The giant panda Jiao Qing in May 2020. Berlin Zoological Garden. Image courtesy of Avda and Wikipedia.

In an online article at Evolution News titled, Is the Panda’s Thumb Suboptimal? (April 4, 2024), Professor Stephen Dilley criticizes Stephen Jay Gould for claiming that the panda’s thumb is suboptimal and, therefore constitutes evidence in favor of naturalistic evolution. In response, Professor Dilley cites “two major studies” that highlighted the functionality and efficiency of the panda’s thumb before concluding that the panda’s “thumb” provides evidence of being engineered:

Gould’s claim is mistaken. The panda’s thumb is not suboptimal. The best studies we have conclude that the thumb is anything but “clumsy” or “highly inefficient.” Instead, they describe it as having “great precision,” “great economy of motion,” and “great dexterity.” It may even rank as “one of the most extraordinary manipulation systems” among mammals. That is quite an accolade.

Indeed, one might rather regard the thumb as positive evidence for intelligent design. A system of such precision, efficiency, economy, and dexterity is a spectacle of a high order. That sounds very much like the kind of sophistication that only engineers produce.

UPDATE: A clarification from Glenn Branch over at Panda’s Thumb

Professor Dilley’s remarks demonstrate that he misunderstands the meaning of the word “suboptimal,” as used by evolutionary biologists. He isn’t the first person to do so; nor will he be the last. Allow me to quote from a science educator who can set him straight on this issue.

In Part Two of a reprinted article (April 18 and 25, 2024) that was originally published on the Metanexus: The Online Forum on Religion and Science website in two parts on August 15 and 16, 2002, Glenn Branch, deputy director of the National Center for Science Education, provides a useful clarification about the meaning of the term “suboptimal,” in response to Robert C. Newman and John L. Wiester, authors of a paperback book in comic form titled, What’s Darwin Got to Do With It? A Friendly Conversation About Evolution, which pokes fun at evolution. The authors cite an article in Encyclopedia Americana, in order to cast doubt on the evolutionist claim that the panda’s “thumb” is suboptimal. Branch responds that the authors misunderstand what evolutionary biologists mean when they describe a structure as “suboptimal”:

Regardless of the far-from-authoritative nature of their source, Newman and Wiester’s emphasis on the efficiency of the panda’s thumb reveals that they have again misrepresented Gould’s argument, apparently due to their conflation of two senses in which it is possible for design (or, if you prefer, “design”) to be suboptimal. In the first sense, a biological feature is suboptimally designed if it accomplishes its function not as efficiently as it (or a plausible substitute) might. Thus in the first sense, to say that the panda’s thumb is suboptimally designed is to say that it is not as useful for stripping leaves from bamboo as it (or a plausible substitute) might be. In the second sense, however, a biological feature is suboptimally designed if it is not designed as well as it might have been, that is, if the process whereby it acquired the ability to accomplish its function was not as efficient as it (or a plausible substitute) might have been. That there is such a sense of what it is for design to be suboptimal is testified to by the computer engineer’s word “kludge,” which refers to a clumsy and inelegant, but not necessarily ineffective, solution to a problem. In the second sense, to say that the panda’s thumb is suboptimally designed is to say that the process whereby it acquired the ability to strip leaves from bamboo was a kludge.

It seems to me that Professor Dilley is guilty of the same fallacy as Newman and Wiester in his Evolution News and Views article, Is the Panda’s Thumb Suboptimal? He takes great pains to show that the panda’s “thumb” functions admirably well. What he fails to demonstrate, however, is that the process whereby the panda acquired this function was maximally efficient.

END OF UPDATE.

What modern authors think of Gould’s claim

It is a great pity that of the two studies Dilley cites to demonstrate the efficiency of the panda’s “thumb,” one (by Schaller et al.) is nearly forty years old while the other (by a Japanese team) is twenty-five years old. A more measured assessment of the panda’s “thumb” can be found in a recent article by Wang, X., Su, D.F., Jablonski, N.G. et al., titled, Earliest giant panda false thumb suggests conflicting demands for locomotion and feeding (Scientific Reports 12, 10538 (2022), https://doi.org/10.1038/s41598-022-13402-y).

The authors suggest that the reason why the “thumb” (actually a radial sesamoid bone) is not longer in modern pandas is that it needs to balance the conflicting demands of bamboo manipulation and weight distribution while walking. A longer “thumb” would be better for the former purpose, but at the cost of impeding locomotion: it would have continually gotten trodden on while the panda walked, making it painful to walk long distances. In their concluding paragraph, the authors approvingly quote from the writings of Stephen Jay Gould, who describes the co-option of the radial sesamoid bone in the panda’s wrist as a “clumsy, but quite workable, solution”:

Steven (sic) J. Gould’s⁴ insightful remarks still stand: “the panda’s true thumb is committed to another role, too specialized for a different function to become an opposable, manipulating digit. So the panda must use parts on hand and settle for an enlarged wrist bone and a somewhat clumsy, but quite workable, solution”. However, he would probably have been delighted to learn that the historic contingency of the panda’s false thumb requires that while being a better finger was favored by selection, it also had to bear the burden of considerable body weight.

Why the panda’s thumb does not exhibit the kind of sophistication that only engineers produce

In real life, the panda’s “thumb” turns out to be highly efficient for the purposes of bamboo feeding. Nevertheless, in an earlier section of the article (Dual functions of false thumb), the authors (Wang et al.) concede that it is suboptimal, while explaining how it could have evolved one step at a time, without the need for engineering:

Such a passive system of gripping, far less effective than that of humans, nonetheless offers the panda the tightness of grip it needs for bamboo feeding. Furthermore, from an evolutionary point of view, such a simple passive mechanism of grasping can be functionally useful even with a slight initial enlargement of the radial sesamoid. Natural selection would be effective from the early stages of enlargement, i.e., even a small, protruding lump at the wrist can be a modest help in preventing bamboo from slipping off bent fingers.

