GEOLOGICAL TIME SCALE OF THE EARTH
- The geological time scale (GTS) divides and chronicles earth’s evolutionary history into various periods from the beginning to the present based on definite events that marked a major change in earth’s physical, chemical and biological features.
- Major changes in earth’s physical and biological history stretch over several millions of years and hence in GTS all the divisions are expressed in ‘million years (mya – million years ago).’
- The primarily defined divisions of time are eons, the Hadean, the Archean, the Proterozoic and the Phanerozoic. The first three of these can be referred to collectively as the Precambrian supereon.
- Each eon is subsequently divided into eras, which in turn are divided into periods, which are further divided into epochs.
SuperEon ==> Eon ==> Era ==> Period ==> Epoch
- The Hadean eon (4,540 – 4,000 mya) represents the time before a reliable (fossil) record of life.
- Temperatures are extremely hot, and much of the Earth was molten because of frequent collisions with other bodies, extreme volcanism and the abundance of short-lived radioactive elements.
- A giant impact collision with a planet-sized body named Theia (approximately 4.5 billion years ago) is thought to have formed the Moon.
- The moon was subjected to Late Heavy Bombardment (LHB – lunar cataclysm – 4 billion years ago).
- During the LHB phase, a disproportionately large number of asteroids are theorised to have collided with the early terrestrial planets in the inner Solar System, including Mercury, Venus, Earth, and Mars.
- Volcanic outgassing probably created the primordial atmosphere and then the ocean.
- The early atmosphere contained almost no oxygen.
- Over time, the Earth cooled, causing the formation of a solid crust, leaving behind hot volatiles which probably resulted in a heavy CO2 atmosphere with hydrogen and water vapour.
- Liquid water oceans existed despite the surface temperature of 230° C because, at an atmospheric pressure of above 27 atmospheres, caused by the heavy CO2 atmosphere, water is still liquid.
- As cooling continued, dissolving in ocean water removed most CO2 from the atmosphere.
- Hydrogen and helium are expected to continually escape (even to the present day) due to atmospheric escape.
- The beginning of life on Earth and evidence of cyanobacteria date to 3500 mya.
- Life was limited to simple single-celled organisms lacking nuclei, called Prokaryota.
- The atmosphere was without oxygen, and the atmospheric pressure was around 10 to 100 atmospheres.
- The Earth’s crust had cooled enough to allow the formation of continents.
- The oldest rock formations exposed on the surface of the Earth are Archean.
- 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, plate tectonics may have been similar to that of the modern Earth.
- Liquid water was prevalent, and deep oceanic basins are known to have existed
- The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria.
- It is the last eon of the Precambrian “supereon”.
- It spans for the time of appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life (such as corals) on the Earth.
- Bacteria begin producing oxygen, leading to the sudden rise of life forms.
- Eukaryotes (have a nucleus), emerge, including some forms of soft-bodied multicellular organisms.
- Earlier forms of fungi formed around this time.
- The early and late phases of this eon may have undergone Snowball Earth periods (the planet suffered below-zero temperatures, extensive glaciation and as a result drop in sea levels).
- The Snowball Earth hypothesis proposes that Earth’s surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya.
- It was a very tectonically active period in the Earth’s history.
- It featured the first definitive supercontinent cycles and modern orogeny (mountain building).
- It is believed that 43% of modern continental crust was formed in the Proterozoic, 39% formed in the Archean, and only 18% in the Phanerozoic.
- The boundary between the Proterozoic and the Phanerozoic eons was set when the first fossils of animals such as trilobites appeared.
- Life remained mostly small and microscopic until about 580 million years ago, when complex multicellular life arose, developed over time, and culminated in the Cambrian Explosion about 541 million years ago.
- This sudden diversification of life forms produced most of the major life forms known today.
- Plant life on land appeared in the early Phanerozoic eon.
- Complex life, including vertebrates, began to dominate the Earth’s ocean.
- Pangaea forms and later dissolves into Laurasia and Gondwana.
