This pose an interesting ethical question: How would we deal with a new "emerging" intelligent species? Are we willing to give them some rights? Are we more prone to put them into slavery?
David Livingston Smith explains that we are willing to inflict harm into those that we consider "other" or "separate aliens" (That's why I hate the "illegal alien" term when talking about immigrants).
It is very easy then to imagine a new world, a reverse version of the "Planet of the apes" where we, humans, would be supervising hordes of humanzees.
After all, we have been feeling the center of the universe from the very beginning of our species. Why would we change?
To me, while intelligence could certainly impact on the ability to suffer, its almost as arbitrary as deciding ethics on the basis of skin colour or sex.
That's true in most cases, and it's even arbitrary historically/culturally wrt which animals we eat. E.g. we westerners don't eat dogs, and Hindus (?) don't eat cows. Our level of empathy with specific animals informs what we choose to slaughter and eat, and how we feel they should be treated.