Evolution of the Biosphere
4.6 billion years of Earth’s story — from a molten hellscape to the only known living planet in the universe. Simplified, story-driven, with memory tricks, Indian connections, mass extinction table, PYQs and MCQs.
Before diving into details, understand the structure and proportions. Precambrian = 88% of Earth’s history. Everything you recognise = the last 12%.
The Geological Time Scale (GTS) divides Earth’s 4.6-billion-year history into hierarchical units: Eon → Era → Period → Epoch → Age. The biggest divisions are Eons. Boundaries are defined by major events — mass extinctions, appearance of new life forms, atmospheric shifts, or geological markers.
Read Earth’s history as a single sentence:
“Hell cooled into a dead rock, then Archean bacteria appeared and slowly created oxygen, then the Proterozoic built complex cells, then the Cambrian Explosion filled the sea with life, which crawled ashore in the Paleozoic, dinosaurs ruled the Mesozoic, mammals inherited the Cenozoic, and humans are now causing the Anthropocene crisis.”
- Largest unit = Eon (then Era → Period → Epoch → Age)
- Precambrian (Hadean + Archean + Proterozoic) = ~88% of Earth’s history ★
- Cambrian Explosion = 541 million years ago — sudden appearance of most animal phyla ★
- Greatest mass extinction = Permian-Triassic (252 Ma) — “The Great Dying” — 96% species lost ★
- 5th mass extinction = K-Pg (66 Ma) — asteroid + Deccan Traps → dinosaurs extinct ★
- Meghalayan Age = most recent geological age (started ~2200 BCE) — defined by a global drought recorded in Mawmluh Cave, Meghalaya — India’s contribution to global geology ★
- Anthropocene = proposed new epoch defined by human impact — not yet officially ratified by ICS
The name says it all: “Hadean” comes from Hades — the Greek underworld. Earth in this eon was a literal hellscape — molten, bombarded, boiling, with no atmosphere we’d recognise today.
Formation of Earth (~4.6 billion years ago)
Earth formed from the accretion of solar nebula — dust and gas from a swirling cloud around the young Sun. Gravity pulled material together, compression heated it, and Earth formed as a molten rock ball.
Moon Formation (~4.5 billion years ago)
A planet-sized body called Theia collided with the young Earth at an angle. The impact vaporised enormous amounts of material that ejected into space and coalesced to form the Moon. This is the Giant Impact Hypothesis (Theia Impact). The Moon’s tidal forces would later play a crucial role in stabilising Earth’s axial tilt — a precondition for stable seasons and complex life.
Late Heavy Bombardment (~4.1–3.8 billion years ago)
A period of intense asteroid and meteorite impacts — the Moon’s craters are largely from this era. These impacts continually re-melted Earth’s crust. But they also delivered water (comets and water-rich asteroids) and organic molecules — the building blocks of life arrived from space. Paradoxically, the bombardment that made Earth hellish may also have seeded it with the ingredients for life.
First Solid Crust
Toward the end of the Hadean, Earth cooled enough for a solid crust to form. Earliest evidence of continental crust: Zircon crystals from Jack Hills, Australia — dated to ~4.4 billion years ago — the oldest known terrestrial material.
Early Atmosphere & Oceans
The Hadean atmosphere was composed mainly of hydrogen, helium, methane, ammonia, and water vapour — completely uninhabitable by modern life. As Earth cooled, water vapour condensed → oceans began to form by the end of the Hadean (~4.0 billion years ago).
The age of the first life. Earth has cooled enough for liquid oceans to exist. Somewhere in these warm seas — perhaps at hydrothermal vents, perhaps in shallow tidal pools — the first life emerges. But it is nothing like what we’d call life today.
First Life: Prokaryotes (~3.5–3.8 billion years ago)
The first organisms were simple prokaryotes — cells without a nucleus. They include the earliest bacteria and archaea. Evidence: microbial fossils (microbialites) in the Pilbara region of Western Australia, dated ~3.5 billion years ago — the oldest direct fossil evidence of life. Chemical signatures in Greenland rocks may push first life back to ~3.7–3.8 billion years ago.
Stromatolites — Life’s First Architecture
Stromatolites are layered mound structures built by communities of cyanobacteria (and other microbes). They are the most important geological record of early life — fossil stromatolites are found worldwide, including in India (Vindhyan Supergroup, central India). Living stromatolites still exist today at Shark Bay, Western Australia.
