Chimps, for example, are a different species; chimps and humans can't have offspring.
Lots of humans can't have offspring. Are they then a different species?
Their brains are obviously quite different.
The point is?
They are also vicious and aggressive animals.
I'd argue that some humans are more vicious and aggressive.
Enlightenment philosophers generally recognized that animals could suffer and that humans had some moral responsibility towards them, but did not generally recognize them as persons.
I'm sure they also didn't recognize blacks as being equal to whites, or that gays should be able to marry. Times change, so does thinking on what's right and wrong.
It seems to me that scientists have judged animals as guilty of being unconscious until proven conscious. This seems backward to me as we know that there is no magical 'stuff' that humans have that animals don't. Also, through evolution consciousness has been built up over the eons through layers in the brain. To say that somehow consciousness was just suddenly switched on 100,000 years ago just seems absurd.