Geologic Time Scale: Eons and Early Earth

• The Geologic Time Scale organizes Earth's 4.6 billion-year history, divided into Eons, Eras, Periods, and Epochs. Each division marks significant geological, climatic, and biological events, including evolution and extinctions.

Eons: The Largest Time Divisions

• Key facts about the Hadean Eon (4.6-4.0 billion years ago):
• Earth formed from a cloud of gas and dust around 4.6 billion years ago.
• The Moon formed after a collision with a Mars-sized body.
• Intense meteorite bombardment and volcanic activity occurred.
• Early Earth was molten with no life.
• First oceans formed as the planet cooled.
• No rock record exists due to Earth's unstable surface.
• Key facts about the Archean Eon (4.0-2.5 billion years ago):
• Earth's crust stabilized; continents and oceans formed.
• First life: Single-celled prokaryotes (including bacteria) evolved.
• Photosynthesis: Cyanobacteria produced oxygen, forming stromatolites.
• The atmosphere was mainly methane, ammonia, and water vapor, with little oxygen.
• Key facts about the Proterozoic Eon (2.5 billion-541 million years ago):
• Great Oxygenation Event (~2.4 billion years ago): Oxygen levels rose due to cyanobacteria.
• The first eukaryotes (cells with nuclei and organelles) emerged.
• Multicellular life: Soft-bodied organisms like the Ediacaran fauna appeared.
• Snowball Earth (~720-635 million years ago): Earth experienced extreme glaciations.
• Marks the lead-up to the "Cambrian Explosion".
• Key facts about the Phanerozoic Eon (541 million years ago-Present):
• Known as the "Eon of Visible Life."
• Life diversified significantly; complex ecosystems emerged.
• It is divided into three Eras: Paleozoic, Mesozoic, and Cenozoic.

