Giant pandas, aka Ailuropoda melanoleuca, have lived with humans for many generations, but it wasn't until 1869 that western travelers first encountered any trace of this animal, and the first actual sighting was 50 years later, in 1916.
Although these animals were classified under the order Carnivora, they mostly eat bamboo. This is because the panda is closely related and looks very similar to two other groups of animals: raccoons and bears. Because they looked so similar, scientists at the time assumed pandas would also be carnivorous. Although pandas were later observed to be omnivores, the assumption that they are carnivores is not without evidence. The digestive system of a panda is almost undeniably that of a carnivore, suggesting that their shift in diet is very recent.
Whether pandas are more closely related to bears or raccoons has been debated for many years. Although pandas look remarkably similar to bears, there seems to be more evidence suggesting otherwise just below the surface. Pandas have better developed molars than other bears, most likely because of their bamboo diet, and they don't hibernate during winter. But the most important piece of evidence is an extension of a wrist bone that acts as an opposable thumb for pandas. This structure allows pandas to grab bamboo shoots with greater ease.
For some information on the ancestors of modern pandas, go here