- 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.
Is it possible to boil water (change the state of water from liquid to gas) at room temperature?
- 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 surface condition during Archean Eon (Tim Bertelink, from Wikimedia Commons)
- 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 stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia.
- The earliest identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria.
Multicellular lifeforms during Proterozoic Eon (Source)
- 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).
Snowball Earth: The Snowball Earth hypothesis proposes that Earth’s surface became entirely or nearly entirely frozen at least once, sometime earlier than 650 Mya (million years ago).
- 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.
- In the late Proterozoic (most recent), the dominant supercontinent was Rodinia (~1000–750 Ma).
Supercontinent Rodinia (John Goodge, via Wikimedia Commons)
- It was also during the Proterozoic that the first symbiotic relationships between mitochondria (found in nearly all eukaryotes) and chloroplasts (found in plants and some protists only) and their hosts evolved.
- 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, begin 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.
- The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic, which are further subdivided into 12 periods.