I think only by one another...
Cannibalism is common to societal collapse...Africa is prone to periodic long-term drought...
People have historically eaten each other for various reasons.
A cow does not have reason...but it has the same primal drive as a human to survive, and can feel the same emotions that we feel, albeit in a more basic format. We can look at their brain and tell that they can emote to some degree.
Severely brain damaged or retarded people may have only that basic desire to survive left.
Sadly, maybe not even that.
We take care of them, of course.
Since the vegetarian thread has started...I try to visualize all the steps and people my food had to have happen to it before it reached me... and mentally thank those people. It's an attempt at more conscious eating.
When you eat the meal, do you ever thank the chicken or the cow for giving up its' life? The meat processors for doing a very dangerous and nasty job? The people who shipped it, stuck it in the supermarket cooler?
Thoughts...
This may interest you.
http://michaelpollan.com/books/the-omnivores-dilemma/
One hour youtube clip: http://www.youtube.com/watch?v=kFpjskn3_Pc
Disclaimer - I haven't actually checked either of these out.