Cyanobacteria in stromatolites performed photosynthesis — producing oxygen as a byproduct. This was the first biological production of oxygen on Earth.
No Oxygen — Anoxic Ocean
Throughout most of the Archean, the atmosphere had virtually no free oxygen (O₂). The early oceans were rich in dissolved iron — an “iron ocean.” Any oxygen produced by early photosynthesis immediately reacted with dissolved iron → iron oxides → sank to the seafloor → forming the world’s major Banded Iron Formations (BIFs). These BIFs are today the primary source of iron ore globally — India’s Odisha and Jharkhand iron ore deposits largely originate from Archean BIFs. ★
Early Continents — Cratons
The first stable continental landmasses — called cratons — formed in the Archean. The Dharwar Craton (Karnataka-Tamil Nadu), Singhbhum Craton (Jharkhand-Odisha), and Aravalli Craton (Rajasthan) in India are some of Earth’s oldest rock formations, dating to the Archean. ★
The most transformative eon. Oxygen accumulates. Complex cells appear. Multicellular life tentatively emerges. The planet freezes completely — twice. This eon changes everything that will come after it.
Great Oxygenation Event (GOE) — ~2.4 billion years ago ★
By the Proterozoic, cyanobacteria in stromatolites had been producing oxygen through photosynthesis for hundreds of millions of years. Finally, enough O₂ accumulated to overwhelm the “oxygen sinks” (dissolved iron in the ocean) → atmospheric oxygen levels began to rise dramatically. This is the Great Oxygenation Event (GOE) — the most important single event in Earth’s atmospheric history.
- What it did to existing life: DISASTER for most Archean microbes — oxygen was toxic to anaerobic organisms. The GOE caused the first mass extinction of life — billions of anaerobic microbes died. Called the “Oxygen Catastrophe” or “Great Oxidation Event.”
- What it enabled: Aerobic metabolism — using oxygen to produce energy far more efficiently than fermentation. This unlocked the energy needed for complex, multicellular life. Without the GOE, there would be no animals, no plants, no fungi — no complex life at all.
- Ozone layer forms: As O₂ accumulated, UV radiation in the upper atmosphere converted O₂ → O₃ (ozone). The ozone layer formed — shielding the surface from lethal UV radiation. Only after the ozone layer formed could life colonise land.
Eukaryotes — Cells with a Nucleus (~2.1–1.8 billion years ago)
The first eukaryotes — cells with a membrane-bound nucleus and organelles — appeared. These are enormously more complex than prokaryotes. The leading theory: eukaryotes formed by endosymbiosis — a prokaryote engulfed another prokaryote, which eventually became the mitochondrion (and later, another engulfment led to the chloroplast in plant cells). Every complex life form on Earth — every animal, plant, fungus — is a eukaryote.
Snowball Earth — ~720 and ~635 million years ago ★
Earth underwent extreme glaciation events — possibly the entire planet froze from poles to equator (or nearly so). Evidence: glacial deposits found at locations that were near the equator at the time. Causes debated: reduction in volcanic CO₂, changes in continental arrangement, or the GOE itself (oxygen destroyed methane — a greenhouse gas — causing cooling).
Escape: Volcanic CO₂ gradually accumulated under the ice (no weathering to remove it) → greenhouse effect eventually melted the ice → rapid thaw → massive carbon cycle disruption → paradoxically triggered rapid biological diversification afterward.
First Multicellular Life — Late Proterozoic (~600–541 Ma)
The Ediacaran Period (635–541 Ma) saw the first multicellular animals — soft-bodied, strange organisms that left impressions in seafloor sediment. No shells, no hard parts. They represent the world before the Cambrian Explosion. The Ediacaran fauna went largely extinct before the Cambrian — replaced by the hard-shelled, complex animals of the Cambrian.
“Phanerozoic” = visible life (Greek). Only 541 million years old — just 12% of Earth’s history — yet contains everything we associate with life on Earth.
