Evolution: A Brief Account of Evolution
The geological time scale includes 4 eras: Proterozoic, Palaeozoic, Mesozoic, and Cenozoic.
1. Proterozoic Era: 2500 - 541 Million Years 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), and 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 evolved.
- Lobefins (stout and strong-finned fishes) could move on land and go back to water. They evolved into the first amphibians (ancestors of modern-day frogs and salamanders).
- In 1938, a lobe-fin called coelacanth fish was caught in South Africa, which was thought to be extinct.
- 320 mya: Seaweeds and a few plants existed.
- Amphibians evolved into reptiles. They laid thick-shelled eggs (do not dry up in the 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), and 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, with huge fearsome dagger-like teeth).
- Toothed birds emerged.
4. Cenozoic Era: 66 - 0 mya
- Age of mammals and angiosperms.
- It has 2 periods: Tertiary (66 mya) and 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 horses, hippopotamuses, bears, rabbits, 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 a lack of competition from any other mammals.
Origin and Evolution of Man
- 15 mya: Dryopithecus and Ramapithecus.
- Hairy. Walked like gorillas and chimpanzees.
- Dryopithecus: Ape-like.
- Ramapithecus: Man-like.
- 3-4 mya: Man-like primates walked upright in eastern Africa.
- Height up to 4 feet. This belief is based on fossils of man-like bones found in Ethiopia and Tanzania.
- 2 mya: Australopithecus.
- Lived in East African grasslands. 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 and 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 and settlements: 10,000 years ago.
Sequence of Human Evolution:
Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens