However, might that mean that if the chimp has NOT been taught sign language, that the action is adjourned until reasonable efforts have been made to teach it sign-language? (Imagine a deaf/mute person who has never been taught to use sign language, read or write).

There is also this Guardian story about moves in Spanish parliaments to grant personhood to great apes, and this about an Austrian case where the personhood and right to hold property of a chimp was dismissed on a technicality because the judge ruled that his human advocates “had no legal standing to argue on the chimp’s behalf” - not that the chimp had no legal standing.

The Spanish initiative allows the right to not BE property, but also own property, and the obligation on the state to protect them like the state would a human with significant cognitive impairment.

I’d also say that it’s not genetic similarity to humans, but capacities (would killing ET be murder? How would we deal with a captured extraterrestrial who had intentionally killed a human?).

The issues are tricky because there is no cognitive gap between human and non-human capacities, but an overlap - apes are smarter than about 15% of humans when measured with “sign-language” IQ tests, only 1 standard deviation below the mean. A “genius” human is only 2 and a bit standard deviations above the mean. It’s not until you get to 3 standard deviations from the mean that a statistician would say “almost everything in that group is included”. By these figures, a genius might have twice the right to treat an average human as a lesser being than the average human has to treat a chimp as a lesser being.

It wasn’t that long ago that non-whites were regarded as subhuman, and could be property rather than own property.