Meyer Marc R, Williams Scott A, Schmid Peter, Churchill Steven E, Berger Lee R
Department of Anthropology, Chaffey College, Rancho Cucamonga, CA 91737, USA.
Center for the Study of Human Origins, Department of Anthropology, New York University, 25 Waverly Place, New York, NY 10003, USA; New York Consortium in Evolutionary Primatology, New York, NY 10024, USA; Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3, Wits 2050, South Africa.
J Hum Evol. 2017 Mar;104:32-49. doi: 10.1016/j.jhevol.2017.01.001. Epub 2017 Feb 9.
Cervical vertebrae are rare in the early hominin fossil record, presenting a challenge for understanding the evolution of the neck and head carriage in hominin evolution. Here, we examine the cervical vertebrae of Australopithecus sediba, which unlike other South African taxa is known from associated cervical vertebrae. The A. sediba cervical vertebrae exhibit human-like values for wedging, pedicle cross-sectional areas, and articular facet heights, indicating reduced ventral loading relative to African apes. These features combine with a pattern of vertebral body bone distribution and caudally progressive size expansion suggesting a mode of cervical lordosis, load mitigation, and head carriage similar to humans and distinct from the cantilevered mode of head carriage of the extant African great apes. Yet these derived features in A. sediba are accompanied by ape-like vertebral body and dorsal pillar sizes, articular facet orientation, and uncinate process morphology signaling reduced lateral and rotational coupled movements between vertebral elements and indicate a considerably stiffer neck than in humans. A primitively long and horizontally-oriented C7 spinous process is likely related to a prognathic viscerocranium, although the complimentary C3 spinous process is short, implying large moments emanating from scapular and shoulder elevators rather than large muscles of head stabilization. Cross-sectional spinous process shape and robust anterior tubercles similarly signal increased arm elevation consistent with climbing behavior in corroboration with arboreal signatures previously observed in the shoulder, arms, and hand of A. sediba. Spinal canal shape and size suggests that A. sediba lacked the cervical spinal cord enlargement of Homo that confers humans with enhanced motor control to the upper limbs. The cervical spine of A. sediba thus presents a mosaic of primitive and derived characters, with anatomical features relating to neck posture and head carriage mirroring humans juxtaposed with most other aspects of functional anatomy that resemble chimpanzees.
颈椎在早期人类化石记录中很罕见，这给理解人类进化过程中颈部和头部支撑方式的演变带来了挑战。在此，我们研究了南方古猿源泉种的颈椎，与其他南非类群不同，它有相关颈椎的化石记录。南方古猿源泉种的颈椎在楔入度、椎弓根横截面积和关节面高度方面呈现出类似人类的数值，这表明相对于非洲猿类，其腹侧负荷有所减轻。这些特征与椎体骨分布模式以及尾部逐渐增大的尺寸相结合，表明存在一种颈椎前凸、减轻负荷和头部支撑的模式，类似于人类，且与现存非洲大猿的悬臂式头部支撑模式不同。然而，南方古猿源泉种的这些衍生特征伴随着类似猿类的椎体和背柱尺寸、关节面方向以及钩突形态，这表明椎体元件之间的侧向和旋转耦合运动减少，并且颈部比人类的要硬得多。原始的长且水平方向的C7棘突可能与前突的面颅有关，尽管与之对应的C3棘突较短，这意味着肩胛和肩部上提肌产生的力矩较大，而不是头部稳定的大肌肉。棘突的横截面形状和粗壮的前结节同样表明手臂上举增加，这与攀爬行为一致，这也与之前在南方古猿源泉种的肩部、手臂和手部观察到的树栖特征相印证。椎管的形状和大小表明南方古猿源泉种缺乏人类所具有的颈椎脊髓增粗，而这种增粗赋予了人类对上肢更强的运动控制能力。因此，南方古猿源泉种的颈椎呈现出原始特征和衍生特征的镶嵌组合，与颈部姿势和头部支撑相关的解剖特征类似于人类，而功能解剖的大多数其他方面则类似于黑猩猩。
