Imagine if when we'd landed on Mars, rather than finding a rubbish empty planet we'd actually found it populated by alien life forms. "How exciting!" the news media would cry as people waited for more information on our planetary neighbours. Some people would change the way they viewed the Universe forever, others would cry "I told you so," and parts of the planet would quake in fear waiting for the inevitable alien invasion. Then the news comes that, "we've found life but it's not what we would call intelligent life". Turns out, in this hypothetical situation, the Martian planet's population is more accurately described as being dumb stupid cattle. Mars is inhabited entirely by cows. Actual, same as we have on earth, cows. And grass, for the cows to eat.
It would still be exciting news but the dissapointment would be more than obvious as extraterrestrial welcome parties were cancelled and scientists got to grips with the fact that for some reason cows were a spicies which had evolved simultaniously on more than one planet.
As the excitement died down and people got used to the fact that we'd found a "Cow Planet", more practical members of the human race might also start to see a solution to the earth's current food problem. "They're exactly the same as our earth cows," the argument would go, "why don't we eat them?". It'd only be a matter of time before the practically minded people of planet earth would push for martian cows to be served up for one and all. The McMartian Burger Happy meal would be a tasty treat I'm sure we'd all enjoy.
Obviously that's a hypothetical situation.
However, lets try a new scenario. Imagine the magic moment many people are waiting for finally comes and instead of us finding extra terrestrials they find us! Excitement abounds as the aliens walk amongst us looking for beings to communicate with; "We've found life but it's not what we would call intelligent life," they communicate back to Zeta Rectal (their star system). "It's inhabited principly by monkeys," they add, "monkeys exactly like the ones we have on our planet". "But we eat them," comes the reply, "what a fantastic solution these things will be for our current food problem".
If we are to speculate about the existence of alien life forms the first thing we have to accept is that they will by their very nature be alien to us. We can't ascribe human motivations to them and as a result the above hypothesis where we become alien food is only important because of what it tells us about humanity. An advanced and intelligent civilisation regarding ours will know or at least be able to deduce that if we could eat them, we would.