Evolutionary relationships can be traced in the following ways:
(i) Study of homologous organs: Some organs in different organisms are similar in structure and design because they are inherited from a common ancestor. For example, forelimbs of horse, wings of bird and arms of man may be functionally different, but because of their similarity in structure, origin and design, they indicate that horse, birds and man are closely linked and had a common ancestor.
(ii) Study of fossils: Fossils are the remains or impressions of organisms that existed in the past, allow us to study organ structure of organisms that are no longer alive. Comparing their organ structure with organ structure of present day organisms also enable us to trace evolutionary relationships.
(iii) Comparing DNA of different species: This will give us a direct estimate of how much the DNA has changed during the formation of these species. This, too, can be used as a criterion to trace evolutionary relationships.
