Over millions of years, organisms have developed a diverse variety of traits due to evolution. Humans look very different than whales, but they are both distant relatives that came from a common ancestor organism.
One way to determine how closely related organisms are is to determine which structures they have in common.
For example, a bird, human, and a dog are more closely related to one another than they are to a plant. One of the reasons you might think birds, humans, and dogs are closely related is that they all have limbs. Birds have wings, dogs have legs, and humans have arms. Structures that organisms have in common with one another are called homologous structures.
Scientists use homologous structures to determine how closely related organisms are to each other.