A brief account of evolution

What does NCERT’s brief account of evolution cover?

Beyond the NCERT text, the modern timeline: microbial evidence reaches back at least about 3.5 billion years, so 2000 mya is NCERT’s date for the first cells, not the modern one (Wikipedia, “Earliest known life forms”, facts in our words). Evolution is a branching history, not a ladder progressing toward mammals or humans; lineages coexist, diversify and become extinct.

First cells and the release of oxygen

• The first cellular forms of life appeared about 2000 mya (page 121); the modern date is older (see above). How non-cellular aggregates of giant macromolecules evolved into cells with a membranous envelope is not known (page 121).
• Some of these cells could release O₂. The reaction could have been similar to the light reaction of photosynthesis, in which water is split with solar energy captured and channelised by light-harvesting pigments (pages 121–122).
• Slowly, single-celled organisms became multicellular life forms. By 500 mya, invertebrates were formed and active (page 122).

Reading note (page 121): NCERT’s Section 6.1 dates life to almost four billion years ago (page 111); 2000 mya is its date for the first cells. In a question built on NCERT’s lines, give NCERT’s dates.

Jawless fish, land plants and the lobefins

• Jawless fish probably evolved around 350 mya, and sea weeds and a few plants existed probably around 320 mya (page 122). Today’s jawless vertebrates are the cyclostomes of Class 11.
• NCERT: the first organisms that invaded land were plants, and they were widespread on land when animals invaded land (page 122). Beyond the NCERT text: microbes had formed land ecosystems far earlier, by at least 2.7 billion years ago, so “first” holds only in comparison with animals (Wikipedia, “Evolutionary history of life”, facts in our words).
• Fish with stout and strong fins could move on land and go back to water, about 350 mya (page 122).
• In 1938, a fish caught in South Africa turned out to be a coelacanth, which was thought to be extinct. These animals, the lobefins, evolved into the first amphibians (pages 122–123). Beyond the NCERT text: tetrapods arose from ancient lobe-finned lineages, but living coelacanths are collateral relatives, not walking ancestors of amphibians; lungfishes are the closest living fish relatives of tetrapods (Wikipedia, “Coelacanth” and “Lungfish”, facts in our words).

NCERT slip (page 122): NCERT dates jawless fish to about 350 mya and sea weeds and a few plants to about 320 mya. Both are far too late, and the plant date clashes with NCERT’s own account: it says plants were already “widespread on land when animals invaded land” about 350 mya, and its Figure 6.9 (page 122) starts the land plants in the Silurian, before the Devonian and Carboniferous. Beyond the NCERT text: the oldest jawless fish fossils come from the Cambrian, about 518 million years ago, and the earliest evidence of land plants is about 470 million years old (Wikipedia, “Agnatha” and “Evolutionary history of plants”, facts in our words). Fossil ages show when a group already existed, not necessarily its exact origin. In a question built on NCERT’s lines, give NCERT’s dates.

Amphibians, reptiles and dinosaurs

• The first amphibians lived on both land and water. There are no specimens of these left, but they were the ancestors of modern frogs and salamanders (page 123). Beyond the NCERT text: their fossils are known; no living ones remain.
• The amphibians evolved into reptiles, which lay thick-shelled eggs that do not dry up in the sun, unlike those of amphibians. We see only their modern descendants, the turtles, tortoises and crocodiles (page 123). Beyond the NCERT text: reptiles, birds and mammals are amniotes, whose embryonic membranes shield the embryo so that it does not depend on open water, and some reptiles bear live young (Wikipedia, “Amniote”, facts in our words); shells do not make eggs immune to drying.
• 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 coal deposits slowly (pages 123–124). Beyond the NCERT text: the great coal deposits formed from wetland “coal forests” that included tree-sized lycophytes, buried as peat (Wikipedia, “Coal”, facts in our words); not every fern became coal.
• Some land reptiles went back into water to evolve into fish-like reptiles, probably about 200 mya, such as the ichthyosaurs (page 124). Beyond the NCERT text: ichthyosaurs first appeared earlier, around 250 million years ago in the Early Triassic (Wikipedia, “Ichthyosauria”, facts in our words).
• NCERT: the land reptiles were the dinosaurs; the biggest, Tyrannosaurus rex, was about 20 feet in height and had huge, fearsome, dagger-like teeth (page 124). Beyond the NCERT text: dinosaurs were one group of land reptiles, not all of them; the sauropods were the largest animals ever to have lived on land (Wikipedia, “Sauropoda”), and Tyrannosaurus stood about 12–13 feet at the hips and was about 40 feet long (Wikipedia, “Tyrannosaurus”, facts in our words). In a question built on NCERT’s lines, give 20 feet.
• About 65 mya the dinosaurs suddenly disappeared. “We do not know the true reason”: some say climatic changes killed them, some say most of them evolved into birds, and “The truth may live in between” (page 124). Small-sized reptiles of that era still exist today (page 124).
• Beyond the NCERT text: the end-Cretaceous extinction, now dated about 66 mya, eliminated the non-avian dinosaurs, and strong evidence implicates the Chicxulub asteroid impact and its environmental effects. Birds already lived alongside the other dinosaurs, and most palaeontologists regard them as the only surviving dinosaurs, not as most dinosaurs suddenly turning into birds (Wikipedia, “Cretaceous–Paleogene extinction event”, facts in our words). In a question built on NCERT’s lines, give NCERT’s 65 mya.

