If simple stone tools always leads to the next step in tool use culture then chimps should not have been left behind. It required a combination of tool use and bipedalism plus some currently unknown other factors based on the long period of slow to stagnant Australopithicine evolution followed by very rapid evolution to Homo erectus, more long stagnation then rapid evolution of neanderthal's and modern man. The ice age was probably the cause of the latter leaps, but I haven't heard an explanation for the rapid rise of Homo erectus.

The context of my comment was both the ETI debates and the theory popular decades ago that toolmaking drove human evolution. It was thought tool use was the first step which drove us to bipedalism and bigger brains in order to become better tool users due to an extreme survival advantage to tool use. This belief was common back when the Drake equation was first formulated. That in combination with the belief that dolphins might match us in intelligence led many "astrobiologists" (a term not then in use) to conclude that chance for evolving intelligence was one once life evolved.

The belief that human level intelligence is some natural consequence of evolution continues among strong ETI believers to this day despite a much clearer view of human evolution which shows it was a quirky branching path to us not some steady positive feedback loop. I have heard it claimed that few anthropologists now think evolving our level of intelligence is anything more than a quirk of evolution. Human evolution is very supportive of Gould's punctuated equilibrium concept. It's too bad he is no longer with us to comment on these issues as more data comes to light.

Regards,
Mark