Geological Time Scale: Hadean Eon

• The geological time scale (GTS) divides and chronicles Earth's evolutionary history into various periods based on major changes in Earth's physical, chemical, and biological features.
• Primarily defined time divisions are eons: Hadean, Archean, Proterozoic, and Phanerozoic, the first three are known as the Precambrian supereon.
• Eons are divided into eras, which are divided into periods, which are divided into epochs.

Hadean Eon

• The Hadean eon (4,540 – 4,000 mya) is the time before a reliable (fossil) record of life.
• Early Earth was extremely hot, and much of it was molten.
• Volcanic outgassing likely created the primordial atmosphere and then the ocean.
• The early atmosphere contained almost no oxygen.
• Over time, Earth cooled, forming a solid crust and leaving behind hot volatiles that probably resulted in a heavy CO2 atmosphere with hydrogen and water vapor.
• Liquid water oceans existed despite a surface temperature of 230° C because, at an atmospheric pressure above 27 atmospheres (caused by the heavy CO2 atmosphere) water is still liquid.
• As cooling continued, dissolving CO2 in ocean water removed it from the atmosphere.
• Hydrogen and helium are expected to continually escape due to atmospheric escape.

Archean Eon

• The beginning of life and evidence of cyanobacteria on Earth dates to 3500 mya.
• Life was limited to simple single-celled organisms lacking nuclei, called prokaryotes.
• The atmosphere lacked oxygen, and the atmospheric pressure was around 10 to 100 atmospheres.
• The Earth's crust had cooled enough to allow the formation of continents.
• Volcanic activity was considerably higher than today, with numerous lava eruptions.
• The oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic.
• By the end of the Archaean, liquid water was prevalent, and deep oceanic basins existed.
• Earliest identifiable fossils consist of stromatolites, microbial mats formed in shallow water by cyanobacteria.

Proterozoic Eon

• The Proterozoic Eon is the last eon of the Precambrian "supereon”.
• It spans from when oxygen appeared in Earth's atmosphere to just before the proliferation of complex life (such as corals).
• Bacteria began producing oxygen, leading to the sudden rise of life forms.
• Eukaryotes (cells with a nucleus) emerged, including some forms of soft-bodied multicellular organisms.
• Earlier forms of fungi formed.
• Early and late phases of this eon experienced "Snowball Earth" periods, where the planet suffered below-zero temperatures, extensive glaciation, and a drop in sea levels.
• Snowball Earth hypothesis: Earth's surface became entirely or nearly entirely frozen at least once around 650 Mya.

Events That Can Cause Global Cooling

Natural Events

• Large Meteor and Asteroid Impacts and Supervolcanoes: Volcanic outgassing spews sulfur dioxide into the stratosphere, combining with water to form sulfuric acid aerosols, reflecting sunlight and cooling Earth's surface (volcanic winter).
• Ash, dust, and soot plumes reduce sunlight reaching Earth.

Anthropogenic Events

• Nuclear winter: A nuclear holocaust could cause a severe and prolonged global climatic cooling effect.
• A nuclear apocalypse can release an immeasurable amount of dust, ash, and soot into the atmosphere, blocking sunlight and causing global cooling.
• Nuclear detonations break down the air and produce large amounts of nitrogen oxides (NOx), causing widespread ozone depletion.
• Thermal conventions and massive smoke plumes transport nitrogen oxides to the stratosphere, catalytically breaking down the ozone layer.

Phanerozoic Eon

• The boundary between the Proterozoic and Phanerozoic eons was set when the first fossils of animals appeared.
• Life remained mostly small and microscopic until around 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago.
• This diversification produced most of the major life forms known today.
• Plantlife on land appeared in the early Phanerozoic eon.
• Complex life, including vertebrates, began to dominate the ocean.
• Life expanded to land and familiar plants, insects, animals, and fungi appeared.
• Birds (descendants of dinosaurs) and mammals emerged.
• Modern animals, including humans, evolved at the most recent phases of this eon (2 million years ago).
• The Phanerozoic eon is divided into three eras:
  • The Paleozoic: Includes arthropods, amphibians, fishes, and the first life on land.
  • The Mesozoic: The rise, reign of reptiles, climactic extinction of non-avian dinosaurs, the evolution of mammals and birds.
  • The Cenozoic: Saw the rise of mammals.
• The Phanerozoic is further subdivided into 12 periods.

