With difficulty.

We humans have a nasty habit of screwing up everything we touch.

Me: Honorable aliens. As you can clearly see, we as the human race have killed millions of our own people throughout history. We will accept the sentence of suicide at our own hands. We will most likely start killing each other off when our population exceeds the resources of our planet.

This way you won't have to waste your precious time or resources in destroying us, we will do your job for you.

If we don't do it in the next 5000 years then feel free to fling a meteor our way, if we can't stop it then human life wasn't worth it anyway

It depends a lot on what authority they use to justify putting us “on trial” and what for. I don’t see how such a thing would be anything short of Hitler’s view of “the Jewish problem”. Those issues aside and in the spirit I think you’re speaking in, I’d point to the holocaust.

I’d show them our music, particularly blues and the swing music of the 40’s. Our need to create beauty in times of misery and sadness is in itself beautiful and interesting. We are feeble and weak, but this need of ours demonstrates that there is something greater in us than we always show...Something worth preserving... it would be hard to convince people that we need to eliminate fireflies and peacocks and that is the center of my defense.

Don’t know that I could, or would. You’ve no doubt seen the star trek ‘the next generation’ trial with ‘Q’ … placing humanity on trial for much of the same.. Seems, ‘by then,’ we’d changed

As far as arguing for our continued existence ‘right now,’ I doubt anyone off planet would care. We’re only hurting ourselves. If we were to begin doing the same with life beyond our own, a different story. Species extinction on earth is tragic, and possibly leading to our own… Rather than be tried, perhaps we simply need guidance … or extermination

Your Grand High God Poobha, I would like to start this evening by saying you look smashing in that...um...thing you are wearing. Absolutely stunning. I have no idea how we win the Miss, Mrs. and Mr. Universe every year. But I kid. Kill them first, please (pointing in some random direction away from myself). They said mean things about you. (It would give me time to hide.)

I would question the alleged basis for a trial of an entire race.
Of what crime(s) are we accused and what party was injured?
Were the only basis for a trial that "they" are "superior", Therefore fit to judge us, Then they would as bad as the worst our species has to offer and no defense would or could be valid. A sham trial in a kangaroo court.
I could not argue that there is some superior, overarching basis of Right that we should be continue to exist unmolested by other species, whatever the source. Our existence is predicated on our ability to survive and sustain ourselves alone.
That a superior race finds us a threat to some pre-existing system of order and agreement among our "space brethren" I would have to plead complete ignorance of any such rule. I believe, contrary to legal maxim, That ignorance of the law is the only possible excuse. We exist as we can. The best that we can. If someone or something has the power to destroy us and a basis in belief or maxim that it should be done, there is little reason to suspect that it will not be done. A trial with a forgone conclusion is but empty ritual. I suppose I would have to close with giving them the "finger".

The same way the religious defend their religion, try to cloud the issue lots and lots of smoke as there is no real defense for humanity.

i couldn't

You've been watching Next Gen, haven't you?

Anyway. I'd say that human history is dark and bloody, but I would challenge the alien race to say that their history is completely pure. I would say that biology drives us to violence, jealousy and evil, and that it's remarkable that even this level of peace has been achieved. I would say that while there is still war, famine, greed, cruelty in the world, humanity is coming through the darkness. Slowly, but surely, civilisation is defeating barbarism.

First I'd say, take the president away. Second, I'd say, humanity is f-ed up! Earth is ruined and take us to Mars.

I’d argue for more time. The nature-nurture thing is still in progress. We haven’t figured out how to control our gene expression yet and besides, we didn’t get to pick our genes......and, the nurture part? We messed it up in the past and are dealing with the consequences now. Psychology and science in general are working on it. Schedule our case for further review in say......600 years? If we make it that far.

Without reading comments, I think it is amazing that the assumption is made that I would be on the defensive side of the argument.

