So how do pandas do it?
Researchers have long speculated that panda intestines must have cellulose-munching bacteria that play a role in digestion. But previous attempts to find such bacteria in panda guts had failed. A new study published recently looks at gene sequences in the droppings from seven wild and eight captive giant pandas—a much bigger sample than what was used in previous panda-poop studies
"We think this may be caused by different diet, the unique inner habitat of the gut, or the unique phylogenetic position of their host," Wei said.
Even with help from gut bugs, pandas don't derive much nutrition from bamboo—a panda digests just 17 percent of the 9 to 14 kilograms of dry food it eats each day. Pandas spend up to 16 hours a day eating, and they could ingest up to 8-14 % of their body weight. To put this into perspective, I eat about a kilo of rice everyday (lunch and dinner), and I weight 60 kilograms. Hence I eat about 1.67% of my body weight each day.
But how and why did pandas became plant-eaters in the first place?
Well, that remains to be seen, though some scientists theorize that, as the ancient human population increased, pandas were pushed into higher altitudes. The animals then adopted a bamboo diet so they wouldn't compete for prey with other meat-eaters, such as Asiatic black bears.
Human population has only increased over the last several thousand years. Prior to that humans were nomadic hunter-gatherers, and civilization has yet to begin. But for pandas to adopt an entirely new diet, or to evolve a digestive system capable of digesting fibrous food, it would require at least several hundred thousand years. Evolution is not an overnight matter. Hence it's unlikely that pandas evolved a vegetarian diet because of humans.
Just my two cents.