The similarities among living organisms indicate that all of them have evolved from some single common ancestor (This is called monophyletic origin). Through the process of gradual modifications, adaptations and natural selection, over a million and a half different species have evolved from a common ancestral form. This process is called evolution or organic evolution. This is described as ‘descent with modification’. Its salient features are:
1. Unicellular organisms were the first to appear.
2. Multicellular organisms evolved from them.
3. Early forms were simple in structure and evolved gradually into more and more complex forms. For example, present day seed plants have evolved from simple seedless plants. Vertebrates have evolved from invertebrates.