This is just speculation, but good speculation. There has been a lot of study on the transition from pre-human to human in evolution, and what was the key factor or cause. There are a number of important developments and variables in play.

Language, certainly is one. Bipedalism is another. (Bipedalism is when the Wright brothers, who operated a small airplane manufacturing shop, one day had the audacious insight that with certain gearing, you could create a two-wheeled ground-based, self-propelled vehicle, which unlike flying cars, would one day be seen as the urban transportation system of the 21st Century.) The third variable is opposable thumbs, which, as we know now, made thumb-wresting possible.

But if you get outside of simple physiology, another variable may be the key one. This is not an original idea with me, but I’m conditionally endorsing it: control of fire. The thinking here is that cooking vastly increased the usable calories available in a largely inedible landscape. Yeah, fire would be transformationally valuable. I’m pretty sure we’ve evolved an attraction to the smell of a cooking fire, and a fascination with flame. But controlling it is hard, as anyone who has ever tried to start a fire by rubbing two sticks together is well aware. You’d need a LOT going on to get fire-making reliable if you were a smartish primate. A bigger brain and some language and opposable thumbs and a would all come in pretty handy. So that’s my candidate. We got good at making fires and keeping them going.

Now we need to learn when it’s time to put them out.