellular life 541 million years ago to present (dinosaurs, mammals)
• Eras subdivide eons represent major shifts in Earth's life and environments
  • Paleozoic marine life explosion to first reptiles 541 to 252 million years ago
  • Mesozoic age of reptiles and dinosaurs 252 to 66 million years ago
  • Cenozoic rise of mammals and modern flora 66 million years ago to present
• Periods further subdivide eras marked by significant geological or biological events
  • Mesozoic era periods Triassic Jurassic and Cretaceous each with distinct fauna and flora
• Epochs subdivide periods reflect more subtle environmental or evolutionary changes
  • Quaternary period epochs Pleistocene (ice ages) and Holocene (current warm period)
• Ages smallest units fine-scale divisions within epochs
  • Pleistocene epoch ages Calabrian and Gelasian represent specific climate phases

Boundaries between major eras

• Precambrian-Paleozoic boundary (541 million years ago)
  • Cambrian explosion rapid diversification of complex multicellular life (trilobites, mollusks)
  • Marks transition from microbial-dominated world to macroscopic animal life
• Paleozoic-Mesozoic boundary (252 million years ago)
  • Permian-Triassic extinction event wiped out ~95% of marine species and 70% of terrestrial vertebrates
  • Set stage for dinosaur dominance in Mesozoic era
• Mesozoic-Cenozoic boundary (66 million years ago)
  • Cretaceous-Paleogene extinction event caused by asteroid impact ended reign of non-avian dinosaurs
  • Paved way for mammalian diversification and eventual human evolution

Major Events in Earth's History

Events of Phanerozoic periods

• Cambrian (541-485 million years ago)
  • Emergence of most animal phyla established basic body plans (arthropods, chordates)
  • Development of hard body parts and exoskeletons enabled fossil preservation
• Ordovician (485-444 million years ago)
  • Diversification of marine invertebrates (crinoids, bryozoans)
  • First vertebrates appeared as jawless fish (ostracoderms)
• Silurian (444-419 million years ago)
  • First land plants colonized shorelines (mosses, liverworts)
  • Formation of coral reefs created new marine ecosystems
• Devonian (419-359 million years ago)
  • Age of Fishes saw diversification of jawed and bony fish (sharks, ray-finned fish)
  • First amphibians and insects ventured onto land
• Carboniferous (359-299 million years ago)
  • Extensive coal forests dominated by giant ferns and horsetails
  • Diversification of amphibians and early reptiles in swampy environments
• Permian (299-252 million years ago)
  • Formation of Pangaea supercontinent altered global climate and ocean circulation
  • Diversification of reptiles including ancestors of mammals (synapsids)
• Triassic (252-201 million years ago)
  • Recovery from Permian extinction led to new ecological niches
  • Appearance of first dinosaurs and mammals as small, nocturnal creatures
• Jurassic (201-145 million years ago)
  • Dominance of dinosaurs saw rise of long-necked sauropods and theropods
  • First birds evolved from theropod dinosaurs (Archaeopteryx)
• Cretaceous (145-66 million years ago)
  • Peak of dinosaur diversity included famous species (Tyrannosaurus, Triceratops)
  • Diversification of flowering plants revolutionized terrestrial ecosystems
• Paleogene (66-23 million years ago)
  • Rapid evolution of mammals filled niches left by extinct dinosaurs
  • Formation of modern continents through continued plate tectonics
• Neogene (23-2.6 million years ago)
  • Evolution of hominids in Africa led to human ancestors
  • Global cooling and expansion of grasslands shaped modern mammal fauna
• Quaternary (2.6 million years ago to present)
  • Ice ages and glacial cycles dramatically reshaped landscapes
  • Evolution and spread of modern humans led to current Anthropocene era

Mass extinctions in Earth's history

• Mass extinctions rapid loss of significant percentage of plant and animal species within short geological timeframe
• Five major mass extinctions recognized in Earth's history
  1. End-Ordovician (444 million years ago) eliminated ~85% of marine species
  2. Late Devonian (375-360 million years ago) wiped out ~75% of species over extended period
  3. End-Permian (252 million years ago) largest known extinction eliminated ~95% of marine species
  4. End-Triassic (201 million years ago) cleared way for dinosaur dominance
  5. End-Cretaceous (66 million years ago) famously ended non-avian dinosaur reign
• Significance of mass extinctions
  • Dramatic shifts in ecosystems and biodiversity reset evolutionary trajectories
  • Creation of evolutionary opportunities for surviving species led to new dominant groups
  • Alteration of global biogeochemical cycles changed Earth's carbon and nutrient flows
• Potential causes of mass extinctions
  • Asteroid or comet impacts (Chicxulub crater linked to End-Cretaceous event)
  • Volcanic eruptions and flood basalt events released massive amounts of greenhouse gases
  • Rapid climate change warming or cooling stressed ecosystems beyond adaptability
  • Changes in atmospheric or oceanic composition altered habitats (ocean acidification)
  • Sea level fluctuations exposed or flooded crucial habitats
• Modern relevance
  • Study of past extinctions informs understanding of current biodiversity crisis and potential outcomes
  • Insights into ecosystem resilience and recovery guide conservation efforts