Douglas Adams writes:
This planet [Earth] has - or rather had - a problem, which was this: most of the people living on it were unhappy for pretty much of the time. Many solutions were suggested for this problem, but most were largely concerned with the movements of small green pieces of paper, which is odd because on the whole it wasn't the small green pieces of paper that were unhappy.
And so the problem remained: lots of the people were mean, and most of them were miserable, even the ones with digital watches.
Many were increasingly of the opinion that they'd all made a big mistake in coming down from the trees in the first place. And some said that even the trees had been a bad move, and that no one should ever have left the oceans.
And then, one Thursday, nearly two thousand years after one man had been nailed to a tree for saying how great it would be to be nice to people for a change, a girl sitting on her own in a small cafe in Rickmansworth suddenly realized what it was that had been going wrong all this time, and she finally knew how the world could be made a good and happy place. This time it was right, it would work, and no one would have to get nailed to anything.
Sadly, however, before she could get to a phone to tell anyone about it, a terrible stupid catastrophe occured, and the idea was lost for ever.
Any one of you with a fairly good sense of literature should know that this is from the introduction to The Hitchhiker's Guide to the Galaxy and that the story itself is not about the girl and her superb idea, but about the terrible stupid catastrophe. Well, never mind the catastrophe, and let us dwell a bit with the girl and her idea.
What I want you to do is to come up with what you think this girl found out. What had gone wrong all this time, and how could the world be made a good and happy place?
Less humans.
Think about it, the animals would be happy... and dolphins would take over the earth. Which is good, because dolphins are cool, and animals don't seem to screw things up like humans do.