Kill Lincoln earlier during the Civil War.

Shoot Arch Duke Ferdinand. Few people even now about it today, but he was subject to a horribly mangled assassination attempt that many scholars belive would have set off a major war in Europe had he died.

Wait until I’m about to die, then go back to when the first land-dwelling animals first started coming ashore. I’d bring a bunch of cockroaches with me and then I’d die there. Either the roaches or my decaying corpse will hopefully cause enough change to the timeline that humanity never develops in the first place.

I think that’s the most I could screw over the (human) world.

I could skip my next union meeting, I guess…

Tell Hitler to focus on Operation Sea Lion and to leave Russia alone

I really like this question. So: rather than killing Hitler, what if, instead, you killed Stalin? Was it inevitable that a strongman dictator would have taken over, and ruined the potential of communism? I guess we have evidence that the answer is “yes,” in the form of Mao, but weren’t the Chinese communist party(s) greatly influenced by the Soviet model? What if Russia had, instead, developed a more democratic system of government - was it possible, and couldn’t it have affected how China’s developed? But, maybe it is always inevitable that dictators emerge from internal revolutions like this.

Here’s another scenario: what if you stopped Oswald, and prevented Kennedy from being assassinated? He was popular, and likely to win a second term. What would 4 (~5) more years of Kennedy look like?

Go back to the stone age and drop modern steel tools and weapons where a tribe could find them. That tribe would likely become dominant and perhaps change everything.

Go to back to the big bang. You presence alone will butterfly effect the earth out of existence.

I’d kill my parents before I was born.

Bring knowledge of CFCs to a time when we’re able to make them but not able to detect the ozone problems they cause.

Go punt kick the first fish back into the ocean as it tries to waddle it’s fat ass out of the water.

I know that’s not how evolution works but I can dream… Life would likely be vastly different given such a long period of time with something like that changing either way.

How much power do I have?

If I could divert the asteroid that resulted in the K-T event, that’d drastically change history. It may not have stopped dinosaurs from eventually going extinct, but it’d have given them 33 million more years more to evolve, and would certainly have affected mammalian evolutionary history. Maybe, just maybe, raptors would have gotten smart enough.

Ooh! Take back a lot of ravens. They’re almost smart enough already. Heck, I wonder if taking ravens back even earlier would be enough for them to evolve into something dominant. Problem is, they’re not particularly social, and I think that’s been our greatest advantage.

Or: introduce modern octopus to ancient oceans.

Stopping the K-T event is my favorite, though. It would absolutely have changed how life on Earth has evolved since.

30M years between extinction events is about all you get, though.

Go back in time and do something to prevent Vasily Aleksandrovich Arkhipov from becoming the Executive Officer on the B-59 Soviet nuclear sub in October 1962. He’s the guy who talked the Captain and the Political Officer out of launching the nukes when they thought they were being attacked by the US Navy during the Cuban Missile Crisis. His persuasiveness is generally considered to have avoided WWIII from starting then.

Hmm I don’t know. Probably go back to Germany 1940 and give them all the information needed to develop nuclear weapons first. You just know that Hitler would’ve pushed the red button a hundred times over if he had the chance.