MALE NARRATOR: The mountain forests of China are the last home of a very peculiar bear indeed, the giant panda. It's one of the most recognizable mammals in the world, with its distinctive black-and-white coat and its adorably round physique. And it's been confusing researchers ever since it was first described by Western scientists in the 1800s. For one thing, the giant panda is mostly a committed herbivore that specializes in eating bamboo, which is not very bear-like behavior. It also shares its name, its diet, part of its habitat, and an enlarged wrist bone that functions kind of like a thumb with the more common red panda. But it turns out the two pandas are actually not that closely related. And while it used to range over much of East Asia, today it's found only in a few pockets of mountain habitat in central China. But oddly enough, its earliest relatives actually lived in Europe, their fossils having been found in Hungary and Spain. Everything we discover about the giant panda just seems to lead to more questions, like why isn't it more closely related to the red panda if they have so much in common? And how does a bear, which is a member of the order Carnivora, evolve into an herbivore? How can it survive on bamboo if it evolved from omnivores? How do pandas even work? Despite how it looks, nothing about the history of the giant panda is black and white, but it is kind of fuzzy.

(Describer) Title: Eons.

Now because the panda is so odd in so many ways, from its very picky diet to its weird extra thumb thing, scientists for a long time wondered, are pandas even bears? And it wasn't until the late 20th century that we finally got an answer. That's how weird these animals are. The giant panda saga started in 1869, when it was given its first scientific name by a French missionary and naturalist based in China. He called it Ursus melanoleucus, literally the "black and white bear." So we know he thought it was a bear because he put it in the genus ursus, with most of the other living bears. But only a year later, another researcher examined a giant panda skeleton and decided that it was more like a red panda than a bear. So he changed its scientific name to Ailuropoda melanoleuca, which is what it's still called today. But the controversy over the panda's bearishness continued for more than 100 years. The experts who thought it was a bear pointed to a ton of similarities between giant pandas and other bears found throughout their entire bodies-- in their teeth, skeletons, muscles, and organ systems. But the scientists who thought it was closer to red pandas focused on their similarities and feeding behavior and their shared habitat. Giant pandas and red pandas live in the same places and eat the same thing, bamboo. It wasn't until the early 1980s that the first genetic analysis of giant pandas was done. It looked at the number of different mutations per gene in pandas and other carnivores. The more differences per gene there are, the longer those two species have been separated reproductively, which means they're more distantly related. These early DNA studies suggested that the giant panda was only a distant relative of the red panda. The red panda lineage actually branched off somewhere between two different families, the one that includes skunks and the one that includes raccoons and their relatives. So, I'm sorry, no, scientifically speaking, it's not super accurate to describe a raccoon as a trash panda. But there is a relationship there, at least with red pandas. It wasn't until studies were done on the giant panda's whole genome in the late 2000s that it was confirmed that the giant panda was indeed a bear. And it turned out that his closest living relative isn't the red panda, it's the South American spectacled bear, which belongs to the second oldest branch on the bear family tree. And the oldest branch of the bear family, that's where pandas came from. According to molecular clock studies of the genomes of the bear family, the giant panda lineage originated around 20 million years ago in the early Miocene Epoch, when its branch split off from all of the other living bears. But we don't know where this happened exactly. Since pandas today are found only in China, for a long time, it was thought that they originated in East Asia. But based on fossils from several different sites, it looks like they might have actually evolved in Europe, or at the very least, pandas had some close European cousins with whom they shared a common ancestor. The oldest, most basal member of the giant panda lineage comes from deposits in northern Spain dating back 11.6 million years. There, scientists have discovered the remains of a bear known as Kretzoiarctos beatrix. Its fossils consist of only teeth and jaws, but its pre-molars have some features, like additional little cusps that researchers think are adaptations to a more herbivorous diet. So it seems that the giant pandas' taste for plants goes back pretty far in its evolutionary history. Just a little younger than Kretzoiarctos is another European panda known as Miomaci