The Paleozoic begins with the Cambrian Explosion and ends with the greatest mass extinction ever — the Permian “Great Dying.” In between: life moves from sea to land, insects take flight, and the first reptiles appear. 6 periods:
The Cambrian Explosion ★ — one of the most important events in the history of life. In a geologically short time (~25 million years), virtually all major animal body plans (phyla) appeared. Before: mostly soft-bodied Ediacaran organisms. After: trilobites, mollusks, echinoderms, chordates, arthropods — all the blueprints of modern animal life. No one fully knows why it happened. Possible causes: rise in oxygen levels, calcium availability for shell-building, ecological opportunities after Ediacaran extinctions.
Great marine biodiversity — corals, mollusks, early fish. End: 1st Mass Extinction — ~86% of species lost. Cause: Gondwana moved over South Pole → global glaciation → sea level fall → ocean habitat loss. ONLY marine organisms affected (land not yet colonised).
Recovery. First jawed fish evolve. First vascular plants colonise land — stabilising soil and beginning terrestrial ecosystems. First land animals (arthropods — scorpions, centipedes) follow the plants ashore. Coral reefs expand dramatically.
“Age of Fish” — enormous fish diversity including lobe-finned fish — whose fins had bone structures homologous to tetrapod limbs. First amphibians evolved from lobe-finned fish — the historic moment when vertebrate life first walked on land (~375 Ma, Tiktaalik). First forests (tree-sized club mosses, ferns). End: 2nd Mass Extinction — 75% of species lost.
Vast tropical swamp forests dominated — tree-sized lycopsids (club mosses), horsetails, ferns forming dense forests. When these forests died and fell into oxygen-poor swamps, they did not fully decompose → accumulated as peat → compressed over millions of years → became the world’s great coal deposits. India’s Gondwana coalfields (Jharia, Raniganj, Bokaro) formed in this era. ★ First reptiles evolved from amphibians — the amniotic egg allowed full independence from water.
Continents united as Pangaea. Dry continental climate — reptiles thrived, amphibians declined. End: THE GREAT DYING — 3rd and worst mass extinction — ~96% of all marine species, ~70% of terrestrial vertebrates lost. Cause: Siberian Traps volcanism — massive volcanic eruptions released enormous CO₂ → rapid warming → ocean acidification → anoxia → most complex life extinguished. Nearly deleted the biosphere entirely.
After the Great Dying, the survivors inherited a nearly empty world. Reptiles — especially the archosaurs — diversified rapidly into dinosaurs, pterosaurs (flying reptiles), and crocodilians. This is the Age of Reptiles. Three periods:
Recovery from Permian extinction. First small dinosaurs appear. First true mammals evolve from mammal-like reptiles (therapsids) — small, shrew-like, nocturnal. Pangaea begins to split. End: 4th Mass Extinction — ~80% of species lost. Cause: massive volcanic activity (Central Atlantic Magmatic Province) and associated climate shifts. ★
Dinosaurs at their peak — sauropods (long-necked giants like Brachiosaurus), predatory theropods (Allosaurus). First birds evolve from feathered theropod dinosaurs — Archaeopteryx is the transitional fossil. Pangaea separates into Laurasia (north) and Gondwana (south). India is part of Gondwana, attached to Madagascar and Africa. ★ First flowering plants (angiosperms) begin to appear toward end of Jurassic.
Flowering plants (angiosperms) explode in diversity — co-evolving with bees, butterflies, and other pollinators. India separates from Antarctica/Madagascar (~120 Ma) and begins its northward journey across the Tethys Ocean — an island continent for ~50 million years → enormous species diversity evolves in isolation. End: 5th Mass Extinction (K-Pg boundary) — 66 Ma ★ — asteroid impact (Chicxulub, Mexico) + Deccan Traps volcanism (India!) → non-avian dinosaurs extinct. Birds and mammals survive. ★
With dinosaurs gone, mammals inherit the Earth and diversify explosively into every ecological niche — the process called adaptive radiation. Three periods: Paleogene, Neogene, Quaternary.
Mammals diversify into every niche vacated by dinosaurs. Early horse ancestors, early whale ancestors (returned to the sea!), early primates evolve. India collides with Asia ~50–45 Ma — ending its island journey. This collision begins the uplift of the Himalayan Mountains — the greatest mountain range on Earth. ★ The Himalayan uplift fundamentally altered global climate — creating the South Asian Monsoon system we depend on today.
