I dreamed about Pandora

[Human No More]

If they develop, it IS a survival/reproduction advantage (in the absence of inbreeding, conscious selective breeding, or geographical/social isolation - and even then, it can be considered one in a more limited sense within selective breeding). An example is how humans are getting taller, because taller people are generally considered more attractive, and, no matter how marginally, more likely to be competed for and successfully reproduce.

Atavism is well known and understood, but only arises in individuals, primarily due to mutation, or in some cases particular rare genetic combinations (such as having a webbed toe). Actually reestablishing a lost phenotype that is still present in DNA is far more difficult.