Art and acts of compassion is the only defense. The only proof we have any value at all

I would ask the aliens why we are on trial and not the birds. We are just one of many species of life on earth. Why should we alone, need to justify our existence? But the fact that the aliens were putting us on trial would indicate that they are just as superstitious, and ignorant of how Nature works, as we are.

Well since I am a human and most of my friends are human I would argue for our continued existence. I'm guessing every intelligent species progresses in that three steps forward then two steps back way like us. Seems like at the moment we've taken four steps back. Anyway why would an advanced alien civilization want to pass judgement on us anyway? What are their motives? Is earth in the way of some interstellar on ramp? Really like @SKDeitch 's idea of keeping us in quarantine for a while longer though.

I couldn't.

As Douglas Adams said we are mostly harmless

I can't

I would defend human by saying that no individualization of humanity ever asked for self-conscious existence; therefore, why would your aliens judge reality's unknown functions. Thus, our continued existence is a function of reality. Why would you want to screw with reality's nature?

I don't know if I would. We haven't been the best of shepherds/guardians/custodians of our planet, have an extremely violent history marked with various degrees of academic suppression, and seem to be inherently violent.

I mean, the usual answer to "what would you do with time travel?" tends to be some variant of "go back and kill Hitler before his rise to power" but how many people have you heard say something like:
-"Prevent the assassination of Archduke Franz Ferdinand of Austria, preventing WW1, thereby preventing the economic/social factors which led to Hitler's rise?"
-"Make sure Hitler got into that Art School he had wanted."
-"Figure out when Hitler was conceived and give his mother a headache or make his father flaccid"
-"Figure out when the Hitlers met and prevent them from ever meeting in the first place."

Yeah, we've done some great things when we work together as a people but we're also guilty of some terrible things so, assuming the aliens have a similar moral framework to ourselves, I'd be hard-pressed for finding a persuasive argument for the continued existence of humanity in it's current form.

But if an option existed for "take the next generation and have them be raised by our best and brightest, freeing them from the taint of our past errors" then yes, I'd defend under the notion of "sins of the father" and "tabula rasa."

I'd need to know what the charge is before I could defend anything. If our crime is the way we treat other people and other living things... well then I have nothing.

Would be hard to do.

Thinking that maybe trying to show the inherant goodness of humanity has most often prevailed. That wars, greed, and abuse of the masses are most often things of governments, and those controling governments. That through
out the history of humanity, the good have raised up to strike down the greedy, and the destroyers. And I would plead for them to watch and see, because I have to believe in the inherent goodness of humanity.

I used to work with this brilliant engineer at Motorola, I was a software engineer there too for 5 years til they laid me off with 60,000 others worldwide. You bring up a great question. Wow
for the most part, I'm not that impressed with mankind anymore, nor of this existence. Music is not fun, women are a pain in the ass.....and women either look like my daughter or they look like my grandmother!! So there ya go. At 65, the dating game has come to end. The bars are full of meth heads......and don't play biker music. So the music business is NOT Fun.

It's really bad when you look at the brutal history of mankind. African Americans, American Indians, HItler and the Jews, Joseph Stalin.....wow insanity. The brutality.

And Trump is a nightmare, but so was Hilary. The politicians are all a bunch of crooks. The ecology is screwed.
"continued existence"??? wow.....

I like being with my grand kids, they give me purpose I guess....but I"m ready to go.....
I had back surgery, the hit me 4 times, and I was in a nursing home 3 times....no more
nursing homes for me....im gonna take some cianide something......wow.....that was 2 years ago.....

It is important to get up and see the beauty in our existence, and let LOVE go thru our veins. It's short and when I die, I dont' want to come back here. That is for sure. Most everything is an illusion anyway. I did not want to come back in my experience in 1994, I wanted to go, but they explained to me it was NOT My time.

Hitler was a nut job, and there were other dictators that killed more people. Just psychopaths.

[variety.com]

This was on PBS a long time ago, and I was lucky enough to catch it. It stuck with me, and your query reminded me of it immediately.