Global cooling continues. Grasslands expand — as forests retreat in some regions, grasses colonise open areas → large grazing mammals evolve (horses, elephants, antelopes, bovids). The Indian subcontinent collides fully with Asia → Tibetan Plateau forms (world’s highest plateau). Apes diversify — leading to the lineage that will eventually produce humans. India’s Siwalik fossil beds (Himalayan foothills) record an extraordinary diversity of Neogene mammals. ★
Recurring Ice Ages (glacial-interglacial cycles driven by Milankovitch cycles — changes in Earth’s orbital parameters). Large mammals (mammoths, woolly rhinos, giant sloths) flourish then go extinct. Genus Homo evolves in Africa (~2–2.5 Ma). Homo sapiens appears (~300,000 years ago). Humans migrate out of Africa (~70,000 years ago). End of last Ice Age (~12,000 years ago) → agriculture, civilisations. The Meghalayan Age (from ~4,200 years ago / ~2200 BCE) — defined by a global drought, with the Golden Spike (GSSP) in Mawmluh Cave, Meghalaya, India ★.
The “Big Five” — each defines a major boundary in the geological time scale. The 6th is happening now.
| # | Name | When | % Lost | Primary Cause | What Survived / What Next | India Connection ★ |
|---|---|---|---|---|---|---|
| 1st | Ordovician-Silurian | ~444 Ma | ~86% marine species | Glaciation (Gondwana over South Pole) → sea level fall + cooling | Only marine life affected — land not yet colonised. Recovery: Silurian reef expansion. | — |
| 2nd | Late Devonian | ~375–360 Ma | ~75% species | Multiple causes — anoxia (oxygen depletion in oceans from forest runoff), volcanic activity, possible asteroid | Fish and early amphibians survive. Insects unaffected. | — |
| 3rd ★★★ | Permian-Triassic (“The Great Dying”) | ~252 Ma | ~96% marine species; ~70% terrestrial vertebrates — GREATEST EVER ★ | Siberian Traps volcanism → massive CO₂ → rapid warming → ocean acidification → anoxia | ~4% of species survived. Archosaurs inherit the world → diversify into dinosaurs. Triassic recovery. | — |
| 4th | Triassic-Jurassic | ~201 Ma | ~80% species | Central Atlantic Magmatic Province volcanism + climate change as Pangaea split | Dinosaurs survive and flourish in Jurassic → “Age of Dinosaurs” | — |
| 5th ★★★ | Cretaceous-Paleogene (K-Pg) | ~66 Ma | ~76% of all species; all non-avian dinosaurs | Chicxulub asteroid impact (Mexico) + Deccan Traps volcanism (India) ★ | Birds (avian dinosaurs), mammals, amphibians, some reptiles survived. Mammals diversify into Age of Mammals. | Deccan Traps volcanism (Maharashtra/Karnataka) contributed to global cooling/warming cycles that stressed life ★ |
| 6th ★ (ongoing) | Anthropocene / Holocene Extinction | ~1500 CE – present (accelerating) | Current rate: 1,000–10,000x background rate. ~1 million species threatened (IPBES 2019) | ENTIRELY HUMAN-CAUSED: habitat destruction, overexploitation, pollution, invasive species, climate change ★ (UPSC 2018) | India: Gharial (CR), Great Indian Bustard (CR), Gangetic dolphin (EN), vulture collapse (99% lost in 10 years) | India hosts 4 biodiversity hotspots — all under severe threat ★ |
“Only Dinosaurs Perish Through Killing” → O=Ordovician · D=Devonian · P=Permian (Great Dying) · T=Triassic · K=Cretaceous (K-Pg). Or remember by cause: Ice → Anoxia → Volcano (Siberian) → Volcano (Atlantic) → Asteroid+Volcano (Deccan).
The Anthropocene (from Greek anthropos = human + kainos = new) is a proposed geological epoch marking the period in which human activity has become the dominant force shaping the Earth system — comparable in geological impact to natural forces.
What Is It — Officially?
The International Union of Geological Sciences (IUGS) and the International Commission on Stratigraphy (ICS) are the bodies that formally define geological time units. As of 2024, the Anthropocene has NOT been formally ratified as an official epoch. The Anthropocene Working Group proposed its formal designation, but the proposal was voted down in 2024 — the primary debate being when exactly it started and which geological marker (GSSP) should define it.
Proposed Start Dates — Why the Debate?
