How are vestigial structures an example of evidence of evolution?

They provide evidence of genes coding for structures that are present not because they are useful, but because they were once useful to an ancestor. 

One example is the plantaris muscle. This is a long, thin muscle in the human foot and calf that serves no significanl purpose in humans ... in fact, it is regularly removed by doctors harvesting muscle tissue to use in reconstructive heart surgery, because its removal leaves no loss of function in walking or balance. 

However this same muscle is found more fully developed in the feet and calves of other primates. It is the muscle used for grasping with the feet.

Proteins are the workhorses of the cell, controlling virtually every reaction within as well as providing structure and serving as signals to other cells. proteins are long chains of  amino acids  , and the exact sequence of the amino acids determines the final structure and function of the protein. instructions for that sequence are encoded in  genes  . to make a particular protein, a messenger ribonucleic acid (mrna) copy is made from the gene (in the process called  transcription  ), and the mrna is transported to the  ribosome  . protein synthesis, also called  translation  , begins when the two ribosomal subunits link onto the mrna. this step, called initiation, is followed by elongation, in which successive amino acids are added to the growing chain, brought in by transfer rnas (trnas). in this step, the ribosome reads the  nucleotides  of mrna three by three, in units called  codons  , and matches each to three nucleotides on the trna, called the anticodon. finally, during termination, the ribosome unbinds from the mrna, and the  amino acid  chain goes on to be processed and folded to make the final, functional protein