Uhm, it's in the Bible, but you will probably not think that argument is valid so I'll come with another argument:
Because we can!
Again, ruling means that we should treat them responsibly, but we can still take from the animals what we need. Food, and in this case, use them as a study object if it is necessary and if there are no real alternatives.
So, instead of examining a gorilla, we could take an alternative by examining a rat or a worm.
I personally don't consider computer animations real alternatives but some people do.
Oh, and what's wrong with examining one worm. Just be honest, do you really care if I kill a worm? I bet you don't.
And if you disagree with me that much, I assume you are a vegetarian? If not, what would give us the right to kill animals? Or plants, for that matter.
Those who are at the top of the foodchain can do with the lower ones whatever they want. And there's nothing anyone can do about it. It's simply that way.