The rise of mammals

• The first mammals were like shrews, and their fossils are small. Mammals were viviparous and protected their unborn young inside the mother’s body, and they were more intelligent, at least in sensing and avoiding danger; when reptiles came down, mammals took over the earth (page 124). Beyond the NCERT text: monotremes, the only living mammals that lay eggs, are an exception (Wikipedia, “Monotreme”, facts in our words), and early mammals, such as a placental-line fossil about 160 million years old, lived in the age of dinosaurs; “more intelligent” is not a universal ranking.
• South America had mammals resembling the horse, hippopotamus, bear, rabbit, etc. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna (page 124). Beyond the NCERT text: the land connection allowed a two-way biotic interchange, with unequal dispersal and extinction in different groups, not a complete replacement of South American mammals (Wikipedia, “Great American Interchange”, facts in our words).
• Due to the same continental drift, the pouched mammals of Australia survived because of “lack of competition from any other mammal” (page 124). Beyond the NCERT text: Australia’s native mammals also include monotremes and placental bats and rodents, the rodents arriving some 5–10 million years ago (Wikipedia, “Fauna of Australia”, facts in our words), and the marsupials competed with one another, so NCERT’s line is a simplification.
• Some mammals live wholly in water: whales, dolphins, seals and sea cows (page 124). NCERT slip (page 124): seals are semiaquatic, coming onto land or ice to breed and rest (Wikipedia, “Pinniped”, facts in our words); in an NCERT-based answer, give NCERT’s four examples.
• NCERT calls the evolution of man, with language skills and self-consciousness, “The most successful story” (page 124). Beyond the NCERT text: human evolution is one branching history, not an objective endpoint of evolution.

Plants and vertebrates through the geological periods

Beyond the NCERT text: these are historical textbook diagrams, not complete modern phylogenies. NCERT sums up the account (page 124) in two sketches. Its Figure 6.9 (page 122) names nine periods in three eras, oldest first:

Beyond the NCERT text: the Paleozoic also includes the earlier Cambrian and Ordovician periods, which this table omits, and the Tertiary is now an obsolete period, roughly the Paleogene and Neogene together, so the modern Cenozoic periods are the Paleogene, Neogene and Quaternary (Wikipedia, “Geologic time scale”, facts in our words).

NCERT Figure 6.10 (page 123) uses only the last seven, Carboniferous to Quaternary, on a 0–350 million year scale, and names no eras.

• Plants: NCERT Figure 6.9 (page 122) starts from green algal (chlorophyte) ancestors and the first vascular plants of the Silurian and Devonian. From these come the lycopods, horsetails (sphenopsids), ferns, the gymnosperm groups (conifers, cycads, ginkgos and gnetales) and the seed ferns. The bryophytes form a separate line, and the angiosperms (dicotyledons and monocotyledons), which branch from the seed-fern line, spread through the Cretaceous and the Cenozoic. Beyond the NCERT text: the diagram is simplified; for example, the traditional dicotyledons are a paraphyletic group: the monocotyledons arose as one branch inside it, so the dicotyledons on their own leave out some descendants of their common ancestor (Wikipedia, “Dicotyledon”, facts in our words).
• Vertebrates: in NCERT Figure 6.10 (page 123), on a scale of 0–350 million years, early reptiles of the Carboniferous split into two lines. The sauropsids lead to turtles, lizards, snakes and tuataras, and through the thecodonts to the crocodiles and the dinosaurs, from whose line the birds branch. The synapsids, with the pelycosaurs and therapsids, lead to the mammals. The early reptiles, thecodonts, dinosaurs, therapsids and pelycosaurs are marked extinct. Beyond the NCERT text: birds are now counted as surviving dinosaurs (see above), and the mammals are the synapsids alive today, so the synapsid line did not die out (Wikipedia, “Synapsid”, facts in our words); “thecodonts” and “pelycosaurs” are paraphyletic, each leaving out some of its own descendants, and most palaeontologists now use the names only informally, if at all (Wikipedia, “Thecodontia” and “Pelycosaur”, facts in our words).

