Human skeletal variation: adaptive responses during growth of the bony pelvis

人类分娩对母亲和新生儿造成的困难和风险的进化论解释集中在母亲骨盆大小和新生儿大小之间的紧密契合，这是由于我们独特的两条腿走路的方法和我们的大大脑造成的。这种“产科困境”认为女性的骨盆形态是两足行走的力学要求和大脑婴儿的产科要求之间的折衷，导致人类和我们的进化史上对女性骨盆产生了强大的选择压力。种群或物种内的物理变异可以受到这种竞争选择性过程的限制(如产科困境假说所预测的那样)，但形态也受到生长和发育过程中的环境刺激的影响，通过生物可塑性。对于骨盆，这些刺激可能包括步态、活动水平和减轻活动的技术手段、饮食和健康。塑料反应在解剖学和生理学上会在个体和群体之间产生差异。因此，人类骨骼的文化生产为生物考古学家提供了一个了解过去种群的活动和适应的窗口。医学模型倾向于将分娩和女性骨盆形态都放在类型学框架内看待，偏离某些标准会增加难产的风险。然而，最近的研究表明，在人类之间和人群内部，盆腔管的形状和容量存在显著差异。此外，历史上关于“产科困境”的先入之见，实际上可能是在不断变化的文化环境中，母亲骨盆和新生儿大小之间的可塑性差异造成的。在这种模式下，人类生态和文化的变化可能在决定骨盆形状方面发挥特殊的作用。然而，到目前为止，很少有研究考察影响骨盆生长和发育的因素。来自各种考古背景和有充分记载的青少年的人类骨骼遗骸为研究在特定生态环境(例如生存、技术和地理位置)下影响生长和发展的因素提供了机会。为了了解生物可塑性在塑造母体骨盆中的潜在作用，我们问：骨盆形状的变异是在什么时候和为什么发生的，生物可塑性在产生与环境刺激有关的变异方面扮演了什么角色？本研究将使用三维(3D)方法从不同的骨骼样本中研究青少年和成人骨盆的形态，以探讨种群的生态和文化属性与骨盆生长发育之间的关系。这代表了一种新的方法来研究骨盆的生长和发育，以及在人类群体中产生形态变异的选择性和可塑性因素。了解形态变异发生过程中的可塑性和约束(进化的、功能的或发育的)之间的相互作用，有助于确定影响适应的环境因素(气候、生态、文化)。这些对于进化论人类学家、古人类学家和进化论解剖学家来说尤其重要。人类骨盆变异的这种模式对于理解现代医学问题也是必不可少的，例如头盆不成比例和难产。对于生物考古学家来说，对适应性反应和限制的认识是重要的，因为随着人口和生态变化的反应，生长过程的结果越来越多地被用作人口健康的指标。

Evolutionary explanation of the difficulty and risk to the mother and newborn associated with human childbirth have focused on the close fit between the size of the maternal pelvis and the size of the newborn as resulting from our distinctive method of walking on two legs and our large brains. This “obstetric dilemma” views female pelvic form as a compromise between the mechanical requirements of bipedalism for a narrow pelvis and the obstetric requirements for a roomy pelvic canal for our large-brained babies, resulting in strong selective pressures acting on the female pelvis among humans and over our evolutionary history. Physical variation within a population or species can be limited by such competing selective processes (as predicted by the obstetric dilemma hypothesis), yet morphology is also influenced by environmental stimuli acting during growth and development, through biological plasticity. For the pelvis these stimuli may include gait, activity levels and technological means of mitigating activity, diet, and health. Plastic responses generate variation among individuals and populations in anatomy and physiology. Hence, the cultural production of the human skeleton provides bioarchaeologists a window on the activities and adaptations of past populations. Medical models have tended to view both childbirth and female pelvic form in a typological framework, with variation from certain norms as increasing risk of obstructed labour. However, recent studies have shown significant variation among and within human populations in pelvic canal shape and capacity. Further, historical preconceptions of an “obstetric dilemma” may actually be the result of differential plasticity between the maternal pelvis and newborn size, in changing cultural environments. Under this model, human ecological and cultural changes may play a particular role in determining pelvic form. Yet, little research has to date examined the factors that influence pelvic growth and development. Human skeletal remains of juveniles from varied and well-documented archaeological contexts provide an opportunity to study factors influencing growth and development in specific ecological contexts (e.g., subsistence, technologies, and geographical location). In order to understand the potential role of biological plasticity in shaping the maternal pelvis, we ask: when and why does variation in pelvic forms arise, and what role does biological plasticity play in generating variation in relation to environmental stimuli? This study will use three-dimensional (3D) approaches to studying the form of juvenile and adult human pelvic bones from diverse skeletal samples to investigate the relationships among ecological and cultural attributes of populations and pelvic growth and development. This represents a novel approach to examining growth and development of the pelvis and the selective and plastic factors that produce morphological variation among human populations. Understanding the interplay between plasticity and constraint (evolutionary, functional, or developmental) in the generation of morphological variation aids in identifying environmental factors (climatic, ecological, cultural) that influence adaptation. These are of particular importance to evolutionary anthropologists, paleoanthropologists, and evolutionary anatomists. This patterning of human pelvic variation is also essential for understanding modern medical problems such as cephalo-pelvic disproportion and obstructed labour. An awareness of adaptive responses and constraints is of importance for bioarchaeologists where the outcomes of growth processes are increasingly being employed as indicators of population health through time in response to demographic and ecological changes.