Geological Time Scale: Eons

• The Geological Time Scale (GTS) is a system to describe Earth's history by dividing it into different time intervals that represent events in geological and biological evolution.

Major Divisions

• The time scale has four main hierarchical levels: Eons, Eras, Periods, and Epochs.

Eons

• These are the longest divisions in the geological time scale.

Hadean Eon (4.6–4.0 BYA)

• Conditions were too extreme to support life.
• Named after Hades, the Greek god of the underworld, due to the hellish conditions.

Archean Eon (4.0–2.5 BYA)

• The first prokaryotic cells (bacteria and archaea) appeared around 3.5 to 3.8 BYA.
• Prokaryotes are simple, single-celled organisms without a nucleus.

Proterozoic Eon (2.5 billion–541 MYA)

• Cyanobacteria oxygenated Earth's atmosphere during the Great Oxygenation Event (~2.4 BYA).
• The endosymbiotic theory suggests the rise of eukaryotes when one prokaryote engulfed another, forming organelles like mitochondria.
• The first known animal life, Ediacaran biota, appeared during this eon.

Phanerozoic Eon (541 MYA–Present)

• This eon shows rapid diversification of plants, animals, and fungi in oceans and on land.
• During this time five major extinctions reshaped dominant life forms, including the rise of mammals after the dinosaurs.
• Continents shifted, climates fluctuated, and ecosystems evolved.
• Mammals thrived, leading to the evolution of primates, early humans, and modern civilizations.

Eras

• These are divisions of Eons

Eras under Phanerozoic Eon include

• Paleozoic (541–252 MYA)
• Mesozoic (252–66 million years ago)
• Cenozoic (66 million years ago–Present).

Paleozoic Era (541–252 MYA)

• The Cambrian Explosion happened about ~541 MYA which saw a rapid diversification of life; first hard-shelled animals.
• First Vertebrates appeared ~525 MYA and were early jawless fish
• First Land Plants appeared ~470 MYA and were simple moss-like plants which colonized land.
• First Land Animals appeared ~430 MYA and were arthropods, such as scorpions and millipedes, move onto land.
• First Amphibians appeared ~365 MYA and are vertebrates that transition from water to land.
• First Reptiles appeared ~310 MYA and emerge from amphibians, better adapted to dry conditions.

Mesozoic Era (252–66 million years ago)

• First Dinosaurs appeared ~230 MYA and are small reptiles which evolve into dominant land predators.
• First Mammals appeared ~225 MYA and are tiny, shrew-like mammals.
• First Birds appeared ~150 MYA and are feathered dinosaurs like Archaeopteryx that take flight.
• Flowering Plants emerged ~130 MYA and were angiosperms beginning to dominate landscapes.

Cenozoic Era (66 million years ago-Present)

• Mammals Diversified (~66 MYA) after dinosaurs were gone and they took over various ecological roles.
• First Primates (~55 MYA) were early ancestors of monkeys and apes evolve.
• First Hominins (~6–7 MYA) and were human-like species that emerge in Africa.
• Early Humans (~300 KYA) and were Homo sapiens.

Periods

• These a divisions of eras

Periods of the Phanerozoic Eon

• Paleozoic Era (541–252 MYA)
  • Cambrian (541–485 MYA) featured the Cambrian Explosion and the first hard-shelled animals.
  • Ordovician (485–444 MYA) featured first vertebrates (jawless fish) and first land plants.
  • Silurian (444–419 MYA) saw first land animals (arthropods) and the development of fish with jaws.
  • Devonian (419–359 MYA) had first amphibians and widespread forests.
  • Carboniferous (359–299 MYA) featured first reptiles, large coal-forming forests, and giant insects.
  • Permian (299–252 MYA) had Pangaea forming and saw first mammal-like reptiles
• Mesozoic Era (252–66 MYA)
  • Triassic (252–201 MYA) had first dinosaurs and first mammals.
  • Jurassic (201–145 MYA) saw dinosaurs dominate and first birds appear.
  • Cretaceous (145–66 MYA) featured first flowering plants, and the asteroid impact that ended dinosaurs.
• Cenozoic Era (66 MYA–Present)
  • Paleogene (66–23 MYA) saw mammals diversify and first primates.
  • Neogene (23–2.6 MYA) featured first hominins and expanding grasslands.
  • Quaternary (2.6 MYA–Present) had ice ages, early humans, and modern civilization.

Epochs

• Divisions of Periods

Epochs of Cenozoic Era

• Paleogene Period (66–23 MYA)
  • Paleocene Epoch (66–56 MYA) saw mammals diversify after the dinosaur extinction.
  • Eocene Epoch (56–34 MYA) featured the first modern mammals (e.g., early primates, whales).
  • Oligocene Epoch (34–23 MYA) had grasslands which spread and first apes evolved.
• Neogene Period (23–2.6 MYA)
  • Miocene Epoch (23–5.3 MYA) saw the expansion of grasslands, first hominins appeared (~7 MYA).
  • Pliocene Epoch (5.3–2.6 MYA) saw early human ancestors such as Australopithecus and the beginning of the Ice Age.
• Quaternary Period (2.6 MYA–Present)
  • Pleistocene Epoch (2.6 MYA–11,700 YA) saw ice ages, and early Homo species evolve.
  • Holocene Epoch (11,700 YA–Present) is when modern humans dominate, alongside agriculture, and civilizations.

The 5 Great Mass Extinctions

• The Earth has experienced five mass extinction events, each dramatically reshaping life.

Ordovician-Silurian Extinction (444 MYA)

• It was the second largest extinction, wiping out 85% of species.
• It was triggered by global cooling and glaciation, leading to sea level drops.
• 85% of species went extinct, including corals, brachiopods, and trilobites due to sea levels dropping dramatically, destroying shallow marine habitats.

Late Devonian Extinction (383–359 MYA)

• Possible causes are volcanism, asteroid impacts, and plant evolution which altering ecosystems.
• Ocean anoxia (oxygen loss) devastated marine life.
• 75% of species were lost, especially marine life like armored fish and reef ecosystems.

Permian-Triassic Extinction (252 MYA)

• Also known as "The Great Dying"
• It was the worst mass extinction, killing 96% of marine species and 70% of terrestrial species.
• The Siberian Traps volcanic eruptions released massive amounts of CO2 and methane, causing extreme global warming, ocean acidification, and oxygen depletion.

Triassic-Jurassic Extinction (201 MYA)

• Eliminated 80% of species, clearing/paving the way for dinosaurs to dominate.
• It was likely caused by volcanic activity (Central Atlantic Magmatic Province), increasing CO2 and global temperatures.
• Ocean acidification made survival difficult for marine life.

Cretaceous-Paleogene Extinction (66 MYA)

• This saw 76% of species go extinct, including all non-avian dinosaurs.
• An asteroid impact near the Yucatán Peninsula, Mexico, triggered wildfires, tsunamis, and global cooling.
• Volcanic eruptions from the Deccan Traps, India, intensified climate instability.