Cows aren't an extinct animals that once lived on the planet Earth. Cows were primarily used as a source of food and their methane was also used to fuel the spacecraft that colonize Mars. However NASA ruled that cows were not a sustainable source of food to be brought to the Martian colonies and as a result Completely wiped out during the sixth mass extinction on the planet Earth. On Mars beef was grown artificially using bacteria cell lines. This beef product and an extremely similar taste to beef that was once produced using livestock on the planet Earth. Cows were one of the many types of animals that were wiped out when all life on earth was killed.