All thumbs

23 August 2013 by Malcolm Campbell, posted in Biology

The work of science is to substitute facts for appearances, and demonstrations for impressions.” John Ruskin (1819-1900)

Spend any time with a dog grappling with a bone, and your appreciation of your opposable thumb is bound to grow. In the absence of an opposable thumb, gripping the bone is a challenge. Holding it in place for a good chew becomes a task of great effort and concentration.

For dogs, the fifth digit isn’t conveniently located adjacent to the paw, as it is in the human hand. Instead, the dog’s thumb emerges some distance from the ground, on the inner part of the lower leg. The dog’s thumb, or “dewclaw”, is not only poorly situated but also has poor functionality. It has little, if any, movement. While some things of appropriate size and shape can be passively restrained by the dewclaw, they cannot be grasped or truly held. For some dog breeds, humans view the dewclaw as beyond useless. For these breeds, the dewclaw is seen as a detriment, and is amputated from puppies when they are days old.

Contrast the dewclaw with your own remarkable, opposable thumb. Hold your hand in front of you, palm upward, and stretch your thumb across your palm to the base of your fourth finger. Touch the tip of your thumb in turn to each of the tips of your other fingers. Together they allow you to powerfully grasp and hang from a tree branch, or to delicately pluck an eyelash from the cheek of a loved one. If a dog possessed the same capabilities, she would be able to hold onto and manipulate a bone for gnawing, or carefully extricate an irritating flea from her coat.

Your opposable thumb has an origin that extends back some 70 million years, as the primate lineage emerged. Consequently, like us, the other apes, and a good many other primates, possess opposable thumbs.  We share a common set of bones, muscles, and nerves that enable independent, opposable action of that fifth digit.

The primate opposable thumb consists of five bones. The first metacarpal bone is closest to the palm. The first metacarpal is attached to the palm at the cluster of bones that make up the carpus. It bends forward from the carpus at the carpometacarpal joint. Moving outward from the palm, the first metacarpal is attached to the proximal phalanx at the metacarpophalangeal joint.  There are two sesamoid bones located at this joint.  As you continue to move outward from the palm, the proximal phalanx is attached to the distal phalanx. The connection between the proximal phalanx to the distal phalanx occurs at the interphalangeal joint, which forms the “knuckle” of the thumb.

The bones of the thumb work in concert with a suite of muscles to give rise to the opposable action. If you think of the bones of the thumb as creating a column, the muscles function as ropes pulling the column in one direction or another. Ropes on one side of the column can pull it in that direction, while those on the far side of the column must be kept taut to prevent the column from toppling over.  Depending on which direction the thumb is pulled, one set of muscles is drawing the bones in that direction, which the other set is holding the column in check. Muscles in the forearm and in the hand proper work together to move the thumb about.

Muscle action on the carpometacarpal joint is most important in generating primate opposable thumb action. Muscles that pull the first metacarpal bone towards the palm, so that the proximal phalanx is extended across the palm is what opposition is all about. The bending that occurs at metacarpophalangeal and interphalangeal joints determines the extent to which we are powerfully gripping or precisely manipulating an object.

An opposable thumb provided our forebears with a tremendous advantage. The capacity to grasp enabled safer navigation of trees and vines, giving them greater accessibility to arboreal environments. The opposable thumb makes the gathering and extraction of food like fruits, nuts and grains easier, thereby broadening and improving diets. Critically, the opposable thumb makes it possible to create and utilise tools – tools that can be used to harvest plants, capture prey, build shelters, defend against enemies, and generally manipulate the environment to one’s advantage. It makes it possible to consider venturing out from the trees into less friendly environments, manipulating those environments to mitigate their unfriendliness. Equally important, opposable thumbs made it possible to better grasp one another, to hold on for travel, to groom, to comfort – to reinforce social bonds and structures. Arguably, opposable thumbs paved the way for the animals we are today.

Notably, these advantages would also benefit other species if they were able to evolve an opposable thumb as well. Imagine the advantage to dogs, and other canines, were they able to get a better grip on the bones they hold so dear. But this is minor in comparison to the advantage an opposable thumb would afford other species. Other species residing in arboreal environments would similarly be advantaged by the dexterity provided by anatomical innovations like an opposable thumb. In keeping with this, evolution has provisioned other species with opposable thumbs.

There are two species that carry the “panda” name that also bear opposable thumbs. Both the giant panda and the red panda have opposable thumbs. From an evolutionary perspective, this makes great sense. Both species are residents of the large bamboo forests of east Asia. What’s more, despite originating from carnivorous lineages, the giant panda and red panda are both largely herbivorous – feeding on the abundant bamboo of the forests in which they reside. Opposable thumbs serve both species exceedingly well – they are able to easily grasp, bend, break and defoliate the round stalks of the bamboo plant – making substantial meals for themselves.

