And likewise, Isaac Newton, who discovered/invented calculus, without which none of this would be possible.
Or, one step before, John Napier, who discovered the logarithmic scale.
Though these mathematical theories seem more like a “product of their time” rather than a specific action by a single person. Apparently a guy named Bürgi had the same ideas as Napier around the same time and might well have brought them into the spotlight had Napier not existed.
If you consider that the Earth is potentially facing irrevocable destruction due to global warming, humans are the largest contributors to that, and the largest single contribution they make is via internal combustion engines, the answer is Nikolaus Otto. There were other contributors, to be sure, but lest you doubt his overall contribution I would point out that all but a handful of gasoline-powered engines are Otto-cycle engines.
This would have had a great impact if McClellan hadn’t waited eighteen hours to act on the intelligence. BTW, John M. Bloss co-discovered the plans, which were wrapped around the cigars (not the other way around).
Well, not really. Newton’s calculus of infinitesimals and Leibniz’ differential calculus essentially appeared simultaneously. If Newton hadn’t written the Principia, We’d still have calculus.
I’d say without Nikola Tesla we’d have to watch TV in the dark. Edison’s DC current could not travel over long distances like Tesla’s AC could. In the end we are all using Tesla’s invention, not Edison’s.
Warwick E. Kerr, or an unnamed beekeeper, had a pretty significant impact, and that is the first thing I thought of when I read the OP. But compared to some of the folks already mentioned, this is still smallish potatoes.
Jonas Salk deciding not to patent the polio vaccine.
re: Vasily Arkhipov
What makes you think the exchange that Arkhipov averted wouldn’t have led to all-out nuclear war?
I’m genuinely asking, not challenging.
This is obviously the answer. The rapid and wide-spread dissemination of ideas following from this enabled practically all these other things, which are just posters’ pet interests, to happen.
Muhammad left Mecca for Medina; once he got there they put him in charge and they got organized. The world turned out pretty different as a result.
Not from the nonexistent Spanish government or from the Castillian government, from Isabella in person. She used personal funds. But then, she’d been throwing tradition and “What The Wise Men Say” off the window for a while at that point.
My Southwestern Pennsylvania homie **kopek ** is hip to the historical significance of the Seven Years’ War, which could be called World War Zero. It redrew the map of world empires not only in America but in India and elsewhere.
What I wanted to underscore is the role of a woman in the negotiation that set up the situation in which Tanacharison acted so rashly. Queen Aliquippa, the Seneca sachem of the lower Monongahela valley, had met 21-year-old George Washington on his first trip to the area the previous winter, and she agreed to side with the British in the upcoming war against the French. On the basis of her agreement, Washington later led the expedition in which Tanacharison got him into hot water and touched off the French and Indian War, which became the Seven Years’ War.
Midgley was a monster.
Sir Frederick G. Banting and John James Rickard Macleod together, along with Charles Best, developed insulin.
Their discovery has saved the lives of millions of diabetics.
I think we have to rule out Princip because while he was the assassin, he doesn’t fit the parameters of the OP: the assassination was not his idea and he, along with two other members of The Black Hand (whose names I can never remember), acted under the orders of Colonel Apis. While the assassination didn’t go according to plan and Princip was the gunman, he wouldn’t have been there at all if The Black Hand hadn’t constructed the plan and smuggled him and the others into Sarajevo.
Yeah, but she had help with the conception part. 
Many years ago I found a book in the library that ranked the most influential people. I found the book and list: 100 most influential people in the world | Biography Online
Actually, I was wrong and found the book and the list.
And sorry for the dupliate post. I was trying to edit. (and now I have a third post)
I’ve read different opinions on the Petrov incident, his job wasn’t to make those sorts of decisions, it was simply to pass his observations up the chain of command for someone else to make the decision and its just as possible someone higher up would have had the same thought processes as him, “Why would the American’s attack with only a handful of missiles?”
I don’t think his passing the information along would have immediately and irrevocably resulted in a Soviet second strike, though it would certainly have left the possibility open.
What we can say for certain is that Petrov’s actions weren’t nearly as clear-cut as those of Arkhipov’s, I think we can also say for certain that we’re glad they both did what they did.