Space beef, also known as Space-cows. The unknown race that Earth had discovered in the third spiral of the milky way, amidst sophisticated self-sustaining machines. Earth assumed that both the aliens, which looked like cows, and the technology belonged to a race that had died or abandoned the planet. So the humans took the technology and the Space-Cows back to Earth, used the machines and ate the Space-Cows, which, oddly enough, tasted like chicken. They didn't realize until they had eaten all of them that the Space-Cows WERE the aliens who had made the technology. The creatures had permitted themselves to be eaten because they had believed in something resembling the bible, from which they adopted a "turn the other cheek" philosophy, so they simply permitted the humans to eat them. It is unknown if any communication attempts had been made on their part.
This paragraph appeared in those sets of Encyclopedia Britannica which were published between the years 2217 and 2252. A version of this paragraph was also posted on an Internet humor site in December of 2217, as the "dumbest planetary extinction of the month".