No one can answer this question with any degree of certainty. However, animals people kill for food and clothing are unquestionably conscious. Therefore, not knowing exactly where the boundary lies should not be an excuse for continuing to slaughter animals for food.
At the same time, it is interesting to speculate about where consciousness begins in the animal kingdom. By “consciousness,” I simply mean the ability to feel sensations such as pain or hunger. Consciousness does not necessarily require reasoning.
A person with a profound intellectual disability is just as conscious as anyone else. Their inability to understand certain calculations does not mean they feel pain or suffering any less than we do. By contrast, a programmable computer might be extremely effective at calculations yet have no consciousness at all.
My personal opinion, although I cannot prove it, is that the boundary of consciousness lies somewhere in the world of insects. I believe that some kinds of insects can feel pain, and for that reason I include insects in my animal-rights philosophy.
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