They also suspect that anyone with a brain this small couldn’t be called sentient – and the idea of natural selection driving a population from sentience to nonsentience bothers them.

I'm a little confused by this use of the word sentient. I understand it to mean "qualia-bearing", and I believe that chimps and other animals probably have qualia. Perhaps they meant that it probably didn't have our depth of human experience, i.e. it probably had a similar degree of consciousness (or qualia) to a chimp.

Incidentally I am reminded of the disturbing science fiction novel Blindsight by Peter Watts, which explores (fictional insight only, of course!) similar ideas.

canine venereal sarcoma, which today is an infectious cancer, but was once a dog.

This is now my favourite fact.

That last sentence just struck me with utter horror.

It's the same kind of horror one feels after reading Eliezer's "Beyond the reach of God". I'd love to know what the neurological difference is between knowing something on a surface level, and actually internalising it such that the full horror of it is felt.