- Industrial Revolution (~1760–1800 CE): Rise in atmospheric CO₂, industrialisation, fossil fuel combustion — marks the beginning of systematic global change
- 1950s — “Great Acceleration”: Post-WWII surge in global population, economic growth, resource extraction, and pollution. Proposed GSSP: Crawford Lake, Ontario (Canada) — varved lake sediments show clear plutonium isotope signals from nuclear weapons testing beginning 1952. ★
- Agricultural Revolution (~10,000 BCE): Humans began reshaping landscapes at scale — some argue this is when human domination began
Signals of the Anthropocene in the Rock Record
- Plutonium-239 from nuclear weapons testing (1952 onwards) — global, synchronous marker ★
- Microplastics — found globally in sediment cores, ice cores, and remote oceans
- Fly ash — from coal combustion — now a global sediment layer
- Elevated CO₂ and methane in ice cores — showing unprecedented rise since industrialisation
- Mass extinction signals — gap in fossil record as species disappear at 1,000x natural rate
- “Technofossils” — concrete, plastics, metals — materials that will persist in the rock record indefinitely
Meghalayan Age — India’s Contribution to Geology ★
While the Anthropocene debate continues, the ICS did officially designate the Meghalayan Age in 2018 — the most recent formally recognised geological age (subdivision of the Holocene epoch). It began ~4,200 years ago (~2200 BCE), triggered by a global mega-drought. The key evidence: Mawmluh Cave (also called Krem Mawmluh) in Meghalaya, Northeast India — a stalagmite in this cave contains the chemical signal of the 4.2 ka drought event. This makes India the Global Boundary Stratotype Section and Point (GSSP) for the Meghalayan Age — India’s single official contribution to defining the global geological time scale. ★
The Precambrian (Hadean + Archean + Proterozoic) spans from Earth’s formation ~4.6 billion years ago to 541 million years ago — a period of ~4.06 billion years out of Earth’s ~4.6 billion year history. That is approximately 88% of all Earth history. The entire Phanerozoic Eon (Paleozoic + Mesozoic + Cenozoic), which contains all visible complex animal life — from the Cambrian Explosion to today — represents only about 12% of Earth’s history. This fact conveys the extraordinary age of the planet relative to the recency of complex life.
1. It caused a mass extinction of most anaerobic organisms
2. It eventually led to the formation of the protective ozone layer
3. It was caused by volcanic activity releasing oxygen
4. It enabled the evolution of organisms using aerobic respiration
Statement 1: CORRECT — The GOE was catastrophic for existing anaerobic organisms — oxygen was toxic to them. This constitutes the first mass extinction of life (of anaerobic microbes), sometimes called the “Oxygen Catastrophe.” Statement 2: CORRECT — As atmospheric O₂ accumulated, UV radiation in the upper atmosphere converted O₂ → O₃ (ozone). The ozone layer formed — shielding the surface from lethal UV radiation. Only after this shield formed could life eventually colonise land. Statement 3: WRONG — The GOE was caused by biological photosynthesis — cyanobacteria in stromatolites releasing O₂ as a photosynthetic byproduct. NOT volcanic activity. Volcanoes release CO₂, SO₂, H₂O — not O₂. Statement 4: CORRECT — Aerobic respiration is far more efficient than anaerobic fermentation. The availability of oxygen enabled far greater metabolic energy, which powered the evolution of complex eukaryotic and multicellular life.
The Cambrian Explosion (~541 million years ago) refers to the sudden, geologically rapid diversification of animal life in the fossil record — over approximately 20–25 million years, most major animal body plans (phyla) appeared. Before the Cambrian: mostly soft-bodied Ediacaran organisms, few hard parts. After: trilobites, mollusks, echinoderms, chordates, arthropods, and virtually all modern animal phyla present. It is called an “explosion” in the geological sense — 20–25 million years seems short when Earth is 4.6 billion years old. The trigger is debated: rise in oxygen, evolution of eyes (creating predator-prey arms races), calcium availability for shell building, ecological opportunities after Ediacaran extinctions.
1. The Permian-Triassic extinction was the largest in Earth’s history, wiping out approximately 96% of marine species
2. The Deccan Traps volcanism in India contributed to the 5th mass extinction event
3. The Ordovician-Silurian extinction affected both marine and terrestrial species equally
4. The 6th mass extinction is driven by human activity including habitat loss, pollution, and climate change
Which of the statements given above are correct?
