As with everything else it comes from us being adaptive, animals express emotions too as anyone that ever had had a pet knows. And it's what makes us being able to function together. I don't think Jessica's words were that wrong, there are a lot of behavioral studies that focus on social traits. The question is if there is a genetic trait to it, if we sort of are evolving emotionally through some sort of genetic selection?
I doubt it, take ten people and place them on a island with limited resources and see what happens :) 'Lord of the flies' is a good example on that. But we have changed over the centuries, in medieval times good fun could be throwing a cat in the fire and watch it burn. But that have more to do with how we objectify people and other things around us and little with emotions per se. Those watching the cat burn could later fall into tears at the theater watching some poor child 'die'.
Emotions are good for us, and help us communicate and help each other achieve common goals, even though not always pleasurable. There are people with brain damage without empathy or with very little of it. They have committed some of the ugliest crimes against other people that I know of. And as everything else related to the brain it seems to be more related to specific areas, which shouldn't be taken as that you will be unable to feel empathy with those areas damaged, The brain is very plastic. But specifically they seem to be localized at the bilateral temporal gyrus and the superior temporal sulcus. Look Here.
But emotions seems to be something we always have been born with, to that we also seem to have instinctive (genes?) crowd behaviors that are the same all over the world as I understands it. I don't think we would have made it so far without them. And what I think have modified our modern behavior is not our genes 'changing' but education. And that's why I think the Internet is the greatest invention since the wheel, as it gives us a constant possibility to learn new things and interact. The more we learn the better we can relate to others and the better our judgment, and I'm not talking about getting a degree, I'm talking about living and learning. And I think that asking questions like you did Colarris, is the first step on an never ending quest for knowledge. If you want to read a really cool book I would recommend "Crowds and Power" by Elias Canetti.
He's a real cool read, and lived in turbulent times, the second world war. He studied the behavior of people and then wrote a book about it, he never received the recognizance that book deserved as it didn't 'fit in' with the theories prevalent under that time but it's very enlightening all the same, and cool.