Limb
It is a simple hypothesis that leads me to believe that humans have a second arm for the sole purpose to destroy the fabric of community and love. A blessing in disguise, our five-fingered fallacy strives only to bring us down and destroy the basis of every friendship and relationship.
There are several reasons why I believe this to be the case. The first and most prominent is that, without a second arm, humans could not work alone. This "evolution of destruction" allows humans to do things like construction and cooking without ever having to be in contact with another member of the same species. By allowing people to go through life never needing another individual is scary, to say the least. Just imagining humans living as individual cells of something more, always bumping into each other but never needing each other is something reminiscent of a nightmare.
The second reason why I believe that a second appendage will destroy us is love. The most basic sign of love is clasped hands, complete with interlocking fingers. As humans are right now, we can hold hands; yet, we have another hand free. This does not symbolize the necessity in love. One may hold hands with another who is holding hands with another. A red-rover-esque orgy of some misogamy could form, humans never seeing the realness of togetherness.
With just one arm, humans could still perform the tasks required for basic survival, but would require others to help in other life endeavors. Humans would not live alone, but in colonies and whole communities, each helping one-another like it should be.