by Greg Mayer
One of the most important lessons of comparative anatomy is that evolution usually proceeds by the modification of pre-existing structures (or, stated more precisely, the modification of the pre-existing developmental programs that produce those structures). Certain changes are easier to evolve because the developmental system can be modified to produce them—evolution follows the developmental path of least resistance. In terms of the skeleton of vertebrates, this means that most evolutionary changes are reduction, fusion, loss, lengthening, shortening, thickening, and narrowing of bones. Evolution uses what’s already there, and rarely do wholly new structures arise.
A paper by John R. Hutchinson and colleagues of the Royal Veterinary College (see also his neat elephant lab website) reports the presence of a sixth ‘digit’ in the fatty pad of the feet of elephants (oddly, they don’t say which species of elephant, at least not in the published paper). Primitively, mammals are flat footed, but as they become more cursorial (i.e., adapted for running), they move to running on their toes, and finally, as in horses, on the tips of their toes.
Elephants are midway between walking on their toes and toe tips (a condition known as subunguligrade), and have a large fatty pad at the bottom of their feet to support their great weight. One of the neatest aspect of this paper is that the relatively good fossil record of elephants allows Hutchinson and colleagues to trace when (about 40 mya) and in what groups this evolutionary change toward subunguligrade locomotion and in the foot skeleton occurred. The sixth ‘digit’ is an enlarged sesamoid bone medial to the first true digit. (A sesamoid is a bone that develops within a tendon.) It is not a true toe, which would have a meta carpal/tarsal and one or more phalanges. Rather, a different bone altogether has been pressed into service to approximate a digit.
Such jury rigged adaptations were made famous by Stephen Jay Gould, who wrote about another modification of a sesamoid bone, the panda’s thumb. The panda uses its enlarged sesamoid on the front limb to aid in stripping leaves off of bamboo. A true thumb, as in primates and certain marsupials, is the first digit. The panda, being a bear, has all five of its flat footed digits, and the sesamoid is pressed into service as a thumb substitute.
Darwin and Gould used such jury-rigged adaptations as powerful evidence for descent with modification, because their imperfection and evident relation to structures of different use in other organisms (what we would now recognize as pre-existing structures) show them to be traces of history, not paragons of design. The ‘one bone-two bones-many bones’ structure of the tetrapod limb, and its manifold modifications for walking, running, digging, grasping, swimming, flying, etc., are the most familiar example of this. This tetrapod unity of type embraces similarities beyond what might be called for for functional reasons, and instead is a marker of ancestry. The limbs undergo a sequence of adaptations to various modes of life, but all are modifications of the immediately ancestral limb.
One of my favorite examples of the sequential adaptation of limbs is Richard Swann Lull’s account of the hind limb in marsupials. Primitive marsupials are arboreal (i.e., living in trees), while more derived forms are quite varied, including the speedy kangaroos. Primitive marsupials have a grasping great toe (digit I), while kangaroos have the elongated limbs and feet and reduced side toes typical of cursorial or saltatorial runners; they have lost the great toe altogether. Lull calls these primary and secondary adaptations. Tree kangaroos have become arboreal again. While it might be nice to have a grasping toe, ancestral kangaroos lacked this toe, and so they made do with what they had: the hind foot becomes broadened and shortened for use on tree limbs, and the claws are used for digging in.
As the Zen poet Donald Rumsfeld might have put it, “You become arboreal with the feet you have, not the feet you want.”
Gould, S.J. 1978. The panda’s peculiar thumb. Natural History 87 (November): 20, 24-30.
Hutchinson J.R., C. Delmer, C.E. Miller, T. Hildebrandt, A.A. Pitsillides, and A. Boyde. 2011. From flat foot to fat foot: structure, ontogeny, function, and evolution of elephant “sixth toes”. Science 334:1699-1703.
Lull, R.S. 1917. Organic Evolution. Macmillan, New York.