Sounds like a cool dude. Modern systems are far too complex to make in a home workshop, but hydraulics are simple at heart if you can use a little logic.

I’m a hydraulic fitter. I repair and maintain hydraulic systems, primarily on earthmoving equipment.

My vote is definitely for fire. Learning to first contain and sustain, then how to create fire. Everything else follows on from that.

Cooking food probably started us down a path to become as intelligent as we are today.

And the light allowed us to have free time at night to try new ideas.

And the warmth allowed us to spread out of Africa.

So many more books : )