What is the origin of x-mark used as a signature of illiterate

I'm not sure that it is the proper site to ask this question, maybe it's an off-topic. However, I've heard it is also a kind of lingual expression used in English/American culture.

I've heard that X-mark is "used as a replacement for a signature for a person who is blind or illiterate and thus cannot write his or her name" (written in Wikipedia.)

I tried to find the origin of the usage, but I can't.

Woud you give me some references about this? If it's impossible, the earliest known example of X-mark used in this usage would be appreciated.

It looks like it took quite some time for the X to be adopted as the one mark you’d use to sign a contract with if you couldn’t sign your name. Before that, an assortment of letters and symbols were used.

Let’s go back to the time of Shakespeare, to my earliest examples. In 1579, John and Mary Shakespeare (parents of William, the playwright you usually think of hearing that name) sold some property. They made it official with a contract, which they “signed” (or more precisely, left their marks on). You can see the contract online, and it has this description:

John’s mark is a cross, no longer clearly visible but shown as such on early photographs. Mary’s mark, more elaborate, is carefully drawn. In 1597 John again used a simple cross for his mark. 

The document itself says something like “mark + of John Shakespeare”. All of the writing other than the marks was written by someone literate.

In later documents, the mark would be placed between the person’s first and last names and signaled with text like “his mark”, “her mark”, or “their mark”, where usually the first word of that was put over the mark, and the second word put under.

• The Legal Genealogist covers the case of a William and Elizabeth Pierce (1745) who used their respective first initials (W, E) as their marks.
• AskHistorians has a response from someone saying that when such documents were copied (by hand or via print), an X would replace the mark originally used in the document. They gave an example of a document with many original marks: some are crosses, some are letters (X is popular but you can see others like P), some that look to be two letters (BH, EC), at least one is a letter written backwards (Я), and there are even some that can’t be written in Unicode (there are several that look like an 𝙸 with a vertical line through it).
• I found an example of a 1902 document having a mark that was “XXX” from someone who was probably too ill to write her name out.
• Here’s an example from 1921 where the mark is just an X.