What took longest to invent?

18th century style crop rotation and other agricultural techniques of the http://en.wikipedia.org/wiki/British_Agricultural_Revolution . No technology needed - you could do this sort of reforms administratively at any era, if only the little birdie were to tell you it’s a good idea.

ETA: no, the wikipedia article does mention a few technological innovations involved. Still, even they don’t sound too complex, even for bronze age situation.

But then, go try convincing a busy Babylonian estate manager he should invest in experimenting with better ideas. The manager has read Paul Graham and knows that ideas are worth nothing. Better read up on Ninurta-the-Farmer’s manual of getting the fields plowed with slaves through the copious application of whip.

I’d consider the printing press among them. The concept was there about 5,000 years before Gutenberg’s printing press, in the form of Mesopotamian cylinder seals. Ink was around for about 3,500 years before the printing press.

Milking. We could have done that at any time, but the first evidence of it is less than 10000 years ago.

Korean Hangul script appeared at a surprisingly late date, especially given that the Japanese had phonetic writing for much longer.

Liquid fueled rockets getting man to the moon instead of gunpowder.
First liquid fuel rocket 1926 as old as man

The horse collar is a similar very useful invention. It went through many refinements of the course of around 1000 years. It greatly increased the land that could be worked by a person when it came into use in the middles ages.

Alternating current is one of the most useful ever

Fair enough.

So, by the OPs specs we need to count from when an axle was technically possible to when it was actually used.

An axle is just a slightly refined log connecting two other logs that have been cut.

The ability to make it existed almost since humans started using tools and human tool use goes back about 3.5 million years (although not sure you would call them what we consider modern humans…I forget where those lines are drawn…more proto-humans).

Perhaps that is pushing it but I bet it was technically feasible to humans 30,000 years ago but what record we have shows no use of a wheel till 8000 BC or so (maybe later).

That’s a good candidate.

As for not needing to move heavy stuff, the plains indians in the US transported their camp goods by pack horse and travois. Wheeled carts probably would have allowed them to cover more ground in a day, but the travois poles could be repurposed for tent poles when the new camp is reached, So you’d have to move the weight of the cart in addition to your tent poles each time, and the cart would be sitting around useless until the next time you moved camp.

Wheels are useful things to be sure, but without pneumatic tires they wear out quickly, wear out roads quickly, and are fairly limited in the sort of surfaces they work well on. Until you are building good roads, and figure out how to vulcanize rubber, I think the wheel is of limited utility.

Wooden cart wheels were used successfully since ancient times. A cart could carry far more goods than a draft animal by itself.

You might be right about kite surfing, but silk at 120,000 years ago? I suppose animal skins could be used. Or bladders, sewn together would be lighter.

How about writing? There is no reason we couldn’t have invented writing as soon as we began speaking. Depending on who you believe, that might have started 1M years ago.

Can’t believe nobody else has mentioned crotchless panties.

Actually, the OP may have hit on the answer himself, pardon the pun. The bone club could have been “invented” by any of our primate ancestors going back at least 50M years. There are always plenty of bones around (or sticks, if you can’t find a bone) and we’ve had opposable thumbs for, like I said, at least 50M years. Hell, you really only need fingers to grasp a club. Opposable thumbs makes it easier, but their not necessary.

So, either clubs or tooth picks. Or chopsticks. Or dildos. Or anything like that.

That reminds me of Sequoyah of the Cherokee, who invented a written Cherokee language. That shows it’s possible to go from no writing to a full fledged written version of the language in one jump if someone has the basic idea.

it took the japanese more than 20 years to construct a walking humanoid robot. experiments and developments were continuous.

This is not true. You might notice an invention only upon needing it, but the actual invention might have taken place at any time. It’s just as accurate to say that invention creates need as it is to say that necessity is the mother of invention.

IIRC the first thing that could be considered a steam engine existed in ancient times. It wasn’t till the Industrial Revolution where it really came into its own.

AFAIK, there was nothing preventing building a gas-discharge laser from the time Einstein first hypothesized the stimulated emission of radiation. But the first lasers weren’t built until the 1960s.

Babbage’s Difference Engine: 1822

Z3 programmable computer: 1941