What specific actions by a single person caused the greatest impact?

Over the weekend I happened to read again about that idiot Eugene Schieffelin, who decided the US should enjoy all of the birds mentioned by Shakespeare, so he released starlings and house sparrows in New York City. I imagine those are 2 of the most common species of birds in a great many parts of the US.

Got me to thinking what an outsized impact this one person had as a result of his discrete acts.

Who can you think of to top that? A couple of things that make me suggest Schiefflin are that he personally did these as isolated acts - as opposed to someone who had an idea, that was later put into effect by a team of people over time. (Yeah, I imagine he didn’t work entirely alone, and had people help him trap and transport the birds.) Also, we can identify the person and the act, as opposed to saying , “the guy who invented the wheel.” Once he took the action of releasing the birds, he was not involved in developing, licensing, or marketing it, as Edison or other inventors were. I guess some political leaders could take specific steps to commit their countries into war or something, but that seems somewhat different. But I’m eager to hear whatever you suggest.

Alexander Fleming noticing that mold in a Petri dish had killed the bacteria around it.

I’m going with Oswald. JFK may have done nothing else. But he had promise.

Vasily Arkhipov.

Christopher Columbus decided to ignore the pretty accurate ancient figures for the circumfrence of the Earth and assume that it was much smaller than it really is.

I second Fleming.

Mr. Gutenberg for inventing movable type.

The doctors in the 1840s who separately realized that the insensibility caused by ether, chloroform, and nitrous oxide could possibly be useful in the operating room.

Had to look him up.

We’re probably all here because of him.

Beat me to it.

Edward Jenner realizing that the common knowledge that infection with cowpox conferred immunity to smallpox might be even more useful than he thought.

But others developed the tech to produce penicilin in large quantities. Without that, it wouldn’t have made much difference.

Yes, but was Oswald’s impact greater than Booth’s or Yigal Amir’s or any of several other assassins? That’s one the the things we can’t say. So this thread probably belongs in IMHO.

A good one, and he probably prevented an exchange of missiles that would have been devastating to several cities. But I’d say Stanislov Petrov’s contribution was slightly greater. His correct identification of a false alarm prevented an all-out second strike plan from being immediately and irrevocably put into action. This was during the years of the cold war stalemate, when mutually assured destruction was the already in place.

But then there’s the woman who gave birth to Ghengis Khan. It’s said that she caused something like 89% of all modern-day humans on the Asian continent.

Gavrilo Princip nominally started WWI when he assassinated Archduke Frank Ferdinand, so maybe that’s worth considering.

On the other hand, I gather that the consensus among historians is that WWI was in the offing no matter what, and that if Princip’s trigger finger hadn’t started The Great War, some other act by someone else would have.

Then there’s Thomas Midgley, who was responsible for putting lead in gasoline AND for putting CFCs in refrigerators. That’s a lot of damage down to one person.

Constantine the Great declaring religious tolerance for Christianity in the Roman empire.

To answer the OP I was going to say Hitler’s decision to invade Poland, kicking of WWII–which fundamentally and permanently changed the entire world’s political landscape. However, I subscribe to the theory the WWII was, essentially, a continuation of WWI so Princip’s assassination of Ferdinand was pretty much directly responsible for much of today’s political world.

I agree that WWI, in some form, would have happened anyway. But it happened the way it did as a direct result of one single person’s actions.

That’s really hard to top.

I was thinking of three:

Stanislav Petrov (already mentioned)

Norman Borlaug: research on wheat saved billions from starvation

A bit US-centric: Barton W. Mitchell, who found D.H. Hill’s cigars wrapped around Lee’s plans before Antietam.

One guy invented both leaded gasoline and CFCs. Thomas Midgley had a huge negative effect on the atmosphere for much of the 20th century.

Let us not forget Harry Burn, who cast the deciding vote in Tennessee to ratify the 19th amendment, giving the vote to women.