I remember a short story about aliens looking for planets to sterilize and colonize, except if the occupants have some redeeming value. They're about to give up on neolithic Earth, populated by warlike brutish cavemen, until they stumble on cave art paintings of animals et al. They have abstract art, but had never tumbled to representational art like pictures of actual things. This is unique and Earth is spared! Possible by Asimov, but I can't find it.
Thanks phantom42 - good points. I think I read it ~20 years ago, tho at age 66 and been reading SF since 13 with subscriptions to Analog, F&SF, etc. and numerous anthologies it gets blurry! I seem to recall it was in an anthology, as I seem to remember Asimov based on its style and subject. Obviously not too helpful.
All that SF corrupted me into an astrophysics PhD and career as NASA scientist. Now during talks about the difficulty of recognizing alien intelligence I often use this story as an example of just how different in unexpected ways even seemingly familiar lifeforms could be. I'd love to be able to get the reference right, and any help would be great!