05 Origin and Evolution of life - part 13 - Geological time scale

05 Origin and Evolution of life - part 13 - Geological time scale



Geological time scale : 

  • The planet earth with its present biodiversity was not so when it was born. 
  • Study of fossils tells us that life forms were not the same millions of years ago (MYA). 
  • Geological time scale is used to understand the sequence of events that to place on the earth in different ages over a period of time.
  • It is divided into six major `Eras' 
  • Eras ended with major environmental changes on earth resulting into extinction and emergence of new species. 
  • The eras are further divided into periods and epochs based on minor but landmark events in each era. 
  • Table 5.15 shows the geological time scale at a glance. 
  • The first life appeared on the earth some 2000 million years ago. 


  • It took billions of years for this process to take place, from protenoids to first cells the transition is still a mystery. 
  • Once formed the living forms diversified into various groups. 
  • Life began in the sea water and plants were the first living beings to adapt to terrestrial life. 
  • Fishes evolved and diversified. 
  • The lobefin group of fishes too got diversified. 
  • Some developed stout and strong fins and could go to land and come back to water. 
  • The coelecanth was considered a living fossils. 
  • It was thought that lobefins are extinct,but the variety of the lobefin fish, called coelacanth was caught in 1938 in South Africa. 
  • Reptiles evolved from amphibians. They are the first true land vertebrates
    . 
  • They do not have to go to water for reproduction. (Hint : think of amphibian and reptilian eggs). 
  • But about 200 million years ago (mya) some reptiles moved back to aquatic mode of life and gained fish like form as seen in Ichthyosaurs. 
  • The giant reptiles like Dinosaurs once dominated the earth but are now extinct. Around 65 million years ago!
  • At around the same time giant ferns were present on earth. However, they also became extinct and got converted to fossil fuels. 
  • Decline of giant reptiles marked the begining of dominance of mammals. 
  • These viviporous organisms were more intelligent. They could avoid danger. 
  • Early mammals were small shrew like organisms, but this group diversified. 
  • Whales, dolphins, seals and sea cow live in water, bats are the flying mammals, Kangaroo rats are fossorial, lemurs are arboreal. 
  • Major physical disturbances led to phenomenon like the continental drift i.e. continents moved from their original place. 
  • As a consequence when south America joined north America, ancestral forms of horse, hippos, rabbits, etc. native to south america were dominated by north American animals. 
  • At the same time, marsupial diversified into different habitats in Australia. 
  • These survived due to lack of competition. 
  • From the fossil records we can trace complete evolutionary history of horse, elephant, dog, etc. Human beings are the most evolved animals on the earth.

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