All living things are identified and categorised on the basis of their body design in form and function. Once a certain body design comes into existence, it will shape the effects of all other subsequent design changes, simply because it already exists. So, characteristics that came into existence earlier are likely to be more basic than characteristics that have come into existence later. This means that the classification of life forms will be closely related to their evolution.
On connecting this idea of evolution to classification, it is seen that some groups of organisms with ancient body designs have not changed very much. But there are other groups of organisms that have acquired their particular body designs relatively recently Since there is a possibility that complexity in design will increase over evolutionary time, it may not be wrong to say that older organisms are simpler, while younger organisms are more complex.