I can tell you how it won't happen. There have been efforts of late to lie about and/or to distort the past to make way for a more politically correct world. People cut adrift from their true history, their true past or being forced to share it with other people who weren't even there at the time will feel lost.
In the novel 1984 the world's history kept on changing to keep up with present day politics. In Great Britain over the last ten years this is precisely what has been happening. The end result? I think it will be disunity on a grand scale as people struggle to get in touch with who they are rather than what some government on some whim might tell them who they are. Work with the truth and the real past and there is a chance of unifying humanity.
Generally speaking we are all united in basic needs such as food, water, clothing and shelter. Any unifying program can start there. In fact, most of the ones that prove to be successful do.
As for a global tribe, I don't know if it is a good idea. A lot of people throughout the world realize that overpopulation is a problem and have done or are doing something about it. Others will not or cannot because of religion and/or custom. How can you penalize the people doing the right thing by our planet by lumbering them in with the people still doing the wrong thing? I don't believe that it would be fair to do so.
As for mankind...maybe best nowadays to say humankind...