1. Hadean Eon (4.6 - 4.0 Billion Years Ago)
- Earth forms from the solar nebula.
- The planet undergoes heavy bombardment by asteroids and comets.
- Formation of the Moon after a giant impact with a Mars-sized body.
- Early atmosphere develops, primarily composed of volcanic gases.
2. Archean Eon (4.0 - 2.5 Billion Years Ago)
2.1 Early Archean (4.0 - 3.6 Billion Years Ago)
- Oldest known rocks form on Earth.
- First signs of life: simple prokaryotic organisms (bacteria and archaea).
2.2 Late Archean (3.6 - 2.5 Billion Years Ago)
- Photosynthesis begins, leading to the production of oxygen.
- Formation of stromatolites, layered structures created by microbial communities.
- Atmosphere remains largely anoxic (lacking oxygen).
3. Proterozoic Eon (2.5 Billion - 541 Million Years Ago)
3.1 Paleoproterozoic (2.5 - 1.6 Billion Years Ago)
- The Great Oxidation Event: a significant rise in atmospheric oxygen due to photosynthetic organisms.
- Formation of banded iron formations as oxygen reacts with dissolved iron in oceans.
3.2 Mesoproterozoic (1.6 - 1.0 Billion Years Ago)
- Emergence of the first eukaryotic cells with complex internal structures.
- Development of early nuclear continents and supercontinents like Rodinia.
3.3 Neoproterozoic (1.0 Billion - 541 Million Years Ago)
- First Multicellular Life: Eukaryotic cells evolved, leading to the development of multicellular organisms, which laid the foundation for complex life.
- The Earth experiences severe ice ages, known as "Snowball Earth" events.
- Rise of the first animal life in the late Neoproterozoic, including the Ediacaran biota.
4. Phanerozoic Eon (541 Million Years Ago - Present)
4.1 Paleozoic Era (541 - 252 Million Years Ago)
Cambrian Period (541 - 485 Million Years Ago)
- The "Cambrian Explosion": a rapid diversification of life forms occurs in the oceans.
- All major animal phyla appear, including arthropods, mollusks, and chordates.
Ordovician Period (485 - 444 Million Years Ago)
- Marine life flourishes with diverse invertebrates and the first vertebrates.
- The first land plants begin to colonize terrestrial environments.
- Ends with a major glaciation event and mass extinction.
Silurian Period (444 - 419 Million Years Ago)
- Evolution of jawed fish and the first bony fish.
- Early vascular plants spread across land.
- Arthropods, including arachnids and myriapods, become established on land.
Devonian Period (419 - 359 Million Years Ago)
- Known as the "Age of Fishes" due to the diversity of fish species.
- First amphibians evolve, marking the transition of vertebrates to land.
- Forests of primitive plants, including ferns and lycophytes, cover the continents.
- Ends with a significant extinction event affecting marine life.
Carboniferous Period (359 - 299 Million Years Ago)
- Extensive swampy forests lead to the formation of vast coal deposits.
- Evolution of reptiles and amniotic eggs, allowing vertebrates to fully exploit terrestrial environments.
- Giant insects and arthropods thrive due to high atmospheric oxygen levels.
Permian Period (299 - 252 Million Years Ago)
- Pangaea, a supercontinent, forms, altering global climate and ecosystems.
- Diversification of reptiles, including early synapsids (mammal ancestors) and diapsids (reptile ancestors).
- Ends with the Permian-Triassic Extinction, the largest mass extinction in Earth's history, wiping out over 90% of marine species and 70% of terrestrial vertebrates.
4.2 Mesozoic Era (252 - 66 Million Years Ago)
Triassic Period (252 - 201 Million Years Ago)
- Recovery from the Permian extinction leads to the rise of new life forms.
- Early dinosaurs and mammals appear alongside other archosaurs.
- Conifers and cycads dominate plant life.
- Ends with a mass extinction event that paves the way for dinosaur dominance.
Jurassic Period (201 - 145 Million Years Ago)
- Dinosaurs become the dominant terrestrial vertebrates.
- The first birds evolve from theropod dinosaurs.
- Mammals remain small and mostly nocturnal.
- Oceans are filled with marine reptiles and early ammonites.
- Pangaea begins to break apart, leading to the formation of the Atlantic Ocean.
Cretaceous Period (145 - 66 Million Years Ago)
- Continuation of dinosaur dominance with diverse groups like tyrannosaurs, sauropods, and ceratopsians.
