The anthropoid postcranial axial skeleton: Comments on development, variation, and evolution

Abstract

Within‐species phenotypic variation is the raw material on which natural selection acts to shape evolutionary change, and understanding more about the developmental genetics of intraspecific as well as interspecific phenotypic variation is an important component of the Evo‐Devo agenda. The axial skeleton is a useful system to analyze from such a perspective. Its development is increasingly well understood, and between‐species differences in functionally important developmental parameters are well documented. I present data on intraspecific variation in the axial postcranial skeleton of some Primates, including hominoids (apes and humans). Hominoid species are particularly valuable, because counts of total numbers of vertebrae, and hence original somite numbers, are available for large samples. Evolutionary changes in the axial skeleton of various primate lineages, including bipedal humans, are reviewed, and hypotheses presented to explain the changes in terms of developmental genetics. Further relevant experiments on model organisms are suggested in order to explore more fully the differences in developmental processes between primate species, and hence to test these hypotheses. J. Exp. Zool. (Mol. Dev. Evol.) 302B:241–267, 2004. © 2004 Wiley‐Liss, Inc.

Gibbons and hominoid ancestry

“…Their characters are as follows: skulls with lightly built crania with relatively prominent brow ridges; variable prognathism from low to high; strong angle between face and skull (klinorhynchy) in Hispanopithecus laietanus; reduced maxillary sinus; broad triangular nose; broad palate; high zygomatic root; primitive teeth in D. fontani, molars with broad basins between cusps elongated molars and premolars in Pierolapithecus; teeth with thick enamel in Anoiapithecus and Pierolapithecus; reduced M3 in the three earlier species but not in H. laietanus and R. hungaricus; orthograde (upright) posture; broad chest region; long clavicle; scapula shifted on to back; stiff lumbar region; mobile elbow joint, stable at full extension; mobile wrist; long slender hand phalanges (short and less curved in some); femur head above greater trochanter; femur neck steeply angled. Not all these characters are known for all species, but where they are known for two or more species the characters are consistent, with the conclusion that upright posture, and/or suspensory locomotion had evolved in some species of dryopithecines, particularly in Hispanopithecus laietanus (Crusafont-Pairo and Hurzeler, 1961;Pilbeam and Simons, 1971;Kretzoi, 1975;Morbeck, 1983;Begun et al, 1990Begun et al, , 2003Moyà-Solà et al, 1993, 2004, 2009a, 2009bKordos, 1991;Begun and Kordos, 1993;Moyà-Solà S. and Köhler, 1993, 1995, 1996, 1997Kordos and Begun 1997;Ungar and Kay 1995;Kordos and Begun, 1997;Kordos and Begun, 1997;Begun, 2002Begun, , 2009Ungar, 2005;Alba et al, 2010;Begun et al, 2012). Some of the characters supposedly indicating suspensory locomotion are absent in gibbons, the most suspensory of the apes, for example the stiff lower back.…”

“…Some of the species extend back into the middle Miocene, for example Sivapithecus sivalensis, and they are similar functionally to middle Miocene European apes, with relatively robust jaws and thick-enamelled teeth. Some have similarities of the skull with the orangutan, but the few postcrania show no suspensory adaptations and indicate a strong element of terrestriality in their locomotion (Pilbeam, 1982(Pilbeam, , 1996(Pilbeam, , 2004Pilbeam et al, 1990;Rose, 1984Rose, , 1986Rose, , 1988Rose, , 1989Rose, , 1994Rose, , 1997. Laccopithecus robustus from late Miocene deposits in China is an ape similar to hylobatids in its skull and dental formation, but a single proximal phalanx is long and curved, like that of Hispanopithecus and gibbons (Wu and Pan, 1984;Meldrum and Pan, 1988;Begun, 2002).…”

Skeletal development in sloths and the evolution of mammalian vertebral patterning

“…Hence, as defined in these studies, the term homeotic is still potentially applicable to both transgenic mice and sloths. Moreover, Buchholtz and Stepien (13) did not include vertebral counts in the caudal region of sloths (which are infrequently available in museum specimens) (24), leaving open the possibility that changes in presacral number were balanced in the postsacral region. Regardless of the strict definition of homeotic, we interpret our data as consistent with the PAS hypothesis of Buchholtz and Stepien (13).…”

Reexamining Human Origins in Light of Ardipithecus ramidus

“…This method is termed referential modeling (1). A central tenet has been the presumption (sometimes clearly stated but more often simply sub rosa) that Gorilla and Pan are so unusual and so similar to each other that they cannot have evolved in parallel; therefore, the earliest hominids must have also resembled these African apes (2,3). Without a true early hominid fossil record, the de facto null hypothesis has been that Australopithecus was largely a bipedal manifestation of an African ape (especially the chimpanzee).…”