Fire isn't an element unless you're posting from the past, but that's a tangent.
While fire isn't an invention, the ability to make fire on demand is an invention. It also requires tools, which were invented for that purpose. Since it provided cooking, heating, lighting, protection from disease and protection from predatory animals, pottery, use of metals (other than as rare jewellery) and engines, the invention of making fire on demand is definitely a world-changing invention and arguably the most world-changing one. The only inventions I can think of that might be on a par with fire on demand are agriculture and writing, but both of those could be considered at least partly dependent on fire on demand. The key crops, the ones that made agriculture of such monumental importance, are grains and cooking was key to their usefulness. They're not very edible to humans without cooking - you can grind them and mix the powder with fluid to ingest it, but that's not obvious, ideal or particularly appealing. If that was the only way to eat grains, people might well not have bothered and instead stuck with the tried and tested lifestyle of small groups of hunter-gatherers. Writing stemmed from large settlements, which stemmed from surplus food, which required agriculture and arguably other applications of fire on demand as well. Maybe it would be possible to build a city with no power, no heating, no lighting and no cooking, but would anyone have bothered even if it was possible? Hunter-gatherer worked just fine.
Move on a bit in time and the fundamentals of more advanced technology come to mind. Euclid comes to mind for that - his intellectual inventions were of immense importance in shaping the world over the last couple of millenia. Or more material inventions, such as concrete. That's about as old, although knowledge of how to make it was lost for a while. Maybe the wheel - that made a lot of difference. Domesticating animals, which ties in with the wheel and with agriculture and communication and infrastructure. Things would have gone very differently without cows and horses, especially cows since they did most of the work until a few centuries ago. Or sheep - wool was of enormous importance in many times and places.
Or maybe epidemiology. Without that invention, we'd probably still be dying of cholera and typhoid and suchlike. Also, on related notes, germ theory, the sanitation movement (I'm not sure of the name, but it was hugely important in the 19th century because it was a practical and working solution to many diseases for both of the competing schools of thought regarding the cause of those diseases) and the medical procedures invented by Lister. Similar procedures had been invented a little earlier in a different place by a different person, but they were suppressed and had no effect. Lister invented similar procedures independently (building on Pasteur's work) and managed to get them implemented, which made all the difference. I'd got "Samwise" in my head for the other guy, but that's obviously wrong
Looking it up...Ignaz Semmelweis. Sad story - he was absolutely right and had evidence to back him up, but he was imprisoned in an asylum and died there soon afterwards, probably killed.