Paleozoic Era

• Six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

Cambrian Period

• The Cambrian period spanned from 541 to 485 million years ago.
• The Cambrian sparked a rapid expansion in evolution known as the Cambrian Explosion.
• Plants like algae evolved, and arthropods dominated the fauna; Almost all marine phyla evolved.

Ordovician Period

• The Ordovician Period spanned from 485 million years ago to 440 million years ago.
• Many species still prevalent today evolved, such as primitive fish and corals.
• The most common forms of life, however, were trilobites, snails and shellfish.
• The first arthropods crept ashore (the beginning of terrestrial lifeforms).
• Glaciation resulted in a major drop in sea level, killing off all life along the coast and causing Snowball Earth.
• This glaciation led to the Ordovician-Silurian extinction (First Mass Extinction).

Ordovician-Silurian extinction (First Mass Extinction)

• Second deadliest event in Earth's history, greatly affecting marine communities.
• It resulted as the southern supercontinent Gondwana drifted over the South Pole, and ice caps formed.
• Agents included lowering of sea level and glacially driven cooling.
• A fall in atmospheric carbon dioxide preceded the late Ordovician glaciation event.
• The above is correlated with a burst of volcanic activity that deposited new silicate rocks, which drew CO2 out of the air.

Silurian Period

• The Silurian spanned from 440 million years to 415 million years ago.
• This period saw the mass evolution of fish.
• The first freshwater fish evolved, though arthropods remained apex predators.
• Fully terrestrial life evolved, including fungi and centipedes.
• The evolution of vascular plants allowed plants to gain a foothold on land.

Devonian Period

• The Devonian spanned from 415 million years ago to 360 million years ago.
• Also known as the Age of the Fish, the Devonian featured a massive diversification in fish.
• On land, plant groups diversified and the first trees and seeds evolved.
• By the Middle Devonian, shrub-like forests of primitive plants existed.
• The first amphibians also evolved, and the fish were now at the top of the food chain.
• Near the end of the Devonian, 70% of all species became extinct in the Late Devonian extinction (second mass extinction).

Late Devonian extinction (Second Mass Extinction)

• The Late Devonian extinction occurred about 376-360 million years ago, mainly affecting marine life.
• Causes are unclear but leading hypotheses include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism.

Carboniferous Period

• The Carboniferous period spanned from 360 million to 300 million years ago.
• Tropical swamps dominated the Earth, with the large amounts of trees creating much of the carbon in coal deposits.
• High oxygen levels caused these swamps to allow massive arthropods, normally limited in size by their respiratory systems, to proliferate.
• Perhaps the most important evolutionary development of this time was the evolution of amniotic eggs, which allowed amphibians to move farther inland.
• Also, the first reptiles evolved in the swamps.

Permian Period

• The Permian Period spanned from 300 million to 250 million years ago.
• All continents came together to form the supercontinent Pangaea, surrounded by one ocean called Panthalassa.
• The earth was very dry during this time, with harsh seasons, as large bodies of water didn't regulate the climate of the interior of Pangaea.
• Reptiles flourished in the new dry climate.
• Creatures such as Dimetrodon and Edaphosaurus ruled the new continent.
• The first conifers evolved, then dominated the terrestrial landscape.
• Nearing the end of the period, Scutosaurus and gorgonopsids filled the arid landmass.
• Eventually, they disappeared, along with 95% of all life on earth, in the Earth's third and the largest mass extinction event, known as the "the Great Dying".

Permian-Triassic extinction event (Third Mass Extinction)

• The Permian-Triassic (P-T) extinction event is also known as "the Great Dying."
• It occurred about 252 million years ago, boundary between the Paleozoic and Mesozoic eras.
• It is the most severe known extinction event, with up to ~96% of all marine species and 70% of terrestrial vertebrate species becoming extinct.
• It is the only known mass extinction of insects.
• Suggested causes include large meteor impact events, massive volcanism such as that of the Siberian Traps, runaway greenhouse effect triggered by the sudden release of methane from the seafloor due to methane-producing microbes (methanogens).
• Possible gradual changes include sea-level change, increasing anoxia and aridity, and a shift in ocean circulation driven by climate change.