- Flowering plants (angiosperms) evolve and spread, transforming terrestrial ecosystems.
- Mammals begin to diversify, although they remain largely small and ecologically niche-bound.
- The period ends with the Cretaceous-Paleogene Extinction, likely caused by an asteroid impact, leading to the extinction of non-avian dinosaurs and many other species.
4.3 Cenozoic Era (66 Million Years Ago - Present)
Paleogene Period (66 - 23 Million Years Ago)
Paleocene Epoch (66 - 56 Million Years Ago)
- Mammals rapidly diversify and occupy ecological niches left vacant by dinosaurs.
- Early primates and other modern mammalian groups begin to appear.
- Climate is warm and humid, with dense forests covering much of the Earth.
Eocene Epoch (56 - 34 Million Years Ago)
- Continued mammalian evolution with the emergence of early horses, whales, and bats.
- Primates evolve further, with the appearance of more modern forms.
- The Earth experiences a warm climate, followed by a cooling trend.
- The formation of the first polar ice sheets begins in Antarctica.
Oligocene Epoch (34 - 23 Million Years Ago)
- Grasses spread, leading to the development of open savannas and grasslands.
- Evolution of large herbivores and predatory mammals adapted to these new environments.
- Further cooling and drying of the global climate.
- Significant geological activity leads to the uplift of mountain ranges like the Himalayas and the Rockies.
Neogene Period (23 - 2.6 Million Years Ago)
Miocene Epoch (23 - 5.3 Million Years Ago)
- Continents take on their modern positions, and the climate continues to cool.
- Expansion of grasslands leads to the evolution of grazing mammals such as antelope, horses, and elephants.
- Apes diversify, and the ancestors of humans appear in Africa.
- The Mediterranean Sea undergoes a dramatic drying event known as the Messinian Salinity Crisis.
Pliocene Epoch (5.3 - 2.6 Million Years Ago)
- Further cooling and drying trends contribute to the expansion of savannas and deserts.
- Hominins, including Australopithecus, evolve, marking an important step in human evolution.
- The Isthmus of Panama forms, connecting North and South America and altering ocean currents.
- The onset of the modern ice age cycle begins, with glaciers advancing and retreating in the Northern Hemisphere.
Quaternary Period (2.6 Million Years Ago - Present)
Pleistocene Epoch (2.6 Million - 11,700 Years Ago)
- Characterized by repeated glacial and interglacial cycles (Ice Ages).
- Massive ice sheets cover much of North America, Europe, and Asia during glacial periods.
- Evolution and global dispersion of Homo sapiens, along with other hominins like Neanderthals and Denisovans.
- Many large mammals, known as megafauna, such as mammoths, mastodons, and saber-toothed cats, roam the continents.
- Ends with the extinction of many megafaunal species, possibly due to climate change and human activity.
Holocene Epoch (11,700 Years Ago - Present)
- A relatively warm and stable climate following the last Ice Age.
- Rise of human civilization: agriculture, cities, and technological advancement.
- Significant environmental changes due to human activity, including deforestation, pollution, and climate change.
- The Anthropocene concept emerges, highlighting the profound impact of humans on Earth's geology and ecosystems.
Major Geological and Biological Developments
Tectonic Activity
- The movement of tectonic plates has shaped continents, created mountains, and triggered volcanic eruptions and earthquakes throughout Earth's history.
Climate Changes
- Earth has experienced numerous climate shifts, from extreme ice ages to warm greenhouse periods, influencing the evolution and distribution of life.
Mass Extinctions
- Five major mass extinction events have drastically altered life on Earth, removing dominant species and allowing new forms to evolve:
- Ordovician-Silurian Extinction (c. 444 million years ago): Eliminated many marine species.
- Late Devonian Extinction (c. 375 million years ago): Affected coral reefs and marine life.
- Permian-Triassic Extinction (c. 252 million years ago): The largest extinction event, wiping out most marine and terrestrial species.
- Triassic-Jurassic Extinction (c. 201 million years ago): Cleared the way for dinosaur dominance.
- Cretaceous-Paleogene Extinction (66 million years ago): Ended the reign of the dinosaurs and paved the way for mammalian evolution.
Evolution of Life
- Life on Earth has progressed from simple single-celled organisms to complex multicellular life, including plants, animals, and fungi.
- Major evolutionary milestones include the development of eukaryotic cells, multicellularity, the Cambrian Explosion of animal diversity, the colonization of land by plants and animals, and the rise of mammals and humans.
