h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I have a couple of points...

The act of flying is extremely comlpex yet birds manage it with a very small brain, so I'm not sure if you could justify human brain evolution on the development of tool use.

There is almost certainly a threshold or limit of brain size above which the development of human intelligence became possible but I suspect that this was reached before our ancestors picked up tools.

The serious introduction and use of tools in our ancestors culture would then have changed their social setting significantly and changed their way of life. A further switch with the use of cultvation further enhanced their social setting.

These substantial changes could then act as a catalyst to release the development of higher intelligence.

The closest proveable analogy comes from the world of programmable gate arrays and genetic algorithms. One example that I remember reading was of a system that was supposed to compare two inputs and produce an output result. The algorithm solved the problem using a relatively small number of logic gates. I can't remember the full details.

On investigation of the final working solution it was found that only about 2/3 of the gates actually did anything, but to remove a redundant logic gate rendered the system unable to solve the problem and additional gates did not improve the operation of the system. Furthermore programmers who examined the problem independently could not build a standard logic circuit to produce the same results without using several times the number of logic gates.

I guess what I'm saying is that the evolution of human intelligence was dependent on at least two factors constantly changing over time. That is the development of the brain to cope with a particular scenario and a subsequent change to that scenario.

For example from prey to predator, from predator to hunter/gatherer, from hunter/gatherer to cultivator, etc.

No other species on the planet has changed itself, its environment and its culture so rapidly or drastically.

However it would be difficult in the extreme to pin down human intelligence to a particular change.