Here's the thing. There is a LOT of evidence that supports evolution by natural selection. Not only is there genetic evidence (which would hold up in any court), but there are obvious things that a person can notice just about our structure and function (look up homologous traits, and you'll see what I mean). Why do you think that our physiology is so closely related to primates, even mammals! hence the class "comparative physiology." Model organisms provide further evidence . . . but enough of that rant, I'll re-focus on your question.
Luck.
There are many viable theories that explain how man evolved (not from apes as many believe, but from a common ancestor of primates). My favorite is the blood cooling theory. If you take into consideration that our semi-circular canals are nearly identical to other apes, as is our color vision, and you really get an idea of us as a type of quadriped, you'll understand that our brain was not always on top of our body. The theory is that as our ancestors began to walk upright (whether to escape predators, or hunt, or to reach food in trees) our brains were able to drain blood faster, effectively cooling the brain!
Why is this important? Ever have a fever? A fever when you're young is common, but a fever as an adult is uncommon and can be very dangerous. The reason why is that proteins tend to "denature" or unravel when heated. Considering this, it makes sense that the proteins associated with brain function may be inhibited by body heat, but if the brain were to cool itself faster (i.e. draining more effectively) then proteins would not only be protected, but could proliferate! There's no telling why hominids began standing upright and no other primate did, but that's my best answer. That also explains why man has so many back problems (the vertical spinal column is NEW compared to other structures). Give it a million years or so, and everyone's back will feel much better :) Also, intelligence has been proven to not be associated with brain size per se, but to surface area. Our brains are much, much, much more convoluted than the brain of an ape, this increases surface area, and therefore allows further intelligence.
Back to my rant- Basically I believe that man, like every animal, has survived because of adaptive traits. OUR ONLY STRENGTH is intelligence. We are slow and weak compared to other animals that would share our trophic level if intelligence were not an issue. Can you think of any benficial trait other than intelligence that we possess? Me either. The human eye is something that people will tell you, is too complex to have happened by accident. Why? The eye of the octopus is actually superior to ours. We have a blind spot due to how the optic nerve enters the back of the eye (our brain has to fill in the image). The octopus does not.
Our semi circular canals are nearly identical to apes (as mentioned before). They provide us the balance needed to walk on two legs, but originally they were used to swing through trees like Tarzan.
If creationists are right (specifically Christian/Catholics), and we were created in His image, why do we share so many traits with apes, and ALL mammals? Why is it that the organs we have are identical to organs of other species? Some of which, we don't even use anymore (Appendix). Why does our hair stand up, and we get goosebumps when we are no longer covered in any measurable fur? Why don't we have some special structure or organ, or ANY special trait that differentiates us from the animals? A organ for our soul, or something? Some say intelligence differentiates us, but I say, it reinforces that we are simply another animal that has a particular trait that has benefited us through time. We're just afraid of death, and afraid of being on the same level as the animals we eat and kill. But there's no difference, other than they do what they do naturally, and we do what we do forcibly, because we're confused about what it is we're supposed to do. What we are here for . . is to feed, procreate, and die. How sad.
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