this implies transness predates humanity, which would imply there are probably trans chimps or bonobos, and possibly other primates or mammals more generally. we know there are lots of homosexual mammals. i wonder if it would be possible to distiguish if an animal is trans through its behavior. maybe they would alter their appearance to look more like their gender or act in ways exclusive to their gender outside of a sexual context.

oh now i think about it a trans animal might act differently than a cis animal in a sexual context too, like how the sexual culture of cis gay men is very different than of cis het people. you might see what appears to be homosexual behavior but is actually heterosexual behavior between a cis partner and a trans partner, so you'd see behavior more typical of cis het partners than cis gay partners. given how cultural gender is in humans it might be really subtle in other animals, depending on how sophisticated their culture is.

all of this would suggest we search for transness in our closest relatives amoung the primates, but also that we should look in the large cetaceans like dolphins and orcas. if we found trans cetaceans it would show pretty definitively that mammals in general at least are trans, and possibly that transness is an inherent feature of the neurology of sexually dimorphic animals, which it probably is.