- Gradually, life expands to land and all familiar forms of plants, insects, animals and fungi begin appearing.
- Birds, the descendants of dinosaurs, and more recently mammals emerge.
- Modern animals—including humans—evolve at the most recent phases of this eon (2 million years ago).
The Phanerozoic eon is divided into three eras:
- the Palaeozoic, an era of arthropods, amphibians, fishes, and the first life on land;
- the Mesozoic, which spanned the rise, reign of reptiles, climactic extinction of the non-avian dinosaurs, the evolution of mammals and birds; and
- the Cenozoic, which saw the rise of mammals
- There are six periods in the Paleozoic era: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian.
- Spans from 541 to 485 million years ago
- The Cambrian sparked a rapid expansion in evolution in an event known as the Cambrian Explosion during which the greatest number of creatures evolved in a single period in the history of Earth.
- Plants like algae evolved, and arthropods dominated the fauna.
- Almost all marine phyla evolved in this period.
- Spans from 485 million years to 440 million years ago
- Many species still prevalent today evolved, such as primitive fish, corals, etc.
- The most common forms of life, however, were trilobites, snails and shellfish.
- More importantly, the first arthropods crept ashore (the beginning of terrestrial lifeforms).
- By the end of the Ordovician, Gondwana had moved from the equator to the South Pole.
- The glaciation of Gondwana resulted in a major drop in sea level, killing off all life along its coast.
- Glaciation caused a snowball Earth, leading to the Ordovician-Silurian extinction (First Mass Extinction).
Ordovician–Silurian extinction (First Mass Extinction)
- This is considered as the second deadliest in the history of Earth.
- This event greatly affected marine communities.
- As the southern supercontinent, Gondwana drifted over the South Pole, ice caps formed on it.
- A combination of lowering of sea level and glacially driven cooling were likely driving agents.
- The dip is correlated with a burst of volcanic activity that deposited new silicate rocks, which draw CO2 out of the air as they erode.
- The Silurian spans from 440 million years to 415 million years ago.
- It saw warming from Snowball Earth.
- This period saw the mass evolution of fish.
- The first freshwater fish evolved, though arthropods, such as sea scorpions, remained the apex predators.
- Fully terrestrial life evolved, which included fungi, and centipedes.
- The evolution of vascular plants allowed plants to gain a foothold on land.
- During this time, there were four continents: Gondwana (Africa, South America, Australia, Antarctica, India), Laurentia (North America with parts of Europe), Baltica (the rest of Europe), and Siberia (Northern Asia).
- The recent rise in sea levels provided new habitats for many new species.
- Spans from 415 million years to 360 million years ago
- Also known as the Age of the Fish, the Devonian features a huge diversification in fish.
- On land, plant groups diversified; the first trees and seeds evolved.
- By the Middle Devonian, shrub-like forests of primitive plants existed.
- This event allowed the diversification of arthropod life as they took advantage of the new habitat.
- 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 an event known as the Late Devonian extinction, which is the second mass extinction known to have happened.
Late Devonian extinction (Second Mass Extinction)
- The Late Devonian extinction occurred about 376–360 million years ago.
- The extinction seems to have only affected marine life.
- The causes of these extinctions are unclear.
- Leading hypotheses include changes in sea level and ocean anoxia (lack of oxygen), possibly triggered by global cooling or oceanic volcanism.
- Spans from 360 million to 300 million years ago
- Tropical swamps dominated the Earth, and the large amounts of trees created much of the carbon that became coal deposits (hence the name Carboniferous).
- The high oxygen levels caused by these swamps allowed massive arthropods, normally limited in size by their respiratory systems, to proliferate.
- Perhaps the most important evolutionary development of the time was the evolution of amniotic eggs, which allowed amphibians to move farther inland. (Amniotic fluid is a clear, slightly yellowish liquid that surrounds the foetus).
- Also, the first reptiles evolved in the swamps.
