Evolution - Notes | Class 12 | Part 5: A brief Account of Evolution



The geological time scale includes 4 eras: Proterozoic, Palaeozoic, Mesozoic & Cenozoic.

1. Proterozoic era: 2500 - 541 million yrs ago (mya)

-   2000 mya: First cellular forms of life appeared.

-   Some of the cells had the ability to release O2 as the light reaction in photosynthesis.

-   Single celled organisms became multicellular organisms.

2. Palaeozoic era (540 - 252 mya)

-   It has 6 periods: Cambrian (540 - 490 mya), Ordovician (490 - 443 mya), Silurian (425 mya), Devonian (405 mya), Carboniferous (360 mya) & Permian (285 mya).

-   500 mya: Invertebrates were formed.

-   450 mya: First land organisms (plants) appeared.

-   400 mya: Arthropods invaded the land.

-   350 mya: Jawless fishes were evolved.

Lobefins (stout & strong finned fishes) could move on land and go back to water. They evolved to first amphibians (ancestors of modern day frogs & salamanders).

In 1938, a lobe-fin called coelacanth fish was caught in South Africa which was thought to be extinct.

-   320 mya: Sea weeds and few plants were existed.

-   Amphibians evolved to reptiles. They lay thick-shelled eggs (do not dry up in sun).

-   Giant ferns (Pteridophytes) were present but they all fell to form coal deposits slowly.

3. Mesozoic era (252 - 66 mya)

-   Age of reptiles and gymnosperms.

-   It has 3 periods: Triassic (230 mya), Jurassic (208 mya) & Cretaceous (144 mya).

-   200 mya: Some of the land reptiles went back into water to evolve into fish-like reptiles (E.g. Ichthyosaurs).

-   The land reptiles were dinosaurs (Tyrannosaurus rex, Triceratops, Stegosaurus, Brachiosaurus etc.)

T. rex was the largest dinosaur (20 feet in height, huge fearsome dagger-like teeth).

-   Toothed birds were emerged.

4. Cenozoic era (66 - 0 mya)

-   Age of Mammals & Angiosperms.

-   It has 2 periods: Tertiary (66 mya) & Quaternary (2 mya - Age of man).

-   65 mya: Dinosaurs suddenly disappeared. Some say climatic changes killed them. Some say most of them evolved into birds.

-   First mammals were shrew-like. Their fossils are small sized.

-   In South America, there were mammals resembling horse, hippopotamus, bear, rabbit etc. Due to continental drift, when South America joined North America, these animals were overridden by North American fauna.

-   Due to continental drift, Australian marsupials survived because of lack of competition from any other mammals.


·   15 mya: Dryopithecus & Ramapithecus.

Hairy. Walked like gorillas & chimpanzee.

Dryopithecus: ape-like.

Ramapithecus: man-like.

·   3-4 mya: Man-like primates walked up right in eastern

Africa. Height up to 4 feet. This belief is based on fossils of man-like bones found in Ethiopia & Tanzania.

·   2 mya: Australopithecus. Lived in East African grass lands. Hunted with stone weapons. Ate fruits.

Homo habilis: First human-like being (hominid).

Brain capacity: 650-800 cc. Did not eat meat.

·   1.5 mya: Homo erectus (Java man). Large brain (900 cc). Ate meat.

·   1 lakh - 40,000 yrs ago: Homo neanderthalensis (Neanderthal man).

Brain capacity: 1400 cc. Lived in East & Central Asia. Used hides to protect their body. Buried their dead.

·   75,000 - 10,000 yrs ago (ice age): Homo sapiens (Modern man).

   Pre-historic cave art developed about 18,000 years ago. E.g. Cave paintings at Bhimbetka rock shelter in Raisen district of Madhya Pradesh.

   Agriculture & settlements: 10,000 years ago.

Sequence of Human evolution:

DryopithecusRamapithecus Australopithecus Homo habilis Homo erectus Homo neanderthalensis Homo sapiens


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