We developed tool use. Canines are of little use in eating fruit, but they are very useful in fighting. The Homo line developed tools to do the fighting for them, and the canines tapered off accordingly.
For that matter, why on earth would we have lost the the fur that covers all the other hominids? What kind of evolutionary advantage would that afford, to be MORE exposed to the elements?
There are a couple of possibilities. One would be change in environment and development of hunting. Running after prey in the open, sunny plains, would require a better cooling system than apes had in the hot but moist forests.
Along with the helplessness of our infants and the burden they place on the mother.
That is also an ape trait.
Humans are not well suited for unprotected life in the wild. It does not make sense that the genetic changes that produced a weaker creature would have allowed the creature to so successfully reproduce and flourish.
The change to "weakness" was allowed by the development of a wide range of tools and culture. It did not happen in isolation.
Brains alone don't ensure survival.
Of course not. But tools and culture are extremely important in our survival, and the degree that humans possess these traits far outstrips the rest of the primates. These traits are almost certainly associated with the increased brain size.
(Good questions by the way.)
Although I don't agree with your conclusion here, I can certainly understand the reasoning behind it
The gradual regression of this genetic expression on the population level, to my understanding, had to be the result of either Genetic Drift, or Natural Selection.
1) Genetic Drift.
According to evolutionary biologist, Douglas Futuyma, Genetic Drift is entirely random, and in those cases where the trait is neutral to survival and reproduction, it is not subject to natural selection1
I think you would agree that isn't the case here, not only because the trait isn't neutral to survival, but because the random nature of the genetic expression would falsify any correlation to the advent of tool use.
2) Natural Selection
This seems to be the only mechanism, along with mutation and/or recombination, that can account for the regression and dominance of the trait within a population, and lend support for a causal relationship (even if by argument) to tool use.
The problem is, since the expression of the trait itself is random, then at any given time, and at any given step of the regression, those organisms will be competing with otherwise normal, fit, organisms of the same species, at the same time, within the same population.
It follows that any morphological change that isn't neutral, which your proposition is not since it effects survival, must be equal to, or greater than, the morphological benefits of a normal, fit organism of the same species it is in competition with. Anything less than equivalence would immediately infer a disadvantage, and grounds for falsification that it would dominate a population.
In short, the adaptability of mutated organism A, must at least be equal to the adaptability of normal organism B, throughout the entire process of morphological change from generation to generation. Each new instantiation must be equal to the one preceding it, reducible of course, to it's normal fit state.
Although it's possible that the trait would dominate a population, I simply don't see any selective advantage that it would do so.
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Sources/notes
1. "You can’t have any evolutionary change whatever without mutation, and perhaps recombination, giving rise to genetic variation. But once you have genetic variation, there are basically two major possibilities: First, there is simply no difference between the different genotypes or different genes in their impact on survival or reproduction, and in that case, you can have random changes of one versus the other type in a population or a species until eventually one replaces the other. That is an evolutionary change. It happens entirely by chance, by random fluctuations. That is what we call the process of genetic drift. * Genetic drift is very different from possibility number two, natural selection..."
Interview with Douglas Futuyma
But would that offset the protective advantages fur would offer. Virtually all the rest of the mammalian predators in nature are fur covered and they seem to have managed, even on the open plains.
Are other ape infants as helpless for as long as human infants? Any I've seen at the zoos are at least clinging to their mothers back at a very young age. Human infants can't even do that. Another disadvantage of fur loss, aside from protection to cold and UV.
If we indeed do use only 10% of our brains, then how is brain size significant? One could conclude that the skull size could be significantly reduced and still produce as intelligent a creature. It would just be a matter of using proportionately more of the brain, even though the same amount would be used in each case. Don't Pygmies have smaller cranial size to match their stature, and are they less intelligent? How do their brain sizes compare to that of mose apes and chimps?