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.