Metameric variation of molar size is in part associated with the dietary adaptations of mammals and results from slight alterations of developmental processes. Humans and great apes exhibit conspicuous variation in tooth morphology both between taxa and across tooth types. However, the manner in which metameric variation in molars emerged among apes and humans via evolutionary alterations in developmental processes remains largely unknown. In this study, we compare the enamel-dentine junction of the upper molars of humans-which closely correlates with morphology of the outer enamel surface and is less affected by wear-with that of the other extant hominoids: chimpanzees, bonobos, gorillas, orangutans, and gibbons. We used the morphometric mapping method to quantify and visualize three-dimensional morphological variation, and applied multivariate statistical analyses. Results revealed the following: 1) extant hominoids other than humans share a common pattern of metameric variation characterized by a largely linear change in morphospace; this indicates a relatively simple graded change in metameric molar shape; 2) intertaxon morphological differences become less distinct from the mesial to distal molars; and 3) humans diverge from the extant ape pattern in exhibiting a distinct metameric shape change trajectory in the morphospace. The graded shape change and lower intertaxon resolution from the mesial to distal molars are consistent with the concept of a 'key' tooth. The common metameric pattern observed among the extant nonhuman hominoids indicates that developmental patterns underlying metameric variation were largely conserved during ape evolution. Furthermore, the human-specific metameric pattern suggests considerable developmental modifications in the human lineage.

中文翻译：

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类人猿上臼齿的同分异构变异及其对臼齿形态发生多样化的影响。

磨牙大小的同分异体变异部分与哺乳动物的饮食适应有关，这是由于发育过程的轻微改变而引起的。人类和大猿在类群之间和跨牙齿类型的牙齿形态上都表现出明显的变化。然而，在猿猴和人类之间通过发育过程中的进化改变而出现的臼齿同分异体变异的方式仍然是未知的。在这项研究中，我们比较了人类上臼齿的牙釉质与牙本质的交界处，该交界处与釉质外表面的形态密切相关，并且受磨损的影响较小，与其他现存类人动物的黑猩猩，黑猩猩，大猩猩，猩猩一样。和长臂猿。我们使用形态计量映射方法来量化和可视化三维形态变化，并应用多元统计分析。结果表明：1）除人类以外，现存的类人动物具有共同的同分异构变异模式，其特征是形态空间呈线性变化；这表明同分异构体臼齿形状的梯度变化相对简单；2）从中磨牙到远侧磨牙，紫杉之间的形态差异变得越来越小；3）人类在形态空间中表现出明显的异聚体形状变化轨迹，与现存的猿