pannonicum, and it comes from deposits dating back about 10 million years in both Hungary and Spain. It, too, is known only from teeth and jaws, but the teeth of Miomaci have microscopic pits and scratches in them that are kind of like the pattern seen on the teeth of modern giant pandas. So some researchers think that it might have had a similar diet and lifestyle as a giant panda. And a third European genus is known from scrappy remains from France, Germany, and Hungary that overlap in time with the other two panda ancestors. So for a few million years, we've got panda relatives living throughout Central and Western Europe. But then something strange and frustrating happens. There seems to be a panda hiatus in the fossil record. The next one to show up is in China millions of years later. There, we found two species belonging to the genus known as Ailurarctos. They date to between 6 million and 8 million years ago, and both are known, again, only from teeth. And those teeth suggest that those bears were smaller than the modern giant panda and were probably more omnivorous. They had broad lower molar crowns, like living bears, but with a wrinkled pattern to their enamel, which is what the giant panda has. So their teeth look like a transitional step between those of more typical bears and those of the highly specialized panda. Then about 2 million years ago, we finally find the earliest-known fossils of bears that truly belong to the same genus as the modern giant panda, the genus Ailuropoda. These fossils come from cave sites in southern China, and the most complete of them is a skull that has a lot of similarities to the skulls of living pandas. This fossil panda was smaller than its modern relatives and maybe had a less powerful bite, but it also had extra cuspy teeth that were specialized for breaking down fibrous foods. But experts aren't sure about when pandas adopted the habit they are most famous for, relying on bamboo as their main source of food. One recent study compared the isotopes of carbon and oxygen preserved in the tooth enamel of modern and ancient pandas. Carbon can tell you about an animal's diet, while oxygen can tell you about the climate it lived in. And the carbon value suggested that ancient pandas were herbivores by at least 2.5 million years ago, but the chemistry wasn't a perfect match with modern bamboo-eating pandas. The teeth also had a much wider range of oxygen values, which means they probably lived in a lot more different kinds of environments than pandas do today. So it looks like ancient pandas had broader diets and were ecologically flexible than their living descendants. But modern pandas are definitely all about that bamboo, which, again, for a bear is just really weird. See, the giant panda has the GI tract, the digestive enzymes, and the gut microbes of a carnivore. It doesn't have the multi-chambered stomach of cattle or produce enzymes that other herbivores use to break down cellulose, which is a key ingredient in bamboo and other plants. So for a long time, the pandas' diet was considered a sort of evolutionary riddle. But a paper published in 2019 seems to have figured out how they've been able to survive on a mostly bamboo diet. Researchers sampled the shoots and leaves of the different kinds of bamboo plants that a select group of pandas was eating throughout the year. Then they sampled the bear's poop for a before and after picture of the nutrients that they were taking in and what they were putting out. And the researchers found that, at their molecular level, the pandas' diet looked more like a carnivore's than an herbivore's. Specifically, their ratio of macronutrients, like proteins, carbs, and lipids, resembled those of meat eaters. It turns out that pandas switch which type of bamboo and which parts of the plant they eat throughout the year to maximize protein and minimize fiber. So the percentage of energy that they get from protein is actually equivalent to the diet of wolves. Pandas are basically vegan gym bros. So maybe their transition from an omnivorous bear-like ancestor to the gentle bamboo eaters we know today wasn't such a stretch for the giant panda after all. From its early Miocene origins near the base of the bear lineage, the giant panda has come a long way. Taxonomically, it went from a red panda relative to a true bear. Geographically, it went from its ancestors' first appearance in Europe all the way to the mountains of China, where it survives today. Physically, it went from a dog-sized omnivore to a, well, bear-sized herbivore that still packs in the protein in a very unexpected way. If the evolutionary history of the giant panda remains kind of fuzzy, its future isn't very clear either. It was once considered to be on the brink of extinction. But in 2014, there were more than 1,800 counted in the wild, a viable, but still vulnerable number. Those false-thumbed bamboo-eating mammals of central China are the last remaining characters in the story of the world's most unbear-like bear.

(Describer) Titles: Blake de Pastino: Co-producer, Script Editor, Host. PBS