Discovery of an extra extraterrestrial being or Alien of any type would most likely mean the destruction of humankind.
I don't think that it is within the capability of the human mind to be able to accept an outside living existence from our own world. Paradigms would be shifted and people's worlds would be turned upside down at the thought that all they had ever known as true in their life is completely different from what they were taught and told growing up.
I don't have faith in humankind could peacefully accept the existence of extraterrestrial being. It's sad but true. Maybe some day our culture and society will become advanced enough that if something on this groundbreaking level of thought did occur we could handle what the consequences would be to our thinking.