To trace back and find common ancestors, we can use the following concepts:


Homologous Characteristics


  • If you look at the above image,
    You will notice that the basic structure of the limbs of frogs, lizards, birds and humans is the same .
  • But, their functions are different .
  • Frogs use their limbs to jump , while lizards also need to climb vertical surfaces. Birds use their limbs to support their wings while humans use limbs to lift things.
  • Different animals hence adopt their structures according to their use over time ( evolution ).

In another example;


  • In Bougainvillea , Axillary buds developed to form thorns to give protection from being eaten by animals.
  • In Cucurbita , Axillary buds form tendrils that help the plant to climb on to a support .
  • Thus they have the same origin but serve different functions .



  • This means they evolved differently from a common ancestor . This is known as Divergent Evolution


Analogous Characteristics

Let’s take a closer look at the wings of Birds and Bats;


  • The structure of bat wings is skin stretched over elongated fingers.
  • In birds , the arms (wing) is covered in feathers .
  • These are called Analogous Characteristics .


  • Their origin and structure are different but function is the same so they are examples of Convergent Evolution .



Evolutionary relations can be traced up to animals that are extinct as well.

We find them with the help of Fossils .

When an organism dies , most of it decomposes and gets broken down into simpler forms.

Traces of parts that  do not decompose quickly, help us to find evolutionary relations .

While making pots, the potter moulds the pot in the desired shape and once the clay is left to dry, it retains the shape.

  • Similarly, if a dead insect gets caught in hot mud it will not decompose easily and once the mud hardens, it retains the impression of the structure.
  • Fossils are formed in the process of formation of sedimentary rocks .


Thus, fossils are preserved remains of organisms that existed on Earth at some point in the past.


Fossil records have been used to study the evolutionary relationship between birds and reptiles .

It also shows how mammals evolved over time .

But how do we find the age of these fossils?


  • Relative depth
    • When we dig to find fossils, if it is closer to the surface then it is a relatively recent fossil
    • If it is found in deeper layers , then it is a relatively old fossil


Carbon Dating

  • This is done by chemical analysis to detect the ratios of different isotopes of the elements the fossil is made of.
  • Living organisms exchange Carbon Dioxide with the atmosphere.
  • When they die, they still have Carbon-12 and Carbon-14 isotopes in them.
  • This Carbon-14 isotope gets disintegrated with time but never gets disintegrated completely.
  • So by measuring the amount of Carbon-14 left in the organism , and its ratio with Carbon-12 we can find the age of the fossil.


Molecular phylogeny

Changes in DNA → Changes in proteins → Changes in characteristics or function

  • These changes get accumulated in the process of evolution
  • Tracing these changes in DNA backwards to know at which point different organisms branched out to display different characteristics or functions is the aim of molecular phylogeny.
  • The thumb rule is; organisms which are most distantly related show more accumulation of changes in DNA.


Evolution by stages

  • Evolution of eyes
    The human eye has a complex structure. It is no doubt a result of evolution. But the evolution did not take place suddenly. It came into existence gradually .
    • Euglena has a small eye-spot but it is hidden.
    • Planaria also has eye-spots that detect light . This gives it a survival advantage.
    • Further evolution over the years resulted in the eyes we see in humans and animals today.


  • Evolution of wings
    • Organs that served organisms for one purpose at a point of time would not be the same after years of evolution.
    • Some dinosaurs had feathers to provide insulation from cold weather ; not for flight .
    • But after years of evolution, birds have the ability to fly with the help of wings.
    • So birds could be closely related to reptiles (dinosaurs).


Now we know what Fossils are and the fact that birds could be related to reptiles.

Lets us now look at an interesting finding based on that;

  • Archaeopteryx the fossil form of an organism that was found as an evolutionary link between reptiles and birds.


  • Artificial Selection - Evolution of wild cabbage
    • Humans have created many varieties of cabbage by manually selecting the desired qualities
    • Artificial selection refers to the process of selecting desired characteristics in plants or animals and then crossing them to produce new organisms of the desired qualities.
    • The following are the desired traits:
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Maninder Singh

CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo