Evolution of the hominoid vertebral column: The long and the short of it

Abstract

The postcranial axial skeleton exhibits considerable morphological and functional diversity among living primates. Particularly striking are the derived features in hominoids that distinguish them from most other primates and mammals. In contrast to the primitive catarrhine morphotype, which presumably possessed an external (protruding) tail and emphasized more pronograde trunk posture, all living hominoids are characterized by the absence of an external tail and adaptations to orthograde trunk posture. Moreover, modern humans evolved unique vertebral features that satisfy the demands of balancing an upright torso over the hind limbs during habitual terrestrial bipedalism. Our ability to identify the evolutionary timing and understand the functional and phylogenetic significance of these fundamental changes in postcranial axial skeletal anatomy in the hominoid fossil record is key to reconstructing ancestral hominoid patterns and retracing the evolutionary pathways that led to living apes and modern humans. Here, we provide an overview of what is known about evolution of the hominoid vertebral column, focusing on the currently available anatomical evidence of three major transitions: tail loss and adaptations to orthograde posture and bipedal locomotion.

Tail function during arboreal quadrupedalism in squirrel monkeys (Saimiri boliviensis) and tamarins (Saguinus oedipus)

“…In apes, an evolutionary increase in grasping ability has been suggested to offset the loss of the tail (Cartmill and Milton, ; Begun et al, ; Kelley, ; Almécija et al, ). Current fossil evidence suggests that many Early‐to‐Middle Miocene hominoid taxa (e.g., Proconsul, Nacholapithecus ) probably relied on generalized forms of arboreal quadrupedalism (Ward, , ; Rose, ; Begun et al, ; Nakatsukasa and Kunimatsu, ; Williams and Russo, ). Given the estimated body sizes for some early hominoids (e.g., Proconsul heseloni , 9–20 kg) (Ruff et al, ; Rafferty et al, ; Harrison, ), having a tail for maintaining stability on narrow (relative to body size) branches would seemingly offer an adaptive advantage.…”

Hominin vertebrae and upper limb bone fossils from Sterkfontein Caves, South Africa (1998–2003 excavations)

“…Most commentators suggest, however, that at least some species of Australopithecus do not possess the full complement of bipedal features observed in the ULs of modern humans (reviewed in Williams & Russo, ). For instance, the caudal body surface areas (CBSAs) of many Australopithecus ULs are small relative to estimated live body weights of those animals (e.g., Dobson, ; Robinson, ; Sanders, ; Shapiro, ; Velte, ).…”

“…Thus, theory predicts that the lumbar lordotic angles (and the bony anatomy that underlie those angles) of women and men should differ (e.g., Whitcome et al, ). There is disagreement (e.g., Been, Pessah, Been, Tawil, & Peleg, ; Vialle et al, ; Williams & Russo, ), but some empirical data seem to support this hypothesis (e.g., Bailey, Sparrey, Been, & Kramer, ; Whitcome, ; Whitcome et al, ). Specifically, the lumbar vertebrae of modern human males show less extreme lordotic body wedge angles, less coronally oriented prezygopophyses, and smaller prezygopophysis surface areas than do the lumbar vertebrae of modern human females.…”

An evolutionary hypothesis to explain the role of deconditioning in low back pain prevalence in humans

“…Larger bodied semi-terrestrial species including extant great apes and dryopithecines instead have a shorter lumbar spine of 3 to 4 lumbar vertebrae [144,[150][151][152]. The transition to bipedalism was accompanied by a lumbar spine consisting of 5 to 6 vertebrae, though there is some disagreement as to whether this constituted a lengthening from shorter backed species or a shortening from longer backed arboreal quadrupeds [133,150,[152][153][154][155][156][157][158][159][160][161][162][163][164]. Nevertheless these anatomical arrangements of vertebral number were also accompanied by structural changes to the vertebral bodies themselves.…”

“…For example there is some disagreement regarding the stages of locomotion presented [131][132][133] with this debate centring on whether the last common ancestor (LCA) of modern humans and the great apes walked with a bent-hip-bent-knee gait when bipedal and whether their lumbar spine was long and flexible (like OWMs) or short and stable (like in the great apes) [133,150,[152][153][154][155][156][157][158][159][160][161][162][163][164]. It is beyond the scope of this article to examine this area of debate but it should be noted that the final circumstance, a long flexible yet unstable lumbar spine in humans being related to LBP, can be supported irrespective of whether an intermediary involving a short back LCA stage in locomotor evolution and its corresponding anatomical evolution occurred [147,150].…”