This very short science fiction story by award-winning writer Terry Bisson appeared in Omni in April of 1991. I've seen it make the rounds on various vegetarian websites over the years and have always gotten a kick out of it. The story was nominated for a Nebula the year it appeared in Omni.
Here's a snippet (or click here to read the entire story):
"They're made out of meat."
"Meat. They're made out of meat."
"There's no doubt about it. We picked up several from different parts of the planet, took them aboard our recon vessels, and probed them all the way through. They're completely meat."
"That's impossible. What about the radio signals? The messages to the stars?"
"They use the radio waves to talk, but the signals don't come from them. The signals come from machines."
"So who made the machines? That's who we want to contact."
"They made the machines. That's what I'm trying to tell you. Meat made the machines."