system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do men appear to have a "stitch line" or "scar line" at the base of their ballsac?

Is this scar line related to that whole "we start out as women" theory?  Is that scar line where our vaginas were?
gpt
Saying it "started out as a vagina" is an overstatement, but it's grounded in truth.

When we're forming in the womb, we start with a shallow slit between our legs. For women, that slit deepens while in men it stitches together.

Calling it a vagina at that stage in development isn't really accurate; a vagina is more than just a gap in skin, it's a structurally complex organ with a dozen different unique cells and its own intricate biology. The prenatal gap superficially, however, *is* closer in appearance to a vulva than to testicles, hence the common narrative that we all start as women. 

EDIT: To add what many other helpful users are posting, we do "default" to female; the activation of the Y-Chromosome causes a release of testosterone which promotes the growth of male sex characteristics. Even if a Y-chromosome is present, if testosterone fails to release then the fetus will progress and develop as basically female.