The only animals that have an opposable thumb and can therefore grab things with their paws are some species of primates, including humans. It’s a piece of information that’s also taught in school, but that might be confusing given a photo of a panda eating: if pandas don’t have opposing thumbs, how do they lift, snap and hold bamboo sticks, the vegetables? – which make up 99 percent of their diet?
The answer is that they have fake thumbs: protuberances on the wrists of the front legs that perform the role of an opposable thumb, while the real thumbs align with all of the other fingers.
How Pandas Grasp Bamboo: Photo by Sharon Fisher shows a panda from the Panda Research Center in Chengdu, China; In the image below, the photo is overlaid by a sesamoid bone in accordance with the false thumb (Scientific Reports)
The bones under the false thumbs are radial sesamoids: they are also found in human hands, more or less where the thumbs meet the hands, and they owe their name to the fact that they resemble sesame seeds in shape. In pandas, however, they are not as small as in human hands, but are greatly enlarged, which is why they can take over the function of thumbs.
They are shorter compared to primates’ opposable thumbs, but still allow pandas to hold on to bamboo sticks while cutting them up with their teeth to eat them. In fact, they have a shape that vaguely resembles a hook and that allows you to lock the bars well.
False panda thumbs are known as an example of evolutionary adaptations that can arise from apparent imperfections, in this case enlarged sesamoids. Noted biologist and science writer Stephen Jay Gould popularized them as such in several of his essays, beginning with the book The Panda’s Thumb (1980). In Bravo Brontosaurus (1991) he explains:
The complex and curious paths of history mean that most organisms and ecosystems cannot be the result of optimal design. In fact, to put it more bluntly, imperfections are the prime evidence of evolution, as optimal designs erase all guideposts of history.
This principle of imperfection has been a main theme in my essays for several years. I call it the panda principle to honor my favorite example, the fake panda thumb. Pandas are herbivorous descendants of carnivorous bears. Their true anatomical thumbs long ago irrevocably committed themselves to performing the limited movements appropriate to this way of life and developed by carnivorous mammals in general. When adaptation to a bamboo diet required greater manipulative flexibility, pandas could not redesign their thumb, but instead had to adapt with a well-fixed replacement: an enlarged radial sesamoid of the carpus (wrist), the panda’s “false thumb.” The sesamoid thumb is a rough, far from optimal structure, but it works. The paths of history (the engagement of the true pulse in other roles during an irreversible past) impose comparable makeshift solutions on all organisms. History makes itself felt in the imperfections of living organisms: this is how we know that modern creatures had a different past, transformed into their present state by evolution.
How are the bones of pandas’ front legs, in this case the left one, compared to a sesamoid leg of Ailurarctos, identified by the letters B and D (scientific reports)
Naturalists have wondered for years why pandas didn’t evolve longer radial sesamoids, allowing them to grip and squeeze bamboo canes even better. A study published June 30 in Scientific Reports now allows us to hypothesize that a longer sesamoid bone would make walking uncomfortable.
The study concerns some fossils of an extinct panda genus, called ancestors of modern-day pandas and ailurarctos, found between 2010 and 2015 in China’s Yunnan region and dating back around 6 or 7 million years. Ailurarctos were smaller than pandas, but as far as we know shared a bamboo-based diet with them.
The fossils—bone of a forelimb, teeth, and an enlarged sesamoid bone—are the oldest evidence ever discovered of a false opposable thumb; Previously, the oldest false thumbs found were tens of thousands of years old and belonged to pandas of the same species as those of today. Compared to the sesamoid legs of modern pandas, that of the Ailurarctos is slightly longer and less curved in the last part.
An illustration of what the Ailurarctos could have been like, created by artist Mauricio Antón (Scientific Reports)
“The false hooked thumb allows for a tighter grip on the bamboo, while at the same time, because it has a less protruding tip, the curvature makes it less of a hindrance to movement,” explains Los Angeles Museum of Natural History paleontologist Xiaoming. Wang, first author of the study published in Scientific Reports: “Imagine a panda stumbling over its false thumb with every move…that’s why we think the false thumb in modern pandas has gotten shorter rather than longer.”
– Also read: The evolutionary dilemma of pandas