The main point is that - in order to make the creative breakthroughs which define genius - a genius needs to be much more relatively intelligent than the average for his population.
Early geniuses will have done things like invented the spear, the spear thrower, or later the bow and arrow.
It is important to recognize what a huge step it was to go from a sharpened stick, to a stick with something like bone or stone to make a harder point. The inventor would not need to have a particularly high absolute intelligence, in terms of - say - the intelligence of the UK or US population of today. But the inventor would surely have had a much higher relative intelligence than was average for his population.
The invention of a spear thrower required a higher absolute intelligence than inventing a spear - because it requires more prior knowledge, is more abstract and technologically complex; and inventing the bow and arrow was a step higher in absolute intelligence.
But probably the inventor of the spear, spear thrower and bow and arrow were all approximately similar in terms of being relatively considerably above the population average - and therefore the technological advance from inventing spear to bow and arrow required a rise in the average intelligence of populations.
However, once these had been invented, once some person had made the creative breakthrough - the discoveries were all very easy to understand, copy and use - and could all be adopted by almost any human society of whatever intelligence.
That is why geniuses are so significant in human history.