Swift
Sorry, but you just made universal morals very relative. Morals happen naturally? Ok, so if you naturally happen to not care to much about what happens to others and you find people that are the same, would it not be possible to become friends?
Slightly different from what I was intending, but I'll answer your question anyways..
If you happen to just not care about anyone, and neither does your new friend? Sure you have that in common, but doesn't it mean you can't really be friends in the purest sense of the word? Surely the definition of a friend is someone you care about, even just a little. You have to care about them enough, to the point of realising that person's survival benefits your own. If you don't have the value system in your head that lets you care for anyone at all (even for purely personal gain through collective gain)... you can't ever truly gain any morals...
As humans ARE without a doubt tribal animals... We function and succeed better in our environment when looking out for each other and the common good, we "naturally" developed morals and values that give some rules as to how we should behave in this society. Some of these rules would have been learned the hard way by trial and error "If I steal that dude's food he will kill me, but if I share my food with him, he will save my butt when a tiger comes!"... I'm not saying we always had morals, but that the development of those morals can and probably has happened regardless of religious teachings. I believe its the natural way for humans to exist. People that acted with good morals and respected their friends did better because they soon became happily part of a successful tribe/group. These kinds of moral behaviours slowly lead to us becoming moral, civilised beings, as we could achieve so much more together as a species (both personally and collectively) when behaving morally (as you get back the good that you give, and more. This creates a solid foundation in the society of respect and value of each person. We can act more confidently and know we are working to the same goal for everyone to benefit much more than they could working by themselves). Whether this is catching fish, hunting mammoth or building the Ancient Greek Empire, or indeed the Empire State Building... its very similar in concept. Even wanting the economy to do better - everyone benefits indirectly.
I'm saying morals benefit everyone, basically. If you act fairly, you will be treated fairly likewise. If you value someone else's life, they will most likely value yours. Its a win-win, and what you can achieve together is greater than the sum of the parts (as proven by the rise of civilisation and the spread of humans to every part of the globe). Thats why I think people naturally gained a moral perspective on life... they soon found it worked for them in the long run (moral thought may have needed a tipping point in human intelligence to work... You need enough brains to think about the future further than "damn I'm hungry, I'll steal your food"). Plus, we NEED to have family and friends close (I think this is in our damned DNA somewhere, its such a strong universal thing) so we have some company and a greater chance of survival. Obviously we need some rules that let us hang onto those people we need to survive.
In some ways, I think religion could be kinda like a way to get people to play by the rules, so that everyone benefits (right from caveman times to now). But if you understand the rules already and the good they do to society (and hence you!) to stick to them, it doesn't neccessitate the need to be religious to act morally.
Danoff is right in that it just takes the first logical idea to realise moral behaviours benefit us. it just takes a little intelligence to think beyond the immediate future into the next day, and then the day after that....