"Humans are Weird" idea???
What if the aliens believe that they are all related to each other, since they are of the same species? Perhaps they think this is true of every species, for example: birds; fish; insects; and so on - under the assumption that ‘They are all of the same genus (or whichever taxonomic rank it is), therefore they must share a common ancestor.’ So, when some of the humans claim that they aren’t related, they get really confused.
- Just imagine two humans meeting back up after a day spent doing their things at opposite ends of the alien ship. They’re hugging, laughing and telling each other about how their day was and what happened that was worth retelling.
- An alien crew member cautiously walks up to them, and they try to hold back their laughter as they listen to them.
- “Are you two siblings?” they ask nervously. The two humans look at each other and chuckle quietly, evidently trying to hold back hysterical laughter but not wanting to seem rude.
- “No, we aren’t,” one eventually manages to say.
- “Oh - cousins, then? Perhaps a few times removed?” the alien tries again, confused.
- “Oh, no, no!” the other human chuckles, slightly flapping their hand as though trying to shoo away the notion. “We aren’t even related!”
- The alien just stands there in shock, mouth slightly agape. Eventually, they swallow dryly and manage to choke out “W-What?! How c-can you not be related?”
- The humans look at each other again and this time they burst into full-on, guffawing laughter. They’re laughing so hard that tears form at the corners of their eyes and they are doubled over, clutching their stomachs.
- “Related? My, if we were all related, we would be very ashamed of family ties to certain people!” one manages between laughs.
- The other wipes their face and exhales deeply to stop making a fool of themselves. “No, most humans aren’t related. The only relations are extended families, and some of those have never interacted with each other at all!”
- “B-But…you’re all of the same species! Don’t you share a common ancestor?” the alien asks, absolutely bewildered at these strange creatures.
- “Eh, who knows? All we know is we evolved from monkey kinda things, or something of the sort.” The human shrugs; the other scratches their head thoughtfully. They chuckle once more, thank the alien for saying hi and walk off to their rooms, leaving the poor thing really questioning its life decisions.