The panda has gut structure, gut function, and gut enzymes like a carnivore.
The panda does not have multiple stomachs, nor an enlarged cecum, nor the gut microbes found in animals that eat diets composed largely of fiber, like cattle and sheep.
A study published in the Proceedings of the National Academies of Science states: "The giant panda genome codes for all necessary enzymes associated with a carnivorous digestive system but lacks genes for enzymes needed to digest cellulose, the principal component of their bamboo diet."
Yet wild giant pandas consume around 20-40 pounds of highly fibrous bamboo stalks and leaves every day.
The San Diego Zoo states that the small Red Panda's diet is 95 percent bamboo.
The Smithsonian National Zoo states that "A wild giant panda’s diet is almost exclusively (99 percent) bamboo."
At the Talk Origins site on human evolution, Douglas Theobald, professor of biochemistry at Brandeis University writes "... even though humans are herbivorous, the small human caecum does not house significant quantities of cellulase-excreting bacteria, and we cannot digest more than but a few grams of cellulose per day." [Emphasis added]
Leaving aside for the moment the interesting fact that this staunch defender of evolutionary theory describes humans as herbivorous, the panda's ability to digest cellulose is similar to humans. The gut microbes of pandas digest very little of the fiber the pandas consume; 92 percent of cellulose a panda ingests ends up eliminated in its feces.
If in pandas gut microbes convert only 8 percent of ingested cellulose into short chain fatty acids, this probably means that the panda gets most of its energy from the digestible carbohydrate and protein provided by bamboo leaves and stalks, not from fat. Does the panda challenge the idea that "the natural diet of mammals is a high-fat diet"?
The pandas provide a striking example of a non-human mammal that has almost none of the genetic or physiological equipment associated with herbivory (only having some dental and grip adaptations), yet it spontaneously lives as an herbivore.
It is believed that the first giant panda ancestor to include some bamboo in it diet was Ailuropoda micrta which existed about 3million years ago. This species appears to have descended from the primal panda, Ailuaractos Lufengensis, an arctoid with a carnivorous diet. The carnivorous arctoids appear first in the evolutionary record about 46 million years ago.
The fact that a member of the Carnivora order has adapted spontaneously to a highly specialized and extremely fibrous, low fat, 99% herbivorous diet without apparent major physiological and genetic changes seems to raise some interesting questions for those who believe that modern humans are optimally adapted to some diet consumed by some paleolithic human ancestors.
The panda also seems to challenge another presupposition of paleolithic diet theory, which goes something like this: Any species that at any time in its evolution adopts a meat-based diet is required thenceforth to always maintain a meat-based diet to sustain health.
This is clearly not true for the panda; despite having physiological equipment that limits its ability to extract nourishment from plants, it has succeeded in its niche for at least a million years.
The panda shows us that when individuals of a species encounter an environmental challenge, they do whatever they can do to succeed in the changing environment without any concern about what their ancestors did, or whether they have the optimal physiology for the new habit. The new feeding strategy may not be optimal, and it doesn’t have to be; it only has to be good enough to allow individuals to survive long enough to reproduce.
Now, why does Professor Theobald say that humans are herbivorous? Because we humans have descended from a very long line of herbivorous ancestors and have a body displaying more features in common with other herbivores than with carnivores, including plantigrade stance, relatively slow sprints (compared, for example, to canines or felines), color vision, nails (rather than claws), small mouth, fleshy lips, non-shearing teeth, carbohydrate taste receptors (rather than amino acid taste receptors found in cats), non-expandable esophagus, haustrated and long intestines, low potency bile, a vermiform appendix, and a sense of fear (those at the top of the food chain are not stalked so do not need fear to enhance survival).
During the last two to four million years, climate changes and migrations put human ancestors in environments where they had to adopt omnivorous diets to survive, despite not being fully adapted to the new foods (meat and animal fat), just as the first panda ancestor to eat bamboo was not fully (physiologically) suited to a bamboo diet. Nevertheless, just as the bamboo diet was/is good enough for the panda lineage to survive, a meat-based diet was/is good enough for the human lineage to survive in plant-food depleted environment.
