Class 12 Biology Evolution A brief account of evolution

A brief account of evolution

  • About 2000 million years ago (mya) the first cellular forms of life appeared on earth.
  • Some cellular forms had the ability to release O2 and slowly single cell organisms became multicellular organisms.
  • By the time of 500 mya, invertebrates were formed and active.
  • Jawless fish probably evolved around 350 mya.
  • Sea weeds and few plants existed probably around 320 mya.
  • First organisms that invaded land were plants.
  • Fish with stout and strong fins could move on land and go back to water was about 350 mya.
  • In 1938, a lobe finned fish caught in South Africa happened to be a Coelacanth which evolved into first amphibians that lived on both land and water and these were the ancestors of modern day frogs and salamanders.

Fig. Coelacanth  

  • The amphibians evolved into reptiles which lay thick shelled eggs which do not dry up in sun.
  • The modern day descendants of reptiles are the turtles, tortoises and crocodiles.
  • 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.

Fig. ferns  

  • Some of the reptiles went back into water to evolve into fish like reptiles around 200 mya.
  • The land reptiles were the dinosaurs and the biggest of them is Tyrannosaurus rex was about 20 feet in height and had huge fearsome dagger like teeth.

Fig. dinosaurs  

  • About 65 mya, the dinosaurs suddenly disappeared from the earth, some of them evolved into birds or might be killed by the climatic changes.
  • The first mammals were like shrews and their fossils were small sized.
  • Mammals were viviparous and protected their unborn young inside the mother’s body.
  • Mammals dominated the earth when the population of reptiles came down.
  • 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 America.
  • Due to the same continental drift pouched mammals of Australia survived because of lack of competition from any other mammal.
  • Some mammals live wholly in water.

Examples- Whales, dolphins, seals and sea cows.

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