What are the most realistic ways to introduce a persistant cosmetic difference into human evolution?

I'd like my world to have humanoid protagonists, while still reinforcing that they are ultimately not human, but have simply evolved in a similar way due to convergent evolution. To provide a constant small reminder of that, I'd like to introduce one or more subtle cosmetic differences to these humanoids that separate them from us -- distinct from and prior to the more rapidly mutable differences in bone structure, pigmentation, and face shape that characterize different ethnicities.

Now I could just give all my humanoids three eyes and vampire fangs, or what have you, and the sky would be the limit. But to keep things as realistic as possible, and to provide the constraints needed to make this a valid StackExchange question, I'd like to conservatively ground the cosmetic difference(s) in actual human evolution, whether by taking up a trait that we once had but lost, or one we didn't develop but that we know (whether from genetics, primate biology, etc.) would have been a very plausible turn.

Here's some more selection criteria here, to help narrow things down. The differences should be ...

• Visible, and easily communicable through text.
• Plausibly constant across all ethnic groups and the entire history of anatomically modern human(oid)s.
• Not a significant change to human(oid) function. Primarily cosmetic.
• Not so extreme as to make the humanoids seem totally alien.
• Not a tail. Already know that one!

In the event of two answers that meet the above criteria equally well, I'll select one based on A) the number of possible differences given, and B) the apparent strength of the argument that the difference(s) are plausible within an otherwise human-ish evolutionary path. Thanks for your time.

posted over 6 years ago

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