Whomever invented and popularized language.
Think about it.
What do we have without words. Without words, we wouldn't be able to organize our thoughts, as they respectively pertain to certain things, ideas, or categories of the world as we know it.
Without language, without words, we would still be living in small tribes - not that there's anything wrong with living in a small tribe.
Most likely, there wasn't just one person who invented and popularized language. But whomever did so most effectively, I think this unsung hero is without a doubt the most influential person in the history of the world.
However, now that I think about this prompt, "history of the world" implies that this person is documented in our history books. Which is not true for the inventor of language. So...I guess this means that I fail at responding to this challenge.
Oh well...I'm submitting it anyway.