Solved examples

Common mistakes to avoid

Quick revision summary

• NCERT’s dates: first cells about 2000 mya; invertebrates by 500 mya; jawless fish about 350 mya; sea weeds and a few plants about 320 mya; lobefins on land about 350 mya; fish-like reptiles about 200 mya; dinosaurs disappeared about 65 mya. Give these in NCERT-based answers.
• Beyond the NCERT text, modern evidence: microbial life before 3500 mya; jawless-fish fossils about 518 mya; land-plant evidence about 470 mya; non-avian dinosaurs extinct about 66 mya, with birds surviving as dinosaurs.
• Plants invaded land first (NCERT); lobefins gave the first amphibians, then reptiles with thick-shelled eggs; beyond NCERT, living coelacanths are tetrapod relatives, not ancestors.
• Mammals took over after the reptiles; continental drift shaped South American and Australian mammals; whales, dolphins, seals and sea cows live in water (NCERT slip: seals are semiaquatic).
• NCERT Figure 6.9 (page 122), periods oldest first: Paleozoic (Silurian to Permian), Mesozoic (Triassic to Cretaceous), Cenozoic (Tertiary, Quaternary). Beyond the NCERT text: these are historical groupings; the modern Cenozoic periods are the Paleogene, Neogene and Quaternary (Wikipedia, “Geologic time scale”).

Questions students ask

Why is the coelacanth called a living fossil?

The coelacanth is called a living fossil because, by our reading of NCERT, its group was known only from fossils until 1938: NCERT says the fish caught in South Africa that year was a coelacanth “which was thought to be extinct” (page 122). Beyond the NCERT text, no coelacanth fossil is known from the Cenozoic era, which began 66 million years ago (US National Park Service, public domain), and the living genus Latimeria is also nicknamed for its slow evolution (Cavin and colleagues 2021, PLOS ONE, CC BY). The same paper reports that its two living species split 30–40 million years ago, so, by our reading, the name does not mean the fish stopped evolving.

Which era is called the age of reptiles?

Beyond the NCERT text, the Mesozoic era, from 252 to 66 million years ago, is called the age of reptiles, and the Cenozoic, from 66 million years ago to today, the age of mammals (US National Park Service, public domain). By our reading, NCERT’s account fits both names: reptiles of different shapes and sizes dominated the earth for about 200 million years, the dinosaurs disappeared about 65 mya, and when reptiles came down, mammals took over the earth (pages 123–124). NCERT’s 65 mya is, by our reading, an older estimate of the same era boundary, which is now dated 66 million years ago.

Which geological period are we living in now?

Beyond the NCERT text, we are living in the Quaternary period of the Cenozoic era, and within it in the Holocene epoch, which the US Geological Survey dates from 11 700 years before AD 2000 (Fact Sheet 2010–3059, public domain). The Quaternary itself began about 2.58 million years ago (US National Park Service, public domain). Beyond the NCERT text, a proposed Anthropocene epoch, marking human impact, was rejected as a formal unit of the geological time scale in March 2024, though the International Union of Geological Sciences expects the word to stay in informal use (IUGS statement, 20 March 2024).

What is the geological time scale?

Beyond the NCERT text, the geological time scale is the calendar geologists use for the earth’s history, split into eons, eras, periods and epochs: its divisions were first ordered from the sequence of rocks and were given reliable ages only after radiometric dating arrived in the middle of the 1900s (US National Park Service, public domain). NCERT dates the earth’s formation to about 4.5 billion years back (page 111) and sets life against parts of the scale in two sketches, NCERT Figure 6.9, “A sketch of the evolution of plant forms through geological periods” (page 122), and Figure 6.10, on the vertebrates (page 123).

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Which organisms were the first to invade land, according to NCERT?

   Show answerHide answer

   Plants (NCERT page 122).

2. Q2. Name the modern descendants of the early reptiles that NCERT gives.

   Show answerHide answer

   Turtles, tortoises and crocodiles (NCERT page 123).

3. Q3. Why did the pouched mammals of Australia survive?

   Show answerHide answer

   Because of continental drift, they survived owing to the lack of competition from any other mammal (NCERT page 124). Beyond the NCERT text: geographic isolation helped their diversification, but they still competed with one another, and other mammal groups also lived in Australia.