It's rather odd. This is the official definition of "humanidad": http://buscon.rae.es/drae/srv/search?id=1XNnYrgXvDXX2WYs5K30
I would say that you can substitute "manifested itself" by "revealed" or "was revealed". If still need more explanations, let me know ;)
Yes, this may well be a good example.
Poor chap is splashing about in the stream an you throw him a death retarding device (DRD) because you're such a good person. When you tell the story to a lady in an after-hours.
Another use, not that polite is to refer ironically to human smell. You enter that classroom just after 4 hours of topology lecture and you may say: huele a humanidad! literally: it smells to humanity