The principles of biological classification and the diversity of organisms.

Put things into groups makes it easier for us to understand them. It also helps us to identify a new object by deciding in which group it fits into. If you find an animal that you have never met before, you can decide, whether it is an insect or a mammal, even if you do not know precisely what is it.
The first person who classificate living organisms was Carl Linnaeus.
Living things evolved over thousand of millions of years. First, single- celled organism appeared. Some of them evolved into more complex organisms.
The things are classified according to how closely are related. E.g.: mammals and birds evolved from reptiles so this three, are put into the same group.
Mammals, birds, reptiles, and insects all have a backbone and five-fingered or pentadactyl limbs. Therefore, it looks as they all evolved from the same group of ancestors, and are quite closely related.
Features like backbone and pentadactyl limbs are called HOMOLOGOUS STRUCUTURES. These are the ones which are built...