The capacity to make large meals of bamboo is key for both species. As a consequence of their carnivorous evolutionary origins, both giant pandas and red panda are relatively inefficient at extracting nutrients from their vegetarian diet. They need to eat a lot. Having opposable thumbs are immensely helpful in that regard. Like us, they are able to capitalise on resources in their environment on account of their amazing thumbs.

Except it’s not as simple as that.

The opposable thumbs of giant pandas and red pandas are not the same as your thumb. Strictly speaking, anatomically they are not thumbs at all. The opposable thumbs of the red panda and the giant panda do not have the same bones that your thumbs have. There is not a metacarpal bone, or a proximal phalanx or distal phalanx, or any other kind of phalanx for that matter. The pandas’ thumbs are entirely different beasts altogether.

In fact, if you were to count up all of their digits, including the opposable thumb, you would find that giant pandas and red pandas have not five, like other mammals, but six. Where did the extra digit come from?

The thumbs of the giant panda and the red panda are made from a bone found in the wrist of most mammals – a carpal bone called the radial sesamoid. In the case of the two pandas, the radial sesamoid has greatly enlarged and acquired an entirely new function – that of an opposable thumb.

The radial sesamoid resides further up the leg from the pad of the paw. In other species, it functions to provide support for the pad. Muscles and ligaments are tied to the radial sesamoid to provide this support. These muscles and ligaments have been repurposed in red pandas and giant pandas to provide the sort of opposable action with which your are familiar with your own thumb.

As with us, evolution has equipped the giant panda and the red panda with an opposable “thumb” suited for their needs. Anatomically their thumbs may differ from our thumb, but they fulfil the same purpose. Evolution has merely provided two routes to the same solution.

And here is the remarkable thing – when it comes to the pandas’ “thumb”, evolution has travelled the same route, independently, on two different occasions.

It turns out that, despite their common names, the red panda and the giant panda are, in fact, only very, very distantly related. In fact, the giant panda is a bear. By contrast, the red panda is the last species in an ancient lineage of mammals that are related to the weasel family.

The red panda, with its raccoon-like striped tail and masked face, and sleek cat-like body is an agile dweller of treetops. The giant panda is a lumbering loiterer of the dense bamboo forest floor. In keeping with this, the ancestors of the giant panda were largely ground dwellers; whereas, the red panda’s ancestors took to the trees. These ancestors had a shared need for grasping bamboo, but for entirely different reasons. The giant panda’s ancestors were harvesting and consuming the thicker stems of the bamboo, while the red panda’s ancestors were making their way about the forest canopy, moving from tender shoot to tender shoot. Despite their difference in grasping need, evolution equipped them with identical means to get their grip on life – an opposable thumb made from a sesamoid bone.

Opposable thumbs provide us with ample evidence of the deceptive nature of appearances. In looking at our thumbs and pandas’ thumbs, we might assume that they are constructed in an identical fashion. In fact, evolution is more flexible than that, and has provided equivalent innovations via entirely different routes. On the other hand, in looking at the opposable thumbs of red pandas and giant pandas, we might expect that they share the same evolutionary origin as they are constructed in an equivalent fashion. Instead, evolution has found the same pathway to innovation, twice, and independently. Fittingly, opposable thumbs enable us to grasp a better understanding of the nature of biological innovation, and the power of evolution.

References:

Davis DD (1964) The giant panda: a morphological study of evolutionary mechanisms. Chicago Natural History Museum.

Fabre AC et al. (2013) Getting a grip on the evolution of grasping in musteloid carnivorans: a three‐dimensional analysis of forelimb shape. Journal of Evolutionary Biology 26: 1521–1535

Flynn JJ, Nedbal MA, Dragoo JW, & Honeycutt RL (2000) Whence the red panda? Molecular Phylogenetics and Evolution 17: 190-199

Gould SJ (1978) The panda’s peculiar thumb. Natural History 87: 105-108

Gould SJ (1980) Pandas Thumb: More Reflections in Natural History. WW Norton & company. New York.

Salesa MJ, Antón M, Peigné S, & Morales J (2006) Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas. Proceedings of the National Academy of Sciences of the United States of America 103: 379-382

Sato JJ et al. (2009) Deciphering and dating the red panda’s ancestry and early adaptive radiation of Musteloidea. Molecular Phylogenetics and Evolution 53: 907-922

Images: All photos by Malcolm M. Campbell, except red panda, which is by Harlequeen via Wikimedia Commons

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2 Responses to “All thumbs”

  1. Amy Prendergast Reply | Permalink

    True thumbs arent restricted to primates - do not possums also have opposable thumbs?

    • Malcolm Campbell Reply | Permalink

      You are correct, Amy! There are non-primate species that have opposable thumbs, like those discussed in this post.

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