This is kind of cool $-$ that’s an article by Joseph Mazur, appeared in The Guardian with a very brief (but interesting) discussion of the origin of mathematical symbols. I had never really thought about it and basically gave the use of familiar symbols for granted.
But, apparently, the first recorded instance of the use of the equality symbol $=$ is in Robert Recorde’s book on algebra (The Whetstone of Witte), in which he says:
_"Howbeit, for easy alteration of equations, I will propound a few examples, because the extraction of their roots, may the more aptly be wrought. And to avoid the tedious repetition of these words: is equal to: I will set as I do often in work use, a pair of parallels, or Gemini lines of one length, thus =====, because no 2 things, can be more equal." _
(incidentally, the equation he referred to was $14x + 15 = 71$).