That is interesting.

Of course, it ignores the social problems raised by turning your neighbors into a food source and all of the extra disease threats that come from cannibalism, but being a cannibal does have a certain panache.

Not to mention the fact that forming a tribe/group allowed humans to be far better predators and armies vs rivals.

it's not a social problem if you only eat your enemies!

like I said, you don't eat your friends. You kill other tribes and eat them. Tribes like this still exist.

 "So we can see over the tall grass" is even more lame. I wonder if someone was smoking some grass when they came up with that crazy answer.

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Also bigger brain doesn't make someone  smarter so it questionable if a big head  has a real advantage. When it comes to evolution you can  pretty much make up  any answer.

no you can't. It's people who are unfamiliar with science that say that. Just because there are a lot of guesses doesn't mean they haven't been very thoroughly researched. 

Evolution comes down to 2 things. Which mutations end up making an animal either a) reproduce more or b) live long enough to reproduce several times.

And actually, the size of our brain cavity is what allows us to have such a large brain. In fact, the most sizable portion of our brain is the cortex, which is notably small or absent in most other creatures. It is this cortex that is involved in highly skilled action, abstract thought, understanding of cause and effect, long term memory, planning, and complex motor skills like multi-tasking.

And you are correct that having a bigger brain than someone else doesn't make someone smarter, but having a bigger brain than another animal is a near infinite difference.

Seeing over tall grass is a very valid possibility. Almost every animal has the ability to stand on hind legs for short durations when scouting out areas.
Even better for humans, the ability to plan out their near future coincides almost perfectly with being able to see the entire environment.

 This is no researching we walk upright because we can see over grass as if every example there are counter examples. Because of so many counter examples found in nature evolution has to cover everything which in turn explain nothing.

 Continuely standing straight up also has it's disadvantage just like a sub continuely using active sonar. Thus other animals can easily see you before you see them.

Cannibalism generally leads to extinction in the long run.

that's a pretty useless counter-example, because subs can get attacked from a great distance once they are spotted, while early humans in the "look over grass" theory would be up against feline predators (the natural predators of the savanna), which are known to only melee, so if those are spotted outside your area of danger they become useless even if they manage to spot you from out of your vision

and these predators easily give up on attacks if the chances of succeeding are bad (if they are spotted early), for burning calories or (even worse) be up against harsh defense which could lead to injuries or broken bones/legs (certain death) is stupid

Fitness and adaptability to an environment.

Using our hands was more advantageous in the environment that our early ancestors lived in, therefore they were more likely to survive and pass on their trait. Eventually we got better and better adapted to using our hands in this way as our need to use them increased using crude tools, etc...), until the point that we didn't need to use our front limbs for walking any more, they were much more important for other manipulating objects and carrying out manual tasks... A few thousand generations later you have us upright walking homo sapiens. Simple.

Well, I may have missed out a few details, but that's a pretty broad overview of what happened.