A Reuters article by reporter Will Dunham titled, For pandas, it’s been two ‘thumbs’ up for millions of years (June 2, 2022), which summarizes Wang et al.‘s findings, quotes another of the study’s authors, Tao Deng, in support of its claim that the panda’s false “thumb,” while quite adequate for bamboo feeding, is far from optimal, in the sense that a much better engineering design was possible:

The false thumb lets pandas hold bamboo to eat but not rotate the food as a true thumb would allow.

“One of the most important features of human beings and their primate relatives is the evolution of a thumb that can be held against other fingers for precise grasping. The panda’s false thumb is far less effective than the human thumb, but it is enough to provide the giant panda with the grasping ability to eat bamboo,” said paleontologist and study co-author Tao Deng of the Chinese Academy of Sciences in Beijing.

Evidently these authors do not share Professor Dilley’s sentiment that the panda’s thumb exemplifies “the kind of sophistication that only engineers produce.”

Suboptimality in the false “thumb” of a six-million-year-old panda

An artistic reconstruction of the extinct panda, Ailurarctos, from Shuitangba, Yunnan Province, China. Image courtesy of Mauricio Antón and Wikipedia.

But wait, there’s more! In their article in Scientific Reports, Wang et al. describe the discovery of a fossil panda named Ailurarctos, dating back six million years, in Yunnan Province, China. It appears that this ancient panda had a false “thumb” too, but it was considerably less efficient than that of the modern giant panda – in other words, suboptimal in the first sense of the term, as distinguished by Glenn Branch in the excerpt quoted above. In the above-cited Reuters article, For pandas, it’s been two ‘thumbs’ up for millions of years (June 2, 2022), reporter Will Dunham catalogues the shortcomings of Ailurarctos’ false “thumb”:

It closely resembled the false thumb of modern pandas, but is a bit longer and lacks the inward hook present on the end in the extant species that provides even greater ability to manipulate bamboo stalks, shoots and roots while eating…

“It uses the false thumb as a very crude opposable thumb to grasp bamboos, sort of like our own thumbs except it is located at the wrist and is much shorter than human thumbs,” said Natural History Museum of Los Angeles County paleontologist Xiaoming Wang, lead author of the research published in the journal Scientific Reports.

Ailurarctos was an evolutionary forerunner of the modern panda, smaller but with anatomical traits signaling a similar lifestyle including a bamboo diet. The modern panda’s false thumb has some advantages over the earlier version.

“The hooked false thumb offers a tighter grasp of the bamboo and, at the same time, its less-protruded tip – because of the bended hook – makes it easier for the panda to walk. Think of the false thumb as being stepped on every time the panda walks. And therefore, we think that is the reason that the false thumb in modern pandas has become shorter, not longer,” Wang said.

Would Professor Dilley care to explain why an engineer (his term) designing the panda’s “thumb” would have to go through an intermediate stage (Ailuractos) that was markedly suboptimal, rather than simply using the current design in the very first giant pandas?

The giant panda’s digestive system: also suboptimal

But wait, there’s even more! In their article in Scientific Reports, Wang et al. highlight the numerous flaws in the giant panda’s digestive system – suboptimality in Branch’s first sense of the term:

Besides having a false thumb, much else about the giant panda is also unusual and/or enigmatic. Pandas traded the high-protein, omnivorous diet of their ursid [bear] ancestors for bamboo, a woody grass of high fiber and low nutrition, but with year-round availability in South China and Southeast Asia. To make this tradeoff work, pandas eat prodigious quantities of bamboo, up to 45 kg/day (depending on the season), and spend ~ 15 h/day eating³⁷. The panda’s short digestive tract, inherited from its carnivoran ancestors, is also poorly suited for extracting nutrients, absorbing less than 20% of digestible dry matter³⁸. Furthermore, pandas lack the high-crowned teeth that most ungulate mammals possess for grinding tough plant fibers into a fine mush and consequently make minimal use of microbials to break down cellulose to extract the structural carbohydrates11. As a result, the panda’s gastrointestinal tract allows a rapid passage of digesta in less than 12 h³⁸, too fast for fermentation when compared to fore- and hindgut fermenting mammalian herbivores, and necessitating an equally prodigious quantity of defecation, up to 100 times/day³⁹…

The panda’s transition from a broad, omnivorous diet to a highly specialized bamboo diet necessitated multiple changes in anatomy and physiology, as well as their genetics underpinning^45,46,47. However, even after at least six million years of a bamboo diet, these transformations are still limited, mostly focused on food handling while the digestive system remains that of a carnivore⁴⁸. The fact that there was no further elongation of the false thumb in the panda lineage after the late Miocene, suggests that an adequate grip for bamboo had been obtained, i.e., good enough for grasping a single stem or small bundle, and that further enlargement was inhibited by countervailing selection for weight-bearing and walking (Fig. 8).

So there you have it: giant pandas have to poop 100 times a day, and after six million years of eating bamboo, their digestive system “remains that of a carnivore,” and makes “minimal use of microbials to break down cellulose,” with the result that the animals absorb “less than 20% of digestible dry matter,” since their carnivoran digestive systems are so “poorly suited for extracting nutrients.” If that’s not suboptimal, then nothing is.

Professor Dilley concludes his article with a proposal: “Perhaps it’s time to champion the panda’s thumb not as an icon for evolution but for intelligent design.” I can only describe this as the assessment of a man wearing rose-colored glasses. Speaking of which, let me leave my readers with Johnny Farnham’s song of that title, from 1969:

I’ll be back wit more tomorrow.