A brief account of evolution through geological time

What this lesson is

NCERT gives a brief account of the evolution of life on the Earth, with dates. This part is not in the NEET syllabus, but NCERT, A-Level and IB teach it. The NEET text lists the evidence from palaeontology and human evolution, and the order of the events is part of the fossil evidence. The dates below are the dates that NCERT prints, and they are rounded. Learn the order first.

From cells to the first land animals

• About 2000 million years ago. The first cellular forms of life appeared. Some of the cells could release oxygen, in a reaction like photosynthesis. These were the first autotrophs, the living things that make their own food.
• Slowly, single-celled organisms gave many-celled organisms.
• About 500 million years ago. Invertebrates were formed and active.
• About 450 million years ago. Plants were widespread on land. NCERT says that plants were the first living things to invade the land.
• About 350 million years ago. Jawless fish probably evolved, according to NCERT. Fish with stout, strong fins could move on land and go back to the water.
• About 320 million years ago. NCERT places seaweeds and a few more plants here.

In 1938, a fish was caught in South Africa that was a coelacanth. It was thought to be extinct. Coelacanths are lobefins, fish with fleshy fins. Lobefins gave rise to the first amphibians, the animals that live on both land and in water. Today we have only a few amphibians, the frogs and the salamanders.

Reptiles and the dinosaurs

Amphibians gave rise to reptiles. Reptiles lay thick-shelled eggs, which do not dry up in the sun as the eggs of amphibians do. Reptiles of many shapes and sizes then dominated the Earth.

• Some returned to the water and became fish-like reptiles, such as the ichthyosaurs, probably about 200 million years ago.
• The reptiles on land were the dinosaurs. The biggest of them, Tyrannosaurus rex, had huge dagger-like teeth.
• About 65 million years ago, the dinosaurs suddenly disappeared. NCERT says that we do not know the true reason. Some say that a change in the climate killed them. Some say that most of them evolved into birds. There is evidence that birds are present day dinosaurs.

The age of the mammals

The first mammals were small, and they looked like shrews. They were viviparous, which means that they carried their young inside the body, where it was protected. They sensed and avoided danger better than the reptiles. When the reptiles declined, the mammals took over the Earth.

Continents move. This is continental drift. South America once carried its own mammals, which looked like horses, hippopotamuses, bears and rabbits. When South America joined North America, these animals were overrun by the animals of North America. The pouched mammals of Australia survived, because no other mammals competed with them.

Dates in other books

Other books give some dates that differ from NCERT. They are the standard values today, and you may meet them in A-Level and IB.

• The first cells are put at about 3500 million years ago.
• Jawless fish are put at about 500 million years ago.
• The end of the dinosaurs is put at about 66 million years ago.
• The geological eras are the Palaeozoic (539 to 252 million years ago), the Mesozoic (252 to 66) and the Cenozoic (66 to now). The Mesozoic is called the age of reptiles, and the Cenozoic the age of mammals.

In an examination, follow NCERT for the dates that NCERT prints, and say "according to NCERT" if you are unsure.

You should see that the dinosaurs vanish near the right end of the strip, 6.5 cm from the present. All of human history would fit in a line thinner than the tip of a pencil.

Try it in Python

The program turns the NCERT dates into distances on the strip.

Practise with new numbers

Common mistakes

• Putting the dinosaurs and early humans in the same time. The dinosaurs were gone about 65 million years ago, and the human line begins millions of years later.
• Reversing amphibians and reptiles. Amphibians came first. Reptiles evolved from them.
• Saying that the mammals appeared after the dinosaurs vanished. The first mammals were small and lived in the age of the reptiles. They took over later.
• Thinking that the coelacanth is an amphibian. It is a lobefin fish. Lobefins gave rise to the amphibians.
• Mixing up the NCERT dates with the dates in other books. Learn which book a date comes from.

Remember these

• Order: first cells, many-celled life, invertebrates, jawless fish, plants on land, fish with strong fins, amphibians, reptiles, mammals.
• Lobefins (coelacanth, caught in 1938) gave amphibians. Amphibians gave reptiles. Reptiles laid thick-shelled eggs.
• Ichthyosaurs went back to the water. The dinosaurs disappeared about 65 million years ago by the NCERT date.
• The first mammals were like shrews and were viviparous. South American mammals were overrun when the continents joined. Pouched mammals survived in Australia.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Put these in the order in which they appeared: reptiles, plants on land, amphibians, invertebrates, mammals.

2. Why did reptiles do better on land than amphibians?

3. What was caught in South Africa in 1938, and why was it important?

4. Why did the pouched mammals of Australia survive?

5. On the 60 cm strip of the last 600 million years, where is an event of 250 million years ago? Measure from the left end.

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 6.
• Reece J. B. et al. (2017). Campbell Biology, 11th edition, Pearson, chapter 25.