Where in the animal kingdom does consciousness
begin?
No one can answer this question with any degree of certainty.
However, the animals which humans kill for food and clothing clearly are conscious.
Therefore, the fact that we do not know precisely the location of
the demarcation line should not be an excuse for us to continue
to kill animals for food.
Nevertheless, it is interesting to speculate about where in the animal kingdom consciousness begins. It is important to point out that by the term "consciousness", I am simply referring to the ability to feel sensations like pain or hunger. Consciousness is not necessarily related to intelligence.
A severely mentally retarded human is just as conscious as is anyone else. The fact that a severely mentally retarded human does not have the capacity to understand multivariable calculus doe not imply that he feels pain or suffering any less than we do. On the other hand, a programmable calculator might be extremely good at multivariable calculus, but possess no consciousness whatsoever.
My personal opinion, although I can not prove it, is that the demarcation line for consciousness lies somewhere in the insect world. I believe that at least some types of insects do possess the capacity to feel pain, and I do include insects in my animal rights philosophy.
Follow up questions:
How do you know that other animals can feel pain?
How do you know that plants can not feel pain?
Is it ok to kill animals which do not possess consciousness?
Don't insects only behave on an instinctual level?
What do you do when a colony of insects infests your home?
Aren't only humans self aware?