What Was Humanity's Greatest Invention?

Before saying anything, I ought to be clear by what I mean by "greatest" (or "great" for that matter). Often when this question is posed, it evokes a kind of recall to some of the more notable moments in our history. Common contenders for answers to the posed question include: fire, the wheel, written language, sea-faring transportation, the telegram, the transistor, and so on. All of these inventions have the same thing in common, and can be thought to be, under isomorphism, really one and the same.

Without [invention] we simply couldn't do [action] because [invention] caused [chain of events] that lead to [what we have now, in whatever form we're familiar with it in]. Thus the "greatest" invention must be the first invention, since without the first thing ever being invented (or at least something that existed in nature already that was utilized/modified consciously by humans at some point in our evolution). Think about it: without [first invention] we couldn't have made it to [second invention], and without [second invention] we couldn't have done [action] that would have led to [third invention], and so on. The convenience that exists in this little arrangement here is that where you draw the line as what could be the first ever invention of mankind is arbitrary, because all other inventions would necessarily succeed it.

I like to compare this with the question of what the best smartphone of all time was. Maybe for some, the gut reaction would be something along the lines of "well, whatever the best phone on the market is now is the best phone, since it has the longest battery to date, best performance specs, and so on." Now, of course on further inspection this does seem to be an unsatisfying answer. The reason: none of the phones that are available today are very groundbreaking or innovative at all. They all follow predictably in the shadow of their predecessors. So, that then must imply that what we're looking for is the most innovative, most groundbreaking, most significant change in the course of the tech industry. It's for these reasons that the iPhone is commonly cited as the single greatest smartphone to date. In many ways it pushed the boundaries of what smartphones were capable of doing at that point, in 2007. It paved the way for what we know now, in 2020, of what a smartphone should be capable of doing.

Using this line of reasoning, what we're looking for in our answer is a single invention (although there are a myriad of candidates) by mankind that has single-handedly propelled us the farthest ahead, expanded our reach the greatest, and paved the way for the future of the species most significantly than any other single invention. Now, how could one choose from a list so vast and distinguished?

Going back to the smartphone analogy, we can make a similar case that the first ever device used for communication was the most important invention. We can make some kind of line of regression like this one:

iPhone > Blackberry > IBM's Simon > Motorola DynaTAC > ... > Transistor > ... > Mining and Smelting Ores > ... > Organized Groups of People Congregated for a Similar Purpose > Oral Communication/Language

I think it's fair to say that in any invention-related regression scheme, you would have to end at oral communication or language. Without this necessary invention, humans wouldn't have been able to communicate concepts to the level where we can collaborate or advance at all technologically. So, the answer to the question "What Was Humanity's Greatest Invention" is simple: language itself.