That does not mean it will produce the best health. A diet does not have to protect individuals from heart disease or cancer, or support maximum longevity, to be good enough to support the continuation of the species. These diseases typically kill people long after they have reached reproductive age.
The discordance hypothesis favored by some Paleolithic diet advocates states that diseases arise as a consequence of an individual adopting a diet, lifestyle, or habitat sufficiently different from the diet, lifestyle, or habitat of its ancestors to create a discordance with the genetic heritage of its species.
The wild giant panda seems to have a diet substantially discordant with its genetic constitution. Yet the panda doesn’t suffer from a host of diet-induced diseases.
Pandas in captivity eat a diet perhaps more discordant with the ancestral panda diet. According to this report of captive giant panda diets in five Chinese facilities:
“Each facility feeds a steamed grain mixture comprising 13–56% of the diet on an as-fed basis, animal products (milk, eggs, and/or meat; 8–25% of the diet), and bamboo (17–82% of the diet). Seasonally available fruits and/or vegetables are sometimes included (0–29% of the diet).”
This mixed and cooked diet deviates from the 100% raw, 99% bamboo diet of wild giant pandas, and includes foods never eaten by wild pandas during the past 3 million years (grains). The San Diego Zoo reports:
“At the San Diego Zoo, pandas are offered bamboo, carrots, yams, and special leaf eater biscuits made of grain and packed with all the vitamins and minerals pandas need.”
It seems the San Diego veterinarians have settled on a vegan diet for the panda. The pandas apparently do quite well on this non-ancestral diet, at least in terms of longevity. According to the SanDiego Zoo, wild pandas live only 14-20 years, but pandas in zoos live 30 years, 50 to 100 percent longer. Apparently, a panda can live much longer when eating an evolutionarily novel diet including steamed grains than when eating only the raw foods eaten by its wild ancestors. This experiment apparently trumps the discordance hypothesis.
Consider that the panda has been isolated to a bamboo forest, and eating a bamboo-based diet, for perhaps one million years, yet this sustained selective pressure to adapt to a bamboo diet has had relatively little impact on its basic anatomy and physiology. Today’s panda still does not have an herbivorous body form despite such a long period of evolution on the 99% bamboo diet, but the diet works good enough for the panda (perhaps as good as it gets in the current niche) to pass the bamboo habit to the next generation.
In contrast, the past two million years of human evolution occurred under much more varied ecological conditions. During human ancestral evolution, the variability of the ice age climate and human mobility led to wide variability of plant-animal ratios in ancestral diets, and this combined with intertribal marriages tended to minimize selective pressures for any specific physiological adaptations to meat-eating and supported retention of the basically herbivorous primate physiology.
The panda seems to challenge the idea that our genes, or the diets of our remote ancestors, determine the optimal diet for present-day humans. Apparently neither ancestral diets, nor genes, nor physiological equipment will necessarily make a meat-based diet a perpetual requirement, or the optimum choice, for any given species, let alone one (such as humans) with an extensively herbivorous ancestry and numerous adaptations to herbivory.
I think the panda shows us quite clearly that if we want to know how to prevent degenerative diseases and maximize healthy longevity, we will want to gather knowledge from the experience and experiments (natural and controlled) of present day humans, rather than assume that we will get the best results by eating only the foods consumed by our remote ancestors.
This post was inspired by similar, but less dramatic examples of non-human animals adapting to non-ancestral foods/diets in some Plant Positive videos, for example:
Primitive Nutrition 60: Ketosis Is Natural. Natural Is Good. Part III
"Polar bears in captivity are not fed a diet like they would consume in the wild. They are actually fed fruits and vegetables. But this isn’t their natural diet! Surely this is a form of animal abuse, right?"
"Actually, polar bears in captivity live considerably longer. But shouldn't an evolutionarily novel diet destroy their health? This is yet another example of how Paleologic is no substitute for experiment and observation."
From Polar Bears International:
"In the wild, polar bears live an average 15 to 18 years, although biologists have tagged a few bears in their early 30s. In captivity, they may live until their mid- to late 30s. Debby, a zoo bear in Canada, lived to be 42."
Thus, an evolutionarily novel diet supports a doubling of lifespan in polar bears.
Thanks to Plant Positive for giving inspiration and sharing suggestions for this post.