The use of '=' to mean 'equals' is one of those minor advances in mathematics that we take for granted. It's so small that we don't really think about it. But Joseph Mazur, the author of Enlightening Symbols A Short History of Mathematical Notation and Its Hidden Powers, has thought about '=' as well as the other symbols with which we pepper our calculations. Mazur has this to say on the history of the equals sign:

The ideas of algebra brought on the symbols, not the other way around. Robert Recorde had written the words "is equal to" almost two hundred times in his book Whetstone of Witte (1557) before noticing that he could easily "avoid the tedious repetition" of those three words by designing the symbol = to represent them. The initial incentive was the need to abbreviate, but once the equal symbol was in place, something else took over. The concise character of the symbol came with an unintended benefit: it enabled an unadorned picture in the brain that could facilitate comprehension.

In other words, how we represent mathematical ideas affects how we think about them. So the timeline below is not just interesting, but perhaps shows how we have changed our very thoughts about mathematics over time: