What were the big historical game changers?
By this I’m talking about things like the discovery of America, the abolition of slavery, freedom of religion… Inventions count too: certainly the invention of the automobile and the airplane changed things a lot. Mass communication changed things, or at least sped up the process enormously. The world wars clearly set big changes in motion.
What events in our history have changed our direction big time?
The first one that comes to mind is the Contraceptive Pill. Freedom from the risk of pregnancy enabled many more women to work.
But the biggest are all modes of communication:
Speech itself.
Writing.
Printing.
Telegraphy / telephony.
Radio / TV.
The Internet.
Each step enables more and more rapid dissemination of information, and more and more independent publication of information.
My first thought was the pill as well.
Agriculture
Adding a few that haven’t been mentioned:
Domesticating animals
Invention of the modern number system
Germ theory
Learning to control electricity
The scientific method. The best source of knowledge ever invented.
And somewhat related, the method of invention; as someone IIRC put it, the greatest invention of the 18th century was invention. Basically, the idea of systematically trying to invent things using known physical laws and/or systematic experimentation, along with patents/copyrights to make it worth the effort.
The concept of progress. This is a subtle one; historically, progress often stalled because it simply didn’t occur to people that it was desirable, or that they should compare methods and choose the most effective ones. And even if it did occur to someone, “It’s traditional” was considered a perfectly good reason for doing something inefficiently. Innovations would appear and then vanish because no one bothered to imitate them. I’ve read about how a water powered version of an assembly line was invented in ancient Rome - and never imitated. How whether the English or French won a battle was largely determined by how many longbowmen the English brought along - and it never occurred to the English to actually make a point to bring along more bowmen. And then there’s all the things that were invented in China, like clocks and then ignored as toys.
Now, people actively look for ways to do things better. Progress is considered a good idea, and good ideas tend to be imitated or outright stolen. Even if something is discovered and thought to be useless, people will fiddle with the idea until they find a use for it - I recall reading about lasers as a child, as being “a solution in search of a problem”; a thousand years ago they’d have been some Emperor or King’s toy and forgotten in a generation.
Not yet mentioned (Some big, some smaller):
Buildings (Architecture)
Division of labor
Cities (Civilization)
Government
Monotheism
Democracy
Gunpowder
Black Death
European colonialism
Steam engine
The Industrial Revolution
Vaccination
[Nothing of any importance happened in the 19th century]
Bakelite
Russian Revolution
Nuclear bomb
Indian Independence Movement (organized civil disobedience, British decolonization)
Establishment of Israel
(American) Civil Rights Movement
End of the Cold War
The programmable electronic computer
The personal computer
Election of Barack Obama (Whoooo! Yeeeeeaaaaaah!)
Ok, maybe it’s a bit too early to tell on that last one…
Eh, swap the order of “vaccination” and “The Industrial Revolution”; also, “[Nothing of any…” should read “[Nothing not yet mentioned of any…”, and be taken entirely at face value, rather than as a statement about the large chronological gap in what came up off the top of my head.
Ack, and now I see there are some pretty egregious editing fuck-ups in the order towards the end. Well, I’m sticking to my theory that the Berlin Wall fell shortly before the development of ENIAC.
To the above I’ll add
Air conditioning (completely changed the design of houses, made the South an economic power house again by opening it to service jobs and factories, etc.)
The elevator (NYC, for example, had pretty much run out of room to grow until “Up” was an option)
The dc battery
The word processor (changed the job of secretaries to admin. assistants in many situations)
The copier
The cannon (made castles obsolete and changed the layout of cities)
The cotton gin (black seed cotton [in which the seed is easily separated from the fiber] was a major commodity but only grew well in swampy environs such as coastal South Carolina and Louisiana, while green seed grew well further inland but it was so labor/time intensive to separate the seed from fiber that it wasn’t grown- the gin made it possible to separate it quickly and efficently [ever seen one in action incidentally? Or the mule powered “blower” variety?). The cotton gin probably changed American history more than any other invention that originated here. (Eli Whitney also designed an assembly line method of rifle making.)
Soybean farming (G.W. Carver was far more important commercially for his soybean uses than his peanut uses [which were largely overrated])
The development of the laws of electrodynamics was one of the nineteenth century’s major scientific contributions.
Discovery of time, or rather, the ability to record it with accuracy - enables navigation, and nowadays many other things too.
Invention of the electrical transformer (this enabled the construction of widespread distribution systems)
Invention of the electronic valve. (and with that the discovery of rectification - which allows the use of just about every form of electronics)
Invention of the transistor.
Take these away and we are back in the days of the industrial revolution, even medicine relies on electricity.
I’ve heard the horse collar being credited as a “game changer”; specifically, it made slavery less profitable and helped bring about it’s loss of acceptability. As I understand it; before the horse collar ( which lets horses use their full strength without strangling, by putting the weight of what they pull on their shoulders and not their throat ) slaves ate less food for the amount of work they did as a horse would. Afterwards, it was the other way around, and horses don’t rebel and cut your throat.
Sewer systems. Besides making cities more pleasant, they make them much healthier places to live - they used to have a death rate that outpaced their birth rate. Which has greatly sped their growth, and helped changed society by urbanization.
I’d just like to ad that part of the problem here, atleast when ancient Rome is concerned, is that such things simply were not needed. One of the biggest problems in roman history was unemployment - there were too many people, and labour, slave or free, was insanely cheap. The unemployed free voters were always a mayor source of unrest and a huge headache for the political and
economic leaders of Rome. In such a circumstance, a device that did the same job with less people would at best be useless, at worst perverse. What they needed was something which allowed them to employ more people with the same amount of money and work.
So, Game Changers: The relatively low population of Europe during late pe-industrial times, leading to a need for innovation.
The Marius reform, creating the efffective and (at the time) nearly endless roman army machine.
Penicillin
Mandatory schooling for children
The ability to travel safely over open sea (as opposed to along the coast)
The invention of the proffessional soldier.
The invention mandatory military service, or the re-invention of the unproffessional army.
They would if they could. 
How about toilet paper? Maybe it’s just a creature comfort, but if civilization ended tomorrow, I could survive without the Internet, fossil fuels, air conditioning, and just about everything else aside from canned food & shotguns. But I don’t think I’d make it without toilet paper. I’m totally dependent on toilet paper.
The invention of paper in general. People always think about the printing press as being a major turning point but without cheap paper to print on, it wouldn’t have gone anywhere.
Some animal-related major inventions: stirrups, the plow, and hay (farmers in Europe used to not bother raising livestock because they couldn’t feed them through the winter).
Guns.
The birth control pill.
Bronze working or civil service. Early axes/maces kick all sorts of ass.
The Internet. I firmly believe that this is the biggest game changer we have ever seen. And we get to be in the middle of it! Exciting!
I tend to agree. But do you really think the impact will be bigger than, say, paper was as Little Nemo pointed out? Or bigger than the appearance of the telegraph/telephone? It seems to me that each advance in communication / mass communication has had an enormous impact. The next one coming around the bend should be no less interesting.