Fingerprint homoplasy: koalas and humans Note 1
Keywords: Dermal ridges, dermatoglyphes, eutherians, evolution, fingerprints, grasping, tree kangaroos, marsupials, prehensile, wombat.
MACIEJ HENNEBERG,* KOSETTE M. LAMBERT, CHRIS M. LEIGHDepartment of Anatomical Sciences, University of Adelaide, Adelaide, SA 5005, Australia, email@example.com
Received February 26, 1997, published March 11, 1997
Summary: Fingerprints (dermatoglyphes) consist of patterns formed by parallel ridges on bare skin of fingertips. They are typical for higher primates, but occur sporadically in other mammals. We have recently observed the presence of dermatoglyphes, microscopically and macroscopically similar to those of humans, on the fingers and toes of the tree-climbing marsupial Phascolarctus cinereus. Presence of nearly identical dermatoglyphes in lineages of mammals undergoing separate evolution for at least 70 million years, but adapting to climbing and grasping, indicates that adaptive regularities may be a common feature of the evolutionary process.
That surfaces of palms, soles, digits, and sometimes prehensile tails, of primates are covered with papillary ridges organised into complex “dermatoglyphic” patterns is well known. Dermal ridges also occur sporadically in non-primate mammalian species (Okajima, 1991). The printable patterns of dermal ridges form during the intra-uterine life, and are known to be highly heritable (Loesch and Przybyla, 1988). Interpretations of the evolutionary origins and functional significance of dermal ridge patterns are still a matter of debate. Interpreting the presence of dermatoglyphes in vastly diverse mammalian lineages may help to resolve the debate. Earlier authors reported that dermatoglyphics are not present in koalas (Dankmeijer, 1938; Okajima, 1991).
Koalas (Phascolarctos cinereus) are members of a marsupial sub-order Vombatiformes, and as the name suggests, are closely related to wombats. Koalas, weighing about 4 to 12 kg, feed by climbing vertically onto the smaller branches of eucalyptus trees, reaching out, grasping handfuls of leaves and bringing them to the mouth.
In an ongoing comparative anatomical study we have observed in koalas dermal ridges very similar in their microscopic morphology, organisation into patterns and pattern uniqueness to an individual, to those of humans (Figures 1 and 2). Ridges were invariably present in a score of individuals examined, be they male, female or juveniles. In contrast to most higher primates, only a portion of the bare skin on hands, feet and digits of koalas is covered with ridges. The remainder display “warts”—roughly circular epidermal elevations. This is similar to an arrangement observed in prosimians (Cummins and Midlo, 1961). Typically marsupials and eutherians have areas of bare skin on distal portions of the extremities covered exclusively by warts (Dankmeijer, 1938; Whipple, 1904).
Figure 1. Standard ink prints of fingertips of third right manual digits of an adult koala (left) and an adult human (right).
Our observations show that dermal ridges are not present on the paws of a close relative of a koala, the hairy nose wombat (Lasiorhinus latifrons), whereas ridges occur in Phalanger maculatus (Dankmeijer, 1938), that is a member of another suborder, Phalangeriformes, but climbs trees.
It seems therefore that dermal ridge patterns are heritable structures occurring on the skin of those mammalian extremities that are prehensile irrespective of the taxonomic affiliation of an animal. An arboreal way of life is insufficient to explain the occurrence of dermatoglyphes, as we have not observed them in tree kangaroos (Dendrolagus linustus), who walk on the surface of larger tree limbs and branches. Their hands and feet are covered by warts resembling those of wombats. Therefore the origin of dermatoglyphes is best explained as the biomechanical adaptation to grasping, which produces multidirectional mechanical influences on the skin. These forces must be precisely felt for fine control of movement and static pressures and hence require orderly organization of the skin surface. Evolutionary paths of marsupials and eutherians have been separated for at least 70 million years (Clemens, 1989) and yet the koala and human dermatoglyphes display remarkable similarity. Adaptive regularities may turn out to be more prevalent in evolution than unique historical events.