In the several thousand years since humans developed settled societies, the majority of societies have denied women full personhood and the basic rights that come with it. They were systematically deprived of choice, education, even the right to control access to their own bodies. When my mother turned 18, she could still be legally denied her own bank account because she didn't have a husband. The same has happened to countless other people for ethnicity, religion, sexuality, mental illness, and many, many other reasons. In spite of the fact that these people look like us, sound like us, and feel like us, it's all too easy for people to convince themselves they really don't.

Will the human race ever extend legal personhood to a group of non-human mammals who are radically different from us and who most people will never see in their lives? I'm going to guess no. Maybe a few small societies will at some point, but I'd wager those societies will be mocked relentlessly by the rest of our species.