Chinese philosopher Lao Tzu once said 'give a man a fish and you feed him for a day. Teach him how to fish and you feed him for a lifetime'. Over the centuries, many have hailed this proverb and we see its message mirrored in the humanitarian aid sector, in an attempt to ensure long-lasting impact on communities.

led to the assembly line and fostered the industrial revolution.

An invention would be something humans created from their own intelligence and effort.

-- Mal

It allowed ancient humans to think during the night and start developing their brains instead of just trying to stay alive.

but only because someone else posted "language"

Clothing. It allowed is to live in and travel to more places and protect against injury. It allowed us to keep more calories and spend less time staying warm or cool so we could pursue other inventions. In other words, we could regulate our body heat better and be less at the mercy of nature.

The Spread of Steam Power

Steam engines found many uses in a variety of industries, most notably mining and transportation, but its popularization shaped nearly every aspect of the industrial society, including where people could live, labor, and travel; how goods were produced, marketed, and sold; and what technological innovations followed.

I'm sure you can figure out why.

Allowing for more efficient verbal communication. Without it, we're still trying to communicate complex ideas with pictures.

I don't think it can be just one.

The plow certainly. The wheel. The sail. The atlatl. Writing. Brewing. There have been many with huge impact.

My suggestions are steam engine, printing press, communications, computer.

One failure might be more advanced weapons of killing.
Thought to be so horrendous in more efficient mass killing that it might discourage war.
It turns out that mankind doesn't really mind more horrendous mass killings.

All the other stuff is obvious, plow, wheel, fire, all would have been discovered at some point in early human history. The transistor was a real development.

Also the microscope and telescope.

All three changed the world.

Not so much an invention as a concept that dogs taught humans when they first domesticated us.

1. Some ancient Egyptian looking at a cat... 7500 BC

2. Some ancient cat looking at an Egyptian... 7500 BC

Which one picks up the other one's poop?

Prior to the invention of the rigid horse collar, your garden-variety horse could do about four times the work of a human. But since they ate about five times the food, they were not economical. Hence, slavery flourished just about everywhere. After the rigid horse collar was invented, the beast could do ten times the work of a human, while still eating only five times as much. This is clearly much better productivity, and universal slavery began to decline at this point except in specific fields where the horse was of no use in production.

A great impact, but I don't know that I'd call it the "largest."

-- Mal