The last line of this has been running through my mind these past few weeks. As such, consider yourself warned for spoilers. Besides. it’s difficult to be coy about a piece less than five hundred words long.
Indeed, the length is difficult to use well. This story takes the form of a message from newly arrived aliens. They claim to be the original humans, the ones who colonised Earth many millions of years ago.
I don’t quite know how to explain it, but from the outset the reader feels like they’re being set up for the ending: be it a twist, a shaggy dog story, or some similar dramatic revelation. There’s a sense of deferred information to the piece, that the big idea is hovering just offstage. In this case, the story is a message in monologue form from newly arrived aliens who claim to be the original humanity. Earth was a colony that has been abandoned for several million years, and they have returned to re-invite us back to galactic society.
Now this is a venerable story trope, especially in 1971. If the aliens are the good guys, it’s a vehicle usually used for critiquing some aspect of our social development (that we’ve invented war, screwed up the environment etc). There’s also the possibility of humanity having grown more powerful than the aliens and attacking them, leading to a “But we created you! / But we are you!” type of ending most common in robot uprising stories.
Clarke does neither. His story, and the setup of the alien precursors, are just a vehicle with which to deliver these last lines:
We have a simple remedy for the offensive yet harmless genetic plague that afflicted so many of the colonists. […] People of Earth, you can rejoin the society of the universe without shame, without embarrassment.
If any of you are still white, we can cure you.
Reunion by Arthur C. Clarke
Availability: print
Word count: 450
First published: Infinity Two, edited by Robert Hoskins, 1971, Lancer Books
Where to find it: The Wind from the Sun, collection, 1990, VGSF / Gollancz
The Collected Stories of Arthur C. Clarke, collection, 2001, Gollancz / Orion
Ten to Infinity 
Your first line caught me when searching for this – its been running round my head all day. Brilliant, and as you say the setup leads to an unusual punchline for these stories. I think it also resonates all the more today for the time it was written.
Regards
When I suspect a person harbours some hidden racist streak – I tell a summary of this story. The person ALWAYS laughs – because they have to do so to be PC. But the QUALITY of their laugh tells you a lot about what they really feel inside! 🙂