NCERT grounding
This subtopic maps directly onto NCERT Class 12 Biology, Chapter 6 (Evolution), Section 6.8 — "A Brief Account of Evolution", supplemented by the geological-time figures 6.9 (plants) and 6.10 (vertebrates). NIOS Biology Lesson 1 (§1.2.2, Evidences of organic evolution) restates the same chronology from a palaeontological angle: "The fossils of the earliest era in the geological time scale were those of bacteria, then invertebrates and then successively of fishes, amphibians, reptiles and lastly of birds and mammals". NEET cross-references both texts.
"About 2000 million years ago the first cellular forms of life appeared on earth … slowly single-celled organisms became multi-cellular life forms. By the time of 500 mya, invertebrates were formed and active."
NCERT XII Biology · 6.8
The 2-billion-year timeline
NCERT opens its brief account with a single sentence whose every word is testable: about 2000 million years ago (2 bya) the first cellular forms of life appeared on earth. These earliest cells were single-celled prokaryotes, non-green and almost certainly anaerobic — the primitive atmosphere was reducing, so respiration without oxygen came first. The mechanism by which non-cellular aggregates of giant macromolecules became cells with a membranous envelope is, NCERT explicitly states, not known.
Some of these primitive cells, however, eventually acquired the ability to release O2. The reaction could have resembled the light reaction of photosynthesis, in which water is split using solar energy captured by light-harvesting pigments. The slow oxygenation of the atmosphere that followed is the chemical pivot of the entire story — every animal lineage downstream depends on it. NEET 2016 (Q.88) tests this directly: the earliest organisms were non-green and presumably anaerobes, and the first autotrophs were chemoautotrophs that did not release oxygen.
First cellular life
Single-celled prokaryotes, non-green, anaerobic. Some later acquired a water-splitting, O2-releasing reaction resembling the light reaction of photosynthesis.
Single-celled organisms then slowly became multi-cellular. NCERT does not date the transition precisely; the textbook's next anchor is 500 mya, when invertebrates were "formed and active" — the Cambrian invertebrate radiation in formal geological terms. Plants, NCERT adds in the very same paragraph, were the first organisms to invade land; animals followed only after plant cover was established. Sea weeds and a few land plants existed by around 320 mya.
From cell to vertebrate land invasion
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2 bya
First cells
Single-celled, non-green, anaerobic prokaryotes; some later split water and release O2.
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Multicellular
Many-celled life
Slowly, single cells became multicellular forms — undated in NCERT, but precedes 500 mya.
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500 mya
Invertebrates
Invertebrates formed and active. Plants are first to invade land.
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350 mya
Jawless fish
Jawless fish probably evolved; lobefins move from water to land in the same window.
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320 mya
Sea weeds, land plants
Sea weeds and a few plants existed, setting up the coal-age fern forests.
Vertebrate march onto land
Jawless fish probably evolved around 350 mya. In the same time window, NCERT records a second decisive event: fish with stout and strong fins could move on land and go back to water, about 350 mya. These are the lobefins. They evolved into the first amphibians that lived on both land and water. No specimens of those first amphibians remain, but their modern descendants are the frogs and salamanders.
The textbook nails this lineage to a single, famous specimen. In 1938, a fish caught in South Africa happened to be a Coelacanth, which was thought to be extinct. The Coelacanth is the textbook-blessed living lobefin — the very link that proves the tetrapod transition. NEET examiners reuse this exact phrasing in matching questions.
Figure 1. A schematic — not to linear scale — of the dates NCERT highlights in §6.8. The leftmost cluster spans the first 1.5 billion years; the rightmost compresses the Tertiary mammalian radiation and the final 65 million years.
The age of reptiles
The amphibians evolved into reptiles. NCERT's reason is a single anatomical fact worth memorising: reptiles lay thick-shelled eggs that do not dry up in the sun unlike those of amphibians. The amniotic egg is the evolutionary key that freed vertebrates from breeding ponds. Modern descendants are the turtles, tortoises and crocodiles.
In the next 200 million years or so, reptiles of different shapes and sizes dominated on earth. Giant ferns — pteridophytes — were present but they all fell to form the coal deposits of that age. Some of these land reptiles returned to water and evolved into fish-like reptiles around 200 mya, for example Ichthyosaurs. The land reptiles were, of course, the dinosaurs. The biggest, Tyrannosaurus rex, was about 20 feet in height and had huge fearsome dagger-like teeth.