- Throughout the Carboniferous, there was a cooling pattern, which eventually led to the glaciation of Gondwana as much of it was situated around the south pole.
- Spans from 300 million to 250 million years ago.
- At its beginning, all continents came together to form the super-continent 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.
- Eventually, they disappeared, along with 95% of all life on Earth in an event simply known as “the Great Dying“, the world’s third mass extinction event and the largest in its history.
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 Ma (million years) ago, forming the boundary between the Paleozoic and the Mesozoic eras.
- It is the Earth’s 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 sea floor due to methane-producing microbes known as methanogens.
- Possible contributing gradual changes include sea-level change, increasing anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
- Spans from 250 million to 66 million years ago
- Also known as “the Age of the dinosaurs“, the Mesozoic features the rise of reptiles.
- There are three periods in the Mesozoic: Triassic, Jurassic, and Cretaceous.
- Spans from 250 million to 200 million years ago
- It is a transitional time between the Permian Extinction and the lush Jurassic Period.
- It has three major epochs: Early Triassic, Middle Triassic and Late Triassic.
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 (release of carbon dioxide or sulphur dioxide) or intense global warming (release of aerosols).
- Spans from 200 million to 145 million years ago, and features three major epochs: Early Jurassic, Middle Jurassic, and Late Jurassic.
- Spans from 145 million to 66 million years ago, and is divided into two epochs: Early Cretaceous, and Late Cretaceous.
Cretaceous–Paleogene extinction event (Fifth Extinction)
- The Cretaceous-Paleogene (K-Pg) or Cretaceous-Tertiary (K-T) extinction, was a sudden mass extinction on Earth approximately 66 million years ago.
- At the end of the Cretaceous, the Deccan Traps and other volcanic eruptions were poisoning the atmosphere.
- As this was continued, it is thought that a large meteor smashed into Earth, creating the Chicxulub Crater (Yucatan Peninsula Mexico) creating the event known as the K-T Extinction.
- Every living thing with a body mass over 10 kilograms became extinct, and the age of the dinosaurs came to an end.
- In its wake, many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species.
- Mammals in particular diversified in the Paleogene, evolving new forms such as horses, whales, bats, and primates. Birds, fish, and perhaps lizards also radiated.
- The Cenozoic featured the rise of mammals as the dominant class of animals.
- There are three divisions of the Cenozoic: Paleogene, Neogene and Quaternary.
- Spans from the extinction of the non-avian dinosaurs, some 66 million years ago, to the dawn of the Neogene 23 million years ago
- It features three epochs: Paleocene, Eocene and Oligocene.
- Spans from 23.03 million to 2.58 million years ago
- It features 2 epochs: the Miocene, and the Pliocene.
- Spans from 2.58 million years ago to the present day
- It features modern animals and dramatic changes in the climate.
- It is divided into two epochs: the Pleistocene and the Holocene.
- Spans from 2.58 million to 11,700 years ago
- Ice ages marked this epoch as a result of the cooling trend that started in the Mid-Eocene.
- There were at least four separate glaciation periods marked by the advance of ice caps as far south as 40 degrees N latitude in mountainous areas.
- Africa experienced a trend of desiccation which resulted in the creation of the Sahara, Namib, and Kalahari deserts.
- Many animals evolved including mammoths, dire wolves, and most famously Homo sapiens.
- 1,00,000 years ago, marked the end of one of the worst droughts of Africa, and led to the expansion of primitive man.
- As the Pleistocene drew to a close, a major extinction wiped out much of the world’s megafauna, including some of the hominid species, such as Neanderthals.
- The Holocene began 11,700 years ago and lasts until to the present day.
- All recorded history and “the history of the world” lies within the boundaries of the Holocene epoch.
- Human activity is blamed for a mass extinction that began roughly 10,000 years ago, though the species becoming extinct have only been recorded since the Industrial Revolution.
- This is sometimes referred to as the “Sixth Extinction“.
- More than 350 species have become extinct due to human activity since the Industrial Revolution.