Statement 1: CORRECT ★ — The Permian-Triassic extinction (252 Ma, “The Great Dying”) is the most severe — ~96% of marine species and ~70% of terrestrial vertebrates lost. Caused by Siberian Traps volcanism → massive CO₂ → global warming → ocean acidification → anoxia. Statement 2: CORRECT ★ — The Deccan Traps (massive volcanic province in western India, now forming the Deccan Plateau covering Maharashtra, Karnataka, Gujarat) erupted around 66 Ma — the same time as the Chicxulub asteroid impact. Most scientists believe both events together caused the 5th (K-Pg) mass extinction. India’s geology is directly linked to the extinction of dinosaurs. Statement 3: WRONG ★ — The Ordovician-Silurian extinction (~444 Ma) affected ONLY marine organisms — land had not yet been colonised by complex animals (only very early plants). It is unique among the Big Five in being purely a marine extinction. Statement 4: CORRECT ★ — The 6th (Anthropocene/Holocene) extinction is caused entirely by human activities: habitat destruction, overexploitation, invasive species, pollution, and climate change. UPSC 2018 directly tested this.
The Meghalayan Age is the youngest of the three sub-stages of the Holocene epoch (the other two being the Greenlandian and Northgrippian). It began approximately 4,200 years ago (~2200 BCE) and continues to the present. It is defined by a global drought event (~4.2 ka event) that affected civilizations across the Eastern Mediterranean, Mesopotamia, Indus Valley, and China simultaneously. The GSSP (the international geological reference point / “golden spike”) was placed in Mawmluh Cave in Cherrapunji, Meghalaya, India — a stalagmite in this cave contains the clearest chemical signal of the 4.2 ka drought in its oxygen isotope record. This makes India the official global reference point for the start of the most recent geological age — a significant scientific honour. The ICS officially ratified this in 2018.
1. The Anthropocene has been formally ratified as an official geological epoch by the International Commission on Stratigraphy
2. The 1950s “Great Acceleration” is one of the proposed start dates for the Anthropocene
3. Plutonium-239 from nuclear weapons testing is considered a strong geological marker of the Anthropocene
Which of the statements given above is/are correct?
Statement 1: WRONG ★ — The Anthropocene has NOT been formally ratified by the ICS. The Anthropocene Working Group voted to propose it, but in March 2024, the proposal was rejected by the relevant ICS subcommission. There is ongoing scientific debate about its formal definition, start date, and GSSP marker. As of April 2026, the Anthropocene remains an informal scientific concept, not an official geological epoch. Statement 2: CORRECT — The 1950s “Great Acceleration” (rapid post-WWII growth in population, economic output, resource use) is one of the leading proposed start dates for the Anthropocene. The proposed GSSP was Crawford Lake, Ontario, which shows a plutonium signal from 1952. Statement 3: CORRECT ★ — Plutonium-239 from atmospheric nuclear weapons testing (beginning 1952) is considered among the strongest candidates for the “golden spike” of the Anthropocene — it is globally synchronous, clearly identifiable in sediment cores worldwide, and would not occur naturally.
Level 1 — The 4 Eons (Precambrian = first 3):
“Hell After Protracted Fights” → Hadean · Archean · Proterozoic · Phanerozoic
Level 2 — The 3 Phanerozoic Eras:
“People Must Come” → Paleozoic · Mesozoic · Cenozoic
Level 3 — The 6 Paleozoic Periods:
“Camels Often Sit Down Carefully, Please” → Cambrian · Ordovician · Silurian · Devonian · Carboniferous · Permian
Level 4 — The 3 Mesozoic Periods:
“The Judge’s Court” → Triassic · Jurassic · Cretaceous
Level 5 — The 3 Cenozoic Periods:
“Please Never Question” → Paleogene · Neogene · Quaternary
UPSC tip: You rarely need to recall ALL periods — focus on the events associated with each. What matters: Cambrian Explosion (Cambrian), 5 Mass Extinctions and their eras, First life on land (Silurian), Coal formation (Carboniferous), Dinosaur era (Mesozoic), Human evolution (Quaternary), Meghalayan Age (Quaternary).
The Chicxulub asteroid struck what is now the Yucatan Peninsula in Mexico, also at ~66 Ma — releasing energy equivalent to billions of nuclear bombs, ejecting massive amounts of dust and debris into the atmosphere, blocking sunlight globally for months to years.
