Human hair, rhino horns, and bird feathers are all made of the same protein: keratin.

Keratin is a member of the intermediate filament protein family and serves as the primary structural component for the outer layer of human skin, hair, and nails. This protein is classified into two types: alpha-keratins, found in mammals, and beta-keratins, which are harder and found in the scales and feathers of reptiles and birds. The strength of keratin comes from the high concentration of the sulfur-containing amino acid cysteine.When cysteine molecules bond, they form disulfide bridges. These are permanent covalent bonds that create a rigid, three-dimensional structure that is highly resistant to tearing and heat. In a 2006 study published in the journal 'Nature Materials,' researchers found that the hierarchical structure of keratin allows it to absorb significant impact without breaking. This explains why it is the ideal material for defensive structures like rhinoceros horns.Unlike true horns which have a bony core, a rhino's horn is composed entirely of keratinized cells and calcium deposits. This was confirmed by researchers at Ohio University using CT scans in 2006. These studies revealed that the center of the horn contains dense mineral deposits of melanin and calcium, which protect the keratin from UV degradation and drying. Because keratin is chemically stable and insoluble in water, it is one of the most common biological materials found in the fossil record alongside teeth and bones.