Discovery of a sixth toe giant panda fossil

Pandas, including giant pandas, are the only bears that feed almost exclusively on plants—in this case, bamboo. However, initially this animal was a large predator. Why did he become a vegetarian? A mystery that could be inextricably linked with the presence of a sixth toe on their paws, or rather, a “false thumb.” We knew that in pandas this growth mimics an atrophied thumb. But it is not clear since when this species developed something similar to the sixth phalanx.

Recently, a team of researchers unearthed fossils about six million years old in Yunnan, southwest China. The latter include the molars and sesamoid bones of the wrist.

Fossils belong to species Ailuarto, an ancestor of the panda, now extinct. This species lived six to eight million years ago.

Giant panda ancestors had a longer thumb

First observation: the researchers found that the “false thumb” had already been developed on the legs of this species. Ailuarto six million years ago. This is the oldest evidence of a sixth toe in modern giant pandas. A notable discovery as the panda is the only bear to have this false thumb. But until now it was not known how and when it appeared in evolution.

Fossils of a “false thumb” at Ailuarto, an ancient relative of the panda. Credits: Xiaoming Wang et al.

The second observation is that the fossil has a longer thumb than modern giant pandas. Its shape is also different: the fossilized thumb is longer, while in the current giant pandas it is curved and wider at the end. Curiosity that intrigued researchers. Why did evolution prefer the twisted “false thumb” shape seen in giant pandas today?

Why do giant pandas have a sixth toe?

The sixth finger of the giant pandas currently living in China is wide, short and curved. In addition, he can only perform limited movements. So what can it be used for?

In fact, this “fake thumb” is very useful for pandas. It allows them to better grip the bamboo stalks in their paws, serving as an additional support on which to hold the stalks. Thus, the presence of this sixth term is precisely related to the diet of pandas.

“The False Thumb at Ailurarktos Reveals for the First Time the Chronology and Likely Evolutionary Stages of Panda Feeding on Bamboo.”evidenced by Wang Xiaoming, co-author of the study and curator at the Museum of Natural History in Los Angeles (USA).

giant panda

The giant panda grabs a bamboo stalk to chew on. His little finger serves as a support for holding the rod. In the enlarged image, the sesamoid bone is placed to show its location within the fleshy pad. Credits: Xiaoming Wang et al.


The giant panda grabs a bamboo stalk to chew on. His little finger serves as a support for holding the rod. In the enlarged image, the sesamoid bone is placed to show its location within the fleshy pad. Credits:
Xiaoming Wang and others.


Pandas have changed their diet to eat almost exclusively bamboo, which is available in China all year round. An adult giant panda can swallow 45 kg per day!

But a new question arises: if this small thumb is so useful to them, why did it become smaller in the course of evolution, and not turned into an opposing thumb, like a human?

According to the authors of the study, this growth should meet two conflicting needs. On the one hand, the thumb serves for feeding, and on the other hand, it stabilizes the supports during plantigrade walking.

However, the wider and more curved thumb distributes the animal’s weight better and provides more stable support while still supporting the hold of the bamboo sticks. According to the researchers, it is for this reason that the thumb has evolved into a curved shape.

So this study tells us that giant pandas changed their diet at least six million years ago. If the discovery of these fossils has made it possible to learn more about the adaptation of the giant panda’s morphology, then the mystery surrounding its vegetarianism has not yet been fully solved.

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