I think I will have to side with several others- in the dim and distant something became a tool user, something made a slightly better tool and was thus less likely to starve, it probably got to a point where things were doing OK but then neurotransmitters get involved for building stuff (similar logic to sex drive being amped all the way up despite it being expensive) meaning better tools get made and a nice feedback loop occurs including increased brain capacity. Fast forward a few million years and new widgets are a way of life, I might have to go a bit further to reconcile that with creation of art and such like but I do not imagine it will be that hard.
Likewise the above might be a tiny oversimplification and I am not sure of timeframes (it might have been the neurotransmitter induced feedback loop appeared way early on instead) not to mention the concept of meta invention (society, farming and such) also playing a huge role.
How it would go from there now laws of the jungle are not quite so applicable is the subject of endless debate although I doubt stagnation will be it (indeed thumbs are larger and there is the whole brown fat thing if you can be bothered to untangle the nonsense from it).
Basically biology and evolution of it.