UrRong, the concept of using a signature as the legally binding marker of one's attestation to a thing is not as arbitrary as you might think. Signatures have been used for attestation purposes for centuries, and this has been so for a few reasons. Firstly, signatures, as opposed to other forms of identification, such as blood, are unique and instantly associable with the individual. This makes them easily recognizable and tamper-proof. Secondly, signatures are more accessible than alternatives, such as blood. Not everyone carries a pin or blood-letting supplies with them, and a signature can be made with just a pen and paper. Finally, signatures are more socially acceptable and less intrusive than alternatives such as blood. For these reasons, signatures have become the universally accepted form of attestation.