Answer: they have all independently evolved bony anatomical structures that function as fingers (or thumbs) that are not derived from tetrapod pentadactyl digits.
So I don’t think I’ll be head hunted by Marks & Spencer for Christmas cracker jokes any time soon, but it is true nonetheless.
The typical body plan of a four-legged land animal (known technically as a tetrapod) consists of four limbs, each limb with an upper bone (in the case of a human arm this is the humerus), two lower bones (the radius and ulna), a group of smaller bones (our wrists) and a variable number of digits, each consisting of several thin cylindrical bones, known collectively as the polydactyl limb.
In our case, the upper limbs have become arms, with four fingers and one opposable thumb (thus we are pentadactyl). The upper limb of bats has become adapted for flight, with elongated ‘fingers’ supporting membranous wings. Some animals have lost digits over evolutionary time, such as hoofed mammals like horses, while others have lost limbs altogether, as in the hind-limbs of Cetaceans (whales and dolphins), or all four limbs in snakes. But all these creatures are still unmistakably related to other tetrapods with more ‘typical’ body layouts.
The Giant Panda (Ailuropoda melanoleuca) possesses the standard pentadactyl limb, with five ‘fingers’ comprising each paw. But it also has a sixth digit in the two front paws that work functionally like opposable thumbs, allowing the panda to grip bamboo shoots more effectively.
This ‘pseudo-thumb’ is not the result of duplicating an extra finger during embryological development. In fact, it is formed by elongating one of the bones found in the panda’s wrist known as a sesamoid bone.
Sesamoid bones are embedded in tendons, and are essentially hardened calcifications of the tendon itself. The largest sesamoid bone in the human body is the patella, which lies suspended in the kneecap between the quadriceps tendon above and the patellar tendon below.
One of the sesamoid bones suspended in tendons around the panda’s wrist has been lengthened over evolutionary time, and supported by additional muscles and nerves, to form its pseudo-thumb. Steven J. Gould wrote about this in his collection of essays, The Pandas Thumb, noting it as a perfect example of what he called an exaptation – the usurping of a piece of anatomy that evolved for one reason into a completely different role to exploit a new environment. It is a great demonstration of the blind opportunism of natural selection; having a lengthened wrist bone for a thumb isn’t perfect ‘design’, but it is what was available and it works, and that’s all that matters.
Indeed, the improvised sesamoid thumb must be advantageous because another group of animals has evolved a very similar structure completely independently. Talpid moles, such as those you’d find in a local golf course, also have a sixth digit in their front paws that acts as a rudimentary thumb. But rather than assist with gripping bamboo, as in the panda, the mole’s ‘thumb’ probably evolved to help with digging. The exact developmental pathway for sesamoid bone elongation and thumb formation has been elucidated in one species of mole, Talpa occidentalis (the Iberian mole). It would be fascinating to learn whether pandas independently hit upon the same developmental pathway, or whether entirely novel developmental processes are at work producing similar results.
Writing this week in the journal Science, anatomists have uncovered a similar bone buried within elephant’s feet. A massive sesamoid is located in all four of the elephant’s foot plates (unlike the panda and mole, which only possess elongated sesamoids in their front limbs). While the five conventional digits in the elephant’s pentadactyl limb point forwards, the extra sesamoid points backwards into the dense heel pad, helping to support the animal’s massive weight. Revealingly, the extra bone only appears in the fossil record from around 40 million years ago, when elephants first started increasing in size.
Evolution does what it has to, and when five digits just aren’t enough, whether it’s for eating bamboo, digging holes or supporting ten tons of herbivorous body mass, natural selection can stumble across unexpected solutions repeatedly and independently, namely by making thumbs out of wrist tendons. As well as exaptation and opportunism, the sesamoid digits exemplify another evolutionary principle, that of Orgel’s Second Law: evolution is cleverer than you are.