What invention had the greatest impact on humanity and why?

[malthaussen]

23. Jerry Pournelle argued it was the rigid horse collar.

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