Eras, Periods, Epochs, and Important Events

• Key facts about the Paleozoic Era (541-252 million years ago):
• Known as the "Age of Ancient Life."
• Consists of six periods from rapid diversification of life to the largest mass extinction.
• Key facts about the Cambrian Period (541-485 million years ago):
• Cambrian Explosion: Rapid increase in biodiversity; most animal phyla appeared.
• Early marine ecosystems had trilobites and brachiopods.
• First chordates (ancestors of vertebrates) appeared.
• First jawless fish at the end of the Cambrian Period.
• No life existed on land; all organisms were marine.
• Key facts about the Ordovician Period (485-443 million years ago):
• A cold climate
• First corals
• Marine algae and sponges appeared.
• First jawless fish evolved into early vertebrates, becoming armored fish.
• Plants began colonizing land (non-vascular plants like mosses).
• Major glaciation and sea-level drop caused the Ordovician-Silurian Extinction, wiping out ~85% of species.
• Key facts about the Silurian Period (443-419 million years ago):
• Vascular plants evolved, enabling plants to grow taller.
• Early terrestrial arthropods colonized land.
• Coral reefs expanded.
• Jawless fish evolved into jawed fish.
• Key facts about the Devonian Period (419-359 million years ago):
• Known as the "Age of Fishes."
• Sea level was high.
• Jawed fishes, sharks, and lobe-finned fishes diversified.
• First forests, ferns and seed-bearing plants, but no flowering plants appeared.
• First tetrapods (proto-amphibians) emerged.
• Trilobites are declining.
• The Late Devonian Extinction likely occurred due to oxygen depletion in oceans (~75% of species lost).
• Key facts about the Carboniferous Period (359-299 million years ago):
• Divided into Mississippian and Pennsylvanian Epochs.
• Vast swamps formed; coal deposits were created from decaying plants.
• Carbon and oxygen levels were high, supporting giant insects like Meganeura
• Early amniotes (reptile-like ancestors of mammals) appeared.
• Key facts about the Permian Period (299-252 million years ago):
• Dry climate.
• Formation of the supercontinent Pangaea.
• Early mammal-like reptiles (therapsids) and the first gymnosperms (conifers).
• Reptiles started to diversify.
• Trilobites were completely wiped out.
• It ended with the Permian-Triassic Extinction (The Great Dying), with ~96% of species lost.
• Key facts about the Mesozoic Era (252-66 million years ago):
• Pangaea was in motion.
• Known as the "Age of Reptiles."
• Dinosaurs, mammals, and birds evolved.
• Key facts about the Triassic Period (252–201 million years ago):
• Recovery from the Permian extinction.
• First dinosaurs and mammals appeared.
• Mammals were competing with reptiles but still overwhelmed
• Crocodiles were abundant.
• Insects evolved from incomplete metamorphosis into complete metamorphosis.
• Ended with the Triassic-Jurassic Extinction (~76% of species lost, possibly due to volcanic activity).
• Key facts about the Jurassic Period (201–145 million years ago):
• The ideal time for dinosaurs.
• Dinosaurs like Stegosaurus and Brachiosaurus thrived.
• First birds (e.g., Archaeopteryx) appeared, due to evolution of insectivores.
• Theropod Dinosaur Hypothesis.
• Oceans were dominated by ichthyosaurs and plesiosaurs.
• Small burrowing mammals underground.
• Conifers continue to flourish ("Age of Cycads").
• Key facts about the Cretaceous Period (145–66 million years ago):
• Flowering plants (angiosperms) evolved and spread.
• Insect diversification.
• Tyrannosaurus rex and Triceratops lived during this time.
• Small birds and mammals also diversified.
• Ended with the Cretaceous-Paleogene Extinction (K-Pg) (~75% of species lost, caused by an asteroid impact forming the Chicxulub Crater).
• Key facts about the Cenozoic Era (66 million years ago-Present):
• Known as the "Age of Mammals."
• Life diversified into recognizable forms today.
• Key facts about the Paleogene Period (66–23 million years ago):
• Mammals diversified after the extinction of dinosaurs.
• Grasslands expanded, ruminants diversify.
• Primates evolved.
• Key facts about the Paleocene Epoch (66-56 million years ago):
• Warm and humid climate, with no polar ice caps.
• Ecosystems recovered after the asteroid impact.
• Mammals diversified and increased in size.
• First primitive primates appeared.
• Flowering plants (angiosperms) continued to dominate.
• First cactus and palm trees.
• Early birds diversified.
• Key facts about the Eocene Epoch (56–33.9 million years ago):
• Significant global warming and the development of many modern mammal groups.
• Very warm, with subtropical conditions even at high latitudes.
• The Paleocene-Eocene Thermal Maximum (PETM) occurred at the start of this epoch.
• The Himalayas began to form.
• Modern mammalian orders diversified.
• The first whales evolved from land-dwelling ancestors.
• Grasslands began to appear.
• First bats appeared.
• Key facts about the Oligocene Epoch (33.9–23 million years ago):
• Cooler and drier than the Eocene.
• Global temperatures decline led to the extinction of some species.
• Formation of the Antarctic ice sheet and ocean currents like the Antarctic Circumpolar Current.
• Evolution of large mammals like elephants and early relatives of rhinoceroses and camels.
• Grasslands continued to influence herbivore evolution.
• Early apes appeared.
• Key facts about the Neogene Period (23-2.6 million years ago)
• Early humans (genus Homo) evolved.
• Climate cooled, leading to an Ice Age.
• Key facts about the Miocene Epoch (23-5.3 million years ago):
• Significant mammal and grassland evolution occurred.
• Warmer than today but with gradual cooling.
• Grasslands and savannas expanded due to drying climates.
• Mammals and birds diversified rapidly.
• Early ancestors of modern giraffes, deer, and elephants appeared.
• First kelp forests supported new marine ecosystems.
• Apes diversified significantly, with early hominins emerging in Africa.
• Key facts about the Pliocene Epoch (5.3-2.6 million years ago):
• Marks the lead-up to the Ice Ages.
• Cooler and drier, with the continued expansion of grasslands.
• Formation of permanent ice in the Arctic.
• Evolution of the genus Australopithecus occurred.
• Large herbivores like mammoths, horses, and antelope thrived in grasslands.
• Marine life adapted to changing ocean circulation patterns.
• Early bipedalism developed in hominins.
• Key facts about the Quaternary Period (2.6 million years ago-Present):
• It is divided into the Pleistocene Epoch (Ice Age) and Holocene Epoch (modern age).
• Humans became the dominant species and developed civilizations.
• Key facts about the Pleistocene Epoch (2.6 million-11,700 years ago):
• Also known as the Ice Age.
• Alternating glacial and interglacial periods occurred.
• Large ice sheets covered much of North America, Europe, and Asia.
• Sea levels fluctuated dramatically.
• Early humans evolved and spread (Homo habilis to Homo sapiens).
• Many large mammals experienced extinction.
• Domestication of fire and development of early tools occurred.
• Key facts about the Holocene Epoch (11,700 years ago-Present):
• Represents the post-Ice Age period.
• Warm and stable climate compared to the Pleistocene.
• Modern coastlines and ecosystems formed.
• Plants and animals were domesticated, initiating the Agricultural Revolution.
• Humans built cities, established trade networks, and developed written languages.
• Rapid population growth and technological advancement occurred.