But our ancestors also had to jump on top of prey, and drive spears into huge animals. This requires great force. The body evolved in a manner that can generate this force.
It's the way we were designed to work, and deviating from natural movement (the way our body was designed by evolution to work) will only impair our abilities and not add to them.
That may be so.... but I doubt it. As Pete said, "internal" mechanics requires rewiring/reprogramming the motor-neural pathways in a specific way, which is counter-intuitive to what you describe as "natural" movement. Whilst it may certainly aid in adaptation for hunting/gathering, the only hunting/gathering activity anyone is inclined to do these days, involves a trip to the supermarket, and wrestling the trolley with the dickie wheel.
I believe talk of the dantain is more confusing then helpful. It's just another vague word that is hard to pin down. Firstly it's from a foreign culture (for most of us). Secondly it's an old world word.
If we want to define amongst ourselves what it means that's great, but otherwise it's just confusing.
If it helps, "internal" means... using your middle, and more pertinently, your lower abdomen, to power ALL movements, supported by the ground, and expressed thru the extremities, in a "loose" (i.e. relaxed but tensioned) manner, and without engaging the use of the large muscle groups. Which would seem hardly natural nor intuitive now, would it?