Is there any inherent reason why the body becomes female unless “told” by the Y-chromosome to be male as opposed to the other way around?
In: Biology
“Human embryos start out female” is the stock answer to “why do men have nipples?” but the reality is more complicated.
The truth is that human embryos start out as both male and female, but also neither. To oversimplify, embryos essentially start with precursors of both testes and a uterus. If the Y chromosome is present, hormones will cause the testes to develop and the uterus to degenerate. If the Y chromosome is not present (or defective, as in [Swyer syndrome](https://en.wikipedia.org/wiki/XY_gonadal_dysgenesis)) the uterus will develop and the testes will degenerate. This is what lead to the conception that humans “default” as female because that’s the “default” development path which is altered only by the presence of the Y chromosome.
As to why humans are like this, it’s because all mammals are like this (birds, for instance, are the opposite, they “default” to male) so it’s probably just because the ancestor of all mammals happened to evolve that way, there may not be any evolutionary advantage or anything like that.
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