Reptile branches in the Mesozoic
Land reptiles (dinosaurs)
~200 mya
Dominant on land, Mesozoic era
- Tyrannosaurus rex — ~20 feet, dagger-like teeth
- Different shapes and sizes
- Suddenly disappeared ~65 mya
- Possible link to birds (NCERT: "some say most of them evolved into birds")
Aquatic reptiles (Ichthyosaurs)
~200 mya
Land reptiles that re-entered water
- Fish-like reptiles, e.g. Ichthyosaurs
- Secondarily aquatic — not dinosaurs
- Co-existed with the dinosaurs
- Small-sized reptiles of that era still exist today
About 65 mya, the dinosaurs suddenly disappeared from the earth. NCERT is unusually candid here: we do not know the true reason. Some say climatic changes killed them; some say most of them evolved into birds; the truth, the textbook concedes, may live in between. Small-sized reptiles of that era still exist today. This sentence — verbatim — has appeared in NEET stems.
Mammals, continents and angiosperms
The first mammals were like shrews — small, with fossils of similarly small size. Mammals were viviparous, protected their unborn young inside the mother's body, and were more intelligent at sensing and avoiding danger. When the reptiles' grip loosened, mammals took over the earth. The geological era after the dinosaurs vanished — the Tertiary — is the age of the mammalian radiation, paralleled by the angiosperm radiation that NCERT figure 6.9 places around 150 mya.
Continental drift sorted the mammalian fauna. NCERT uses two contrasting cases to make this textbook point — Australia and South America are paired in almost every NEET-style match question.
South America
Once had mammals resembling horse, hippopotamus, bear, rabbit.
Continental drift joined it to North America; native fauna was overridden by North American mammals.
NCERT 6.8 — continental driftAustralia
Pouched mammals (marsupials) survived in isolation.
Cause: lack of competition from any other mammal — the continent never reconnected.
NEET 2023 — Australian marsupialsAquatic mammals
Some mammals went wholly back to water: whales, dolphins, seals and sea cows.
Same descent-with-modification logic Darwin used for lobefins.
NCERT 6.8 — secondary aquaticAngiosperms (150 mya)
NCERT figure 6.9 places flowering plants at ~150 mya, mid-Mesozoic.
Their radiation co-occurs with the late dinosaurs; their dominance follows mammals.
NCERT figure 6.9 — plant timelineNCERT closes the section with a one-line acknowledgement that evolution of horse, elephant and dog are special stories reserved for higher classes, and that "the most successful story is the evolution of man with language skills and self-consciousness". The thread is picked up in §6.9, Origin and Evolution of Man — about 15 mya Dryopithecus and Ramapithecus existed; 3–4 mya man-like primates walked in eastern Africa; Homo habilis (650–800 cc), Homo erectus (~900 cc), Neanderthal man (1400 cc), and finally modern Homo sapiens during the ice age between 75,000–10,000 years ago. That lineage is examined in detail on its own page; here it is the closing chapter of the brief account.
Figure 2. Vertebrate branching as set out in NCERT figure 6.10. Lobefins, not the lineage of jawless fish itself, gave rise to amphibians; reptiles radiated into dinosaurs (extinct), Ichthyosaurs (secondary aquatic), birds and mammals.
Worked examples
Arrange the following NCERT 6.8 milestones in correct chronological order (oldest to youngest): (a) jawless fish (b) first cellular life (c) extinction of dinosaurs (d) invertebrates active.
Answer: (b) → (d) → (a) → (c). First cellular life ~2000 mya; invertebrates by 500 mya; jawless fish ~350 mya; dinosaurs vanished ~65 mya. NEET sequence questions on this chapter test exactly this ladder.
A "fish caught in South Africa in 1938" that proved a thought-extinct lineage survives — name it and the lineage.
Answer: The fish is the Coelacanth; the lineage is the lobefins, fishes with stout, strong fins that gave rise to the first amphibians around 350 mya. The 1938 catch is the textbook evidence that lobefins are not entirely extinct.
Why did pouched mammals survive in Australia but not in South America? Give the NCERT reasoning in one line.
Answer: Continental drift joined South America to North America, so its native fauna (horse-, hippo-, bear-, rabbit-like mammals) was overridden by North American placentals. Australia remained isolated, so its pouched mammals survived "because of lack of competition from any other mammal".
A student writes that Ichthyosaurs were dinosaurs. Correct the statement.
Answer: Ichthyosaurs were fish-like reptiles that evolved when some land reptiles went back into water around 200 mya. Dinosaurs are the land reptiles of the same era; Ichthyosaurs are a separate, secondarily aquatic branch — not dinosaurs.