Some structures, like the leg bones in the modern day whale, no longer have a useful
function but it is believed that once upon a time they did have a specific function. These
type of structures are called:

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.