Most paleontologists now believe both events together caused the K-Pg mass extinction (5th). The Deccan Traps volcanism was already stressing global ecosystems through climate disruption (warming from CO₂, acidification from SO₂ becoming acid rain). The asteroid impact then delivered the coup de grâce. Some argue the asteroid actually destabilized the magma chamber and intensified the Deconian eruptions — the two events may be causally linked.
India’s unique position: The Deccan Traps make India’s geology directly connected to the end of the dinosaur age. When you stand on the black basalt lava flows of the Maharashtra plateau (road cuts along Mumbai-Pune Expressway, for example), you are looking at the rock that helped kill the dinosaurs. India’s Deccan Plateau exists because of this ancient volcanism.
Anthropocene — proposed new epoch that reflects human dominance of the Earth system. As of 2024, it has NOT been formally ratified. The concept is widely used in scientific and policy discussions (IPCC reports, environmental policy), but it lacks the formal ICS status required to be part of the official Geological Time Scale. The ICS subcommission voted against formal adoption in March 2024 — primarily because of disputes over the precise start date and GSSP.
Why UPSC cares:
1. UPSC has tested Meghalayan Age and Mawmluh Cave directly (current affairs, 2018–2019). Know it as India’s official geological contribution.
2. UPSC has tested the Anthropocene in the context of the 6th mass extinction (2018 PYQ — answer = D, all human causes). Know that the Anthropocene represents the current era of human-caused ecological change.
3. The distinction (one is official, one is proposed) is potential MCQ material — “Which of the following geological epochs has been officially recognised by the ICS?” → Meghalayan Age (YES) vs Anthropocene (NO, not yet).
The journey: India was once part of the supercontinent Gondwana, attached to Africa, Antarctica, Madagascar, and Australia. When Gondwana broke up (~180 Ma, Jurassic), India began separating. By ~120 Ma (Cretaceous), India was a completely isolated island continent — drifting northward across the Tethys Ocean toward Asia at a rate of ~15–20 cm/year (geologically fast). For approximately 50–70 million years, India was an island.
Consequence for biodiversity — Island Evolution: Isolated on its island, India’s species evolved independently from the rest of the world. This is why India has so many endemic species (found nowhere else on Earth) — 33% of India’s plants and 12% of its mammals are endemic. The Western Ghats, which are ancient mountains that survived the isolation as a refugium for species, have extraordinary endemism — ~5,000 flowering plant species, many endemic. The lion-tailed macaque, Malabar giant squirrel, Nilgiri tahr — all products of this ancient isolation.
When India collided with Asia (~50–45 Ma): The collision wasn’t just about mountains. It created a land bridge between the isolated Indian fauna and the diverse Asian fauna — producing an extraordinary mixing event. Indian species spread to Asia; Asian species colonised India. The Himalayan uplift then created new altitudinal zones — temperate and alpine environments in a tropical subcontinent — adding further layers of diversity. India became the biodiversity hotspot it is today through this extraordinary geological history.
How they formed: In the Archean and early Proterozoic, the oceans contained large amounts of dissolved ferrous iron (Fe²⁺) — no oxygen to oxidise it. When cyanobacteria began producing oxygen through photosynthesis, that oxygen immediately reacted with dissolved iron → ferric iron (Fe³⁺) → iron oxides (hematite, magnetite) → precipitated to the seafloor in layers → BIFs. The banded pattern may reflect seasonal cycles of oxygen production (more in summer → more iron oxidised → darker band).
India connection: India has enormous BIF deposits in its ancient Archean cratons:
— Singhbhum Craton (Jharkhand-Odisha): The Jharkhand-Odisha iron ore belt contains world-class BIF-hosted iron ore — Noamundi, Kiriburu, Bolani, Barajamda mines. India’s #1 iron ore producing region. 3.5–3.8 billion year old Archean rocks. ★
— Bailadila Range (Chhattisgarh): Among Asia’s finest iron ore deposits — BIF origin.
— Dharwar Craton (Karnataka): Hospet iron ore deposits (around Bellary) in BIF-type rocks.
So when UPSC asks about India’s iron ore mineral distribution — the geological answer is Archean BIFs. These ancient rocks, formed by cyanobacterial photosynthesis 3.5+ billion years ago, are literally the foundation of India’s steel industry today.
