ah, a nice discussion about something not useless! pity I'm again late to give my opinian.
What justifies our existence? well, nothing I think. We are just here, nothing we can do about it (but that's already clear here)
But, like all animals we want to continue our specie, that's in our genes. We also have got the brains and the ability to think further of the consequences of our actions. Ofcourse most people don't think of that. So eventually we will destroy our own specie with that and make us an evolutionary failure
I agree with warthog that searching for an answer of this is useless. Thinking of just things only have lead to things like Gods and everything.
My justification to be here is having a good time. have others a good time and eventually reproduce so more people can have a good time!
about the horny bacteria:sexual reproduction had without doubt an advantage to exist. ofcourse, we wouldn't be here if that wasn't the case. And I'm glad it is the way it is!