When was steel invented?

Steel is the bacKBOne of the modern world and used in houses, skyscrapers, automobiles and more. But steel isn't found in nature, so when did humans invent steel?

It turns out that this sturdy modern metal dates back at least 2,000 years, though archaeologists don't have a precise date for when it emerged.

Steel is an alloy of iron and carbon; it's mostly iron, but the crucial addition of between 0.2% and 1% carbon makes it harder, stronger and more resistant to rusting than pure iron.

Related: Why doesn't stainless steel rust?

Archaeologists think this innovation was independently discovered in several places and then spread through the ancient world.

"Steel developed through the first millennium B.C. across much of the Old World," Paul Craddock, an expert in ancient metallurgy at the British Museum, told Live Science.

Some of the earliest were "crucible" steels made by melting iron and carbon together, possibly in parts of Central and South Asia, he said.