Of the many peculiarities that enable the modern giant panda (Ailuropoda melanoleuca) to adapt to life as a bamboo eater, its extra ‘thumb’ is the most celebrated yet enigmatic. In addition to the normal five digits in the hands of most mammals, giant pandas have a greatly enlarged wrist bone — the radial sesamoid — that acts as a sixth digit, an opposable ‘thumb’ for manipulating bamboo. In a new paper published in the journal Scientific Reports, paleontologists report the earliest functional opposable ‘thumb’ in the ancestral panda Ailurarctos from the Late Miocene site of Shuitangba in Yunnan Province, China. This discovery suggests that the origin of the panda’s dedicated bamboo diet goes back to as early as 6-7 million years ago.
An artist’s reconstruction of Ailurarctos from Shuitangba, Yunnan Province, China. The grasping function of its false thumb (shown in the right individual) has reached to the level of modern pandas, whereas the radial sesamoid may have protruded slightly more than its modern counterpart during walking (seen in the left individual). Image credit: Mauricio Antón.
In addition to five digits on their hands, modern giant pandas have an enlarged wrist bone with a thumb-like structure that they use to manipulate bamboo.
Previous research documented evidence of the thumb-like structure to just 100,000 to 150,000 years ago.
“Deep in the bamboo forest, giant pandas traded an omnivorous diet of meat and berries to quietly consuming bamboos, a plant plentiful in the subtropical forest but of low nutrient value,” said Dr. Xiaoming Wang, a vertebrate paleontology curator at the Natural History Museum of Los Angeles County and a researcher with the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences.
“Tightly holding bamboo stems in order to crush them into bite sizes is perhaps the most crucial adaptation to consuming a prodigious quantity of bamboo.”
Comparison of the radial sesamoid in the basal ursoid, Ailuropoda, and Homo and the positioning of the radial sesamoid; illustrations are of left hands: (A) a basal ursoid from the early Oligocene of North Dakota showing the primitive condition of an unenlarged radial sesamoid; (B) grasping hand in extant Ailuropoda; (C) grasping hand of modern human; (D) walking hand of extant Ailuropoda in a plantigrade posture; (E) external ventral surface of the hand of Ailuropoda showing a fleshy, plantar pad that corresponds to the radial sesamoid (red dash lines). Image credit: Wang et al., doi: 10.1038/s41598-022-13402-y.
Dr. Wang and colleagues examined the wrist bone of an individual from the ancestral panda genus Ailurarctos that was discovered at Shuitangba, a site near the city of Zhaotong in the Chinese province of Yunnan.
The researchers compared the shape and size of this bone to previously published data on the wrist bones of modern giant pandas and Indarctos arctoides, an ancient bear that lived 9 million years ago and may share the same common ancestor as giant pandas.
They found that the modern giant panda’s thumb-like structure has the same distinctive shape as Ailurarctos’ wrist bone but not Indarctos arctoides’, which was larger, wider and more hooked.
This indicates that, while the thumb-like sixth digit was not present in Indarctos arctoides or the common ancestor it shares with pandas, it has been present within the panda lineage — and used to grip bamboo — for at least 6 million years.
This image shows a male giant panda named Xiao Liwu at San Diego Zoo. Image credit: Sci-News.com.
Although the sixth digit was present in both modern giant pandas and Ailurarctos, the scientists observed differences in its size and shape.
The modern giant panda’s digit significantly shorter than Ailurarctos’ in relation to its body size and has a hook on the end of it and a flattened outer surface, while Ailurarctos’ does not.
The authors propose that the hook may help modern pandas to better grasp bamboo, while the shorter length and flattened outer surface may assist with weight distribution when walking.
“Five to six million years should be enough time for the panda to develop longer false thumbs, but it seems that the evolutionary pressure of needing to travel and bear its weight kept the ‘thumb’ short — strong enough to be useful without being big enough to get in the way,” said Dr. Denise Su, a researcher in the Institute of Human Origins and the School of Human Evolution and Social Change at Arizona State University.
“Evolving from a carnivorous ancestor and becoming a pure bamboo-feeder, pandas must overcome many obstacles,” Dr. Wang added.
“An opposable ‘thumb’ from a wrist bone may be the most amazing development against these hurdles.”
_____
X. Wang et al. 2022. Earliest giant panda false thumb suggests conflicting demands for locomotion and feeding. Sci Rep 12, 10538; doi: 10.1038/s41598-022-13402-y
Source link: https://www.sci.news/paleontology/ailurarctos-extra-thumb-10957.html
