iPhone. No question. Although I suppose that is a product rather than a technology. If it were a technology, I guess it would be electricity.

Imma say tie between crop rotation and sanitation systems, with penicillin just after. Although the fridge and the printing press are also my favourites, for hopefully obvious reasons.

The steam and the combustion powered engine. They do all the work.

Fire. Makes the indigestible digestible, the uninhabitable, habitable, and keeps the predator from predating.

Mathematics

Haber process (nitrogen fixation) behind the combustion engine.

about half of the nitrogen atoms in the body of an average person living in a developed country once passed through a chemical plant and participated in the nitrogen-to-ammonia Haber-Bosch reaction. Perhaps no other human invention has had a more dramatic impact on Earth than Haber-Bosch chemistry. — Steven K. Ritter

Article: https://pubs.acs.org/cen/coverstory/86/8633cover3box2.html

Legumes (see crop rotation) and lightning are the pre-industrial nitrogen fixers.

Running water.

Mathematics — TheMadFool

I would dispute that mathematics constitute a technology. 'Technology' is derived from the Greek 'techne' which has a connotation of 'craft' or 'artisanship' i.e. something manufactured or made. I would argue that mathematics enables technology but in its own right doesn't fall under the heading of technology.

I would argue that mathematics enables technology but in its own right doesn't fall under the heading of technology. — Wayfarer

Isn't math an invention? Anyway, my next choice is computers.

Isn't math an invention? — TheMadFool

That's a thread in itself? Math: Invented or Discovered?

Electricity, Crop Rotation, Sanitation Systems, Penicillin, Printing Press, Mathematics, Haber Process, Fire, Steam and the Internal Combustion Engine are all life changing technologies.

The printing press was a critical invention, no doubt. So was the steam engine and electricity. The Haber Process might be a sleeper, along with sanitation. These two probably have lengthened life for more people than modern medicine has.

Bazalgette's London sewer system (1866) was tremendously transformative, but so was the Victorian urban dweller's tolerance of their own feces. The Romans, however, beat Bazalgette by about 2000 years. The Romans valued sanitation, but we probably wouldn't find the Roman baths all that inviting -- the bath water was pretty dirty.

The iPhone isn't that big an innovation. Apple combined phone, radio, and electrical technology--all a century old--with computer technology.

Contraceptives. They were instrumental in getting us out of the Malthusian trap.

Photography was pretty important --1839-- without it, scratch cameras, movies, Kodak, great and fine art photography, snapshots, porn, and National Geographic.

The first high-speed communication invention was the telegraph (and Morse Code)--that was in 1834. 25 years later, President Lincoln was hanging around the War Department's telegraph room to get the latest (and unfiltered) reports.

Gunpowder transformed a lot of lives, many of them right out of existence. Still works wonders.

The sail, was around 5500 years ago. Boats? That was big, wasn't it? (5500 years ago was the age of a tablet found in Kuwait depicting a two-sail ship.)

Writing?

Are you sure we are out of it?

I'm not sure.

The answers show that "one technology" is an ambiguous idea.

All technology traces itself back to the one critical formal thought of "a machine". That is nature constrained in a particular mechanical fashion so that it does useful work. An engine is a machine to give useful shape to an explosion. A city's sewerage system is a machine that gives useful shape to another kind of "explosion". :)

To then single out just one most important machine in all history requires some defendable framework of judgement. What are we actually awarding points for? The OP asks us to target "quality of life".

Most would say health - broadly construed - is our number one priority when it comes to quality of life. It could be power, or self-actualisation, or growth. But health is still arguably tops.

That then narrows the field to the usual replies like public sanitation as the "machine thinking" that has had the most general impact.

Fire maybe did have an even greater impact on general health, but was not itself a conscious product of technical thought in the same way as the theory-led design of sewers as a solution in squalid big cities.

And the advances of the agricultural revolution seem only semi-technological. In some ways, crop rotation, hoes, paddy fields, are the imposition of mechanical thinking on the landscape. But as technology, it is more heuristic, less mathematically imagined. In a ranking, early agricultural technology would lose marks on that particular score.

And also it might be questioned how much it improved individual health or quality of life. Turning to farming wore out paleolithic bodies rather faster. It also led to crowded settlements and stratified societies.

The computer is clearly another impactful machine with also questionable outcomes in terms of the generalised quality of life.

Municipal utilities perhaps are the only unadulterated good here - unless you are a wee fish that has to live downstream of the local sewage and stormwater outfalls.

Ya, I'm not sure either. Birth rates in poorer countries are better than they were, but they're still too high. China's growth rate is 0.43% while India's is 1.19% - 2016. In parts of the Middle East it's much higher (they're breeding so fast just to terrorize us).

The computer is clearly another impactful machine with also questionable outcomes in terms of the generalised quality of life. — apokrisis

Computers certainly alter the quality of life, not necessarily in a positive way. But what do we count as "a computer"? Are we counting Hollerith's punch card sorters which go back to the late 1800s? They certainly represented a great advance in data processing. The Enigma machine which was first used for commercial communication encryption goes back to the 1930s, and Turing's machine to decode it were mechanical computers of sorts. ENIAC?

