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
No.
Pretty much every [sex-determination system](https://en.wikipedia.org/wiki/Sex-determination_system) you can imagine is found somewhere within the animal kingdom. There are species that default to female and have a trigger for maleness, and other species that default to male and have a trigger for femaleness. Many species (including humans) don’t precisely default to either: if we artificially inhibit both male and female sex hormones, we get an organism that’s infertile and at least somewhat intersex.
There *is* an evolutionary logic for why most animals do *have* triggers. Intersex organisms tend to suffer fertility problems: for example, it’s better to have a single gene telling the organism to be male, than to have one gene telling it to grow testes and another gene telling it to grow a penis, and risk having an individual uselessly inherit one of those genes but not the other. But there’s no reason why it has to be a trigger for maleness rather than a trigger for femaleness.
There’s some evidence that what happened in our particular lineage is that the ancestral amniote (that is, the common ancestor of humans and alligators, but not of frogs) defaulted to male with an environmental trigger for femaleness, perhaps because trying to lay eggs is a bad investment in cold environments but a better investment in warm ones. But then when the ancestral therian (that is, the common ancestor of humans and possums, but not of platypuses) started gestating embryos internally rather than laying eggs, *all* of our embryos started detecting warm environments and turning female, at which point we evolved a genetic trigger for maleness in order to restore a 50-50 sex ratio. So we ended up with both female and male sex hormones, rather than having either sex as a pure default, as well as a male-only chromosome.
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