system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What muscles are the least used and why do humans still have them?
gpt
The palmaris longus.  It is a muscle in our forearm.  It doesn't do anything.  14 percent of people don't even have it.  Those that do have one have it because it is a vestige of days when claws and climbing were important for survival (and not a hindrance to handling tools).  It is still there in most people because having the muscle doesn't hurt anything in terms of survival, so there is not pressure on the trait.  In other mammals, it serves to retract skin to expose the claws.