Mesozoic Era

• The Mesozoic era spanned from 250 million to 66 million years ago and is also known as "the Age of the Dinosaurs".
• The Mesozoic era features the rise of reptiles and is separated into the Triassic, Jurassic, and Cretaceous periods.

Triassic Period

• It spanned from 250 million to 200 million years ago.
• transitional time between the Permian Extinction and the lush Jurassic Period, with three major epochs: Early, Middle and Late Triassic.
  • Early Triassic: Spanned from 250 to 247 million years ago. Dominated by deserts due to Pangaea.
  • Middle Triassic: the ecosystem recovered from the Great Dying, with new marine and terrestrial creatures evolving.
  • Late Triassic: Characterized by many temperature rises, and the first true dinosaurs evolved, eventually resulting in the Triassic-Jurassic extinction event which many large amphibians became extinct.

Triassic-Jurassic extinction event (Fourth Mass Extinction)

• It marks the boundary between the Triassic and Jurassic periods, 201 million years ago.
• This event happened in less than 10,000 years and occurred just before Pangaea started to break apart.
• On land, all archosaurs except a few and many of the large amphibians became extinct.
• This event vacated terrestrial ecological niches, allowing the dinosaurs to assume the dominant role.
• Gradual climate change, sea-level fluctuations, oceanic acidification reached a tipping point.
• Massive volcanic eruptions might have caused intense global warming (releasing carbon dioxide or sulphur dioxide into the troposphere) or extreme global cooling (releasing aerosols into the stratosphere).

Jurassic Period

• The Jurassic Period spanned from 200 million to 145 million years ago and features three major epochs: Early, Middle, and Late Jurassic.
• Early Jurassic: the climate was much more humid than the Triassic, resulting in a tropical environment, a variety of marine and terrestrial reptiles evolved.
• Middle Jurassic: featured flourishing dinosaurs and Conifer forests became a large portion of the world's forests.
• Late Jurassic featured a a massive extinction of sauropods and ichthyosaurs, separation of Pangaea into Laurasia and Gondwana

Cretaceous Period

• The Cretaceous spanned from 145 million to 66 million years ago that featured an Early and Late division.
• Early Cretaceous: The early Cretaceous saw the expansion of seaways, with the first true birds evolving, competing and pushing out pterosaurs.
• Late Cretaceous: Featured a cooling trend that eventually featured the evolution of the first flowering plants before the Cretaceous-Paleogene extinction event occurred.

Cretaceous-Paleogene extinction event (Fifth Mass Extinction)

• The Cretaceous-Paleogene (K-Pg) or Cretaceous-Tertiary (K-T) extinction was a mass extinction on Earth approx. 66 million years ago.
• K-T extinction: the age of the dinosaurs came to an end (everything >10kg became extinct), while mammals diversified in the Paleogene.
• Deccan Traps and other volcanic eruptions poisoned the atmosphere, followed by a large meteor that smashed into the Earth.

Cenozoic Era

• The Cenozoic era featured the rise of mammals, having three divisions: Paleogene, Neogene and Quaternary.

Paleogene Period

• Spans from the extinction of the non-avian dinosaurs, some 66 million years ago (K-T extinction event), to the dawn of the Neogene 23 million years ago.
• It features three epochs: Paleocene, Eocene and Oligocene.

Neogene Period

• Spans from 23.03 million to 2.58 million years ago with two epochs: the Miocene and the Pliocene.

Quaternary Period

• Spans from 2.58 million years ago to the present day and featuring modern animals and dramatic changes in the climate.
• Divided in the the Pleistocene and the Holocene.

Pleistocene

• Spans from 2.58 million to 11,700 years ago, marked by ice ages due to cooling.
• Many animals evolved, the most famous being Homo sapiens.

Holocene

• The Holocene began 11,700 years ago and lasts until the present day, with human history lying in this epoch..

Holocene Extinction

• Also referred to as the "Sixth Mass Extinction or Anthropocene Extinction," blamed on human activity that features the disappearance of megafauna.
• Began around 10,000 years ago
• Today, more than 322 species have become extinct due to human activity since the Industrial Revolution, with a rate of 100 to 1,000 times higher than background extinction rates.