We don't know when, but humans recognized that fire is hot, hazardous, and helpful if managed 250,000 to 1.5 million years ago.
The Neolithic Revolution transformed humans from gatherers and hunters into farmers, allowing them to grow food instead of hunting or gathering game.
Around 9,000 years ago, mankind learnt to use metals. Starting with bronze changed the world forever.
The Roman Empire crumbled in 476 AD to European barbarian tribes. This event ushered in the European nations we know today and plunged the world into the Dark Age.
Some say the Dark Ages lasted from 476 AD to 1300. People were essentially illiterate, medicine was nonexistent, and they lived like there was no tomorrow.
The Dark Ages ended around 1300 when Italians valued beauty. The artists Dante, Boccaccio, and others recognized life was more than dirt, death, and sin.
The 1439 printing press changed the world by spreading information. Johannes Gutenberg developed previous concepts to create a mechanical device that could print text or voice faster than writing.
In 1492, Columbus discovered America without knowing it was a new continent, but it quickly became a land of wealth and opportunity for the Old World's elite.
First Industrial Revolution, from 1760 to 1820, sped up manufacturing and left millions of people unemployed.
That is undoubtedly the most important invention of the 20th century, and modern people cannot ignore it.