The Defining Moment of Mankind

I recently sat down to dinner with three close friends.  After some time, the conversation entered into some intriguing domain.  The following question was posed: “What moment in history best defines mankind?”  There are so many instances where man has done something of great significance.  How could one choose just one defining moment?  It would need to be an event of upmost importance – one that changed the course of history in a major way.  It should also reflect the nature of our species and set us apart from other ones.  After some pondering, each of us expressed our respective view on the matter.