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.
· 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:
Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens