Why have humans evolved conciousness?
There have been many proposals over the years as to why human consciousness has emerged and how, or even what it is.  

Most of us will not be surprised to know that there is no consensus about an answer here.  Its hard to draw a trend from a single example.  

Here is a little survey of the one's I've heard. 


We are intelligent because we have opposable thumbs and can use tools, which evolved our brains.  This is an old one - maybe as old as Darwin's time.  I think its mostly ignored now because so many animals - including pandas and other non primates have opposable thumbs or use tools or both.  (monkeys and chimps use tools, and recently crows have also been observed to use tools.)
brain size.  We do have a large brain to body size ratio.  There are other animals with similar ratios and they don't have internets.  
We are 'self-aware'.  That is to say we recognize ourselves as an individual.  It was proven that other primates can recognize themselves in mirrors (putting a red dot on their forehead. when they see their reflection, they touch their own forehead).  So that's not exclusive either.  its not a bad idea, but so far it doesn't seem to make the 
'We ate something' I once read a discussion of how humans might have ingested psychotropic plants or fungi.  As you can see, this was 2010.  Its not the worst idea, but its hard to prove.  Lots of experiments on cats since the '60s have not produced cat's who care to tell us if they are intelligent.  Maybe they are just too smart in the first place.  


These theories, from when I was a student are so discounted now, you can't find too much record of anymore since they were pre-internet and also they seem so unlikely now.  

More modern theories have locked onto social configurations of human societies which drive intelligent adaptations.  This is stemming from observations that human evolution (the rate at which genetic variants are retained in the gene pool) are accelerating since humans have become social.  

These new theories while standing on some amazing evidence are still feeling around in the dark - stuff about how social interactions have created a preference for intelligence-like traits. There is probably something to that - once intelligence gets a toehold it is a clear advantage.  But how did it show up only once and why us? I think that's the hard part that science has, for the moment, given a rest.