Fire, Wheels, Electricity.. No..
The biggest one is the Written Language.. Imagine a world where there is only speech and there is no form of written communication like you see in animal world.. The learning of an individual is confined to the individuals he/she meet and that's it.. Today what humans experience is collective learning, the learning is not just shared across millions but passed on to generations where one start from the point where another has concluded without which we all would have been just animals..
So if human race transformed because of writings of a learned, what are we doing about it, Is it not gross injustice to the human race if we don't take this collective learning to the next level.. So let’s Read, Think, Do and Write before we leave this world..
I may be the last person to find this out but happy to be one among them!!..