The combination of scientific discovery and its application through invention of new devices doesn't seem to go back all that far. When did this begin -- after the Renaissance? or later, in the 1600s on into the 1700s? This seems to be the era when technology really started to take hold. (Maybe invention preceded scientific discovery in some ways. I suspect that the steam engine stimulated studies of materials and pressure--sort of like rocket science got going after a few rockets went up in the air.

But what do we count as "a computer"? — Bitter Crank

A von Neumann machine or programmable computer. That is what made Turing computation practical.

But then that throws the spotlight on to transistors and the technology to implement a digital logical circuit. And then in turn, it was the photographic approach to printing chips that paved the way for miniaturisation.

So it becomes like asking which was more important, the invention of electricity or the invention of the switch?

Remember that at the dawn of computing - in the 1940s - analog was bigger than digital. There were also plenty of non-Turing notions of computation. And many thought those were the future. But programmable switches that could encode information digitally were an idea that could be cost-effectively shrunk towards the limits of physics. And the software could be written out in ways that could also be mass produced without limit.

So "true computation" was about this double thing of separate programs and separate logic circuits. Which Turing imagined and von Neumann turned into a practical design with a few clever tricks to fool programs into thinking they had an infinite memory and an infinite time to do their stuff.

The combination of scientific discovery and its application through invention of new devices doesn't seem to go back all that far. When did this begin -- after the Renaissance? — Bitter Crank

It was happening tacitly. And then Newton made the strong connection between maths-strength theories and the inventions that could ensue. Newton saw the Universe as a machine, wrote that down in terms of equations. Then invention became engineering and not just craft or trial and error learning.

Actually I think we ought to mention TCP/IP. Without that, no internet. It was designed in the 1960's and 70's to provide a bomb-proof means of distributed communication, and is a work of utmost genius, mainly due to Vicent Cerf and Bob Cahn. One of my first tech writing assignments involved putting together a narrative history of computer networking, which is what lead me to appreciate the enormous genius of TCP/IP. (Yes, it has many huge security holes in it, due to it not being designed originally for internet commerce and social media, but even so...)

One of my fav nerd jokes:

(Y)

What is the most life changing technology to effect the quality of human life so far? — Ponderer

Whatever has reduced or eliminated maternal mortality (the death of women due to pregnancy and childbirth) and infant mortality.

It's a whole series of things, not one thing. Among them...

hand washing and antiseptics
trained midwives
pre-natal care
antibiotics
C-sections can help, but only in surgical setting, and not just for the convenience of the doctor.
better diet (healthier mothers)
breast feeding
better sanitation (fewer infant GI infections)
electrolyte drink (salt, a little sugar, clean water) for infant diarrhea

Stuff like that

It's a whole series of things, not one thing. Among them...

hand washing and antiseptics
trained midwives
pre-natal care
antibiotics
C-sections can help, but only in surgical setting, and not just for the convenience of the doctor.
better diet (healthier mothers)
better sanitation (fewer infant GI infections)
electrolyte drink (salt, a little sugar, clean water) for infant diarrhea

Stuff like that — Bitter Crank

I would say that those and anything else that has reduced or eliminated maternal mortality and infant mortality has changed the lives of humans more than anything else.

I don't have exact numbers, but it is my understanding that before all of that men lived longer than women (that has flipped) and most newborn children--even if we control for infanticide--did not live past a very young age.

Life expectancy was so short in centuries past because so many children died in the first few years of life. That drove the average down. Many women died in their late teens, early 20s--the beginning of their childbearing years. Any problem delivery (lots of them are) had a good chance of resulting in death of the mother and/or the baby.

Everybody died of infections at a high rate: Small pox, TB, polio, syphilis, staph and strep infections, rabies, tetanus, whooping cough, measles, pneumonia, influenza, and so on. Then there was cancer and other kinds of tumors, and finally, accidents.

Better prenatal care in Europe and North America improved maternal and child survival. Then (1860- 1875) Pasteur, Lister, and Koch clearly revealed the role of bacteria in infection and discovered ways (antisepsis) to reduce it.

Up until the 20th century, infection was the leading cause of death. Vaccinations greatly reduced death from viral diseases, like small pox, rabies, diphtheria, tetanus, and whooping cough, antiseptics helped, and finally antibiotics came along.

Men who were not laborers did tend to live longer than their wives. Laborers, on the other hand, had proportionately high rates of death from accident and the aftermath (like infections, blood clots, excessive bleeding, etc.) of accidents. Better safety procedures and mechanization did a lot to reduce accidents.

I'm a big fan of air-conditioning

Anything that causes death: flame throwers, bullets, guns, arrows and bows, nuclear devices, electric chair. I say this because the biggest change you can experience in life is becoming dead.

You are either dead or you are alive and you can only experience life. To be honest even saying, "you are dead" is an odd phrase that does not reflect reality because there is no more "you". "You" can never actually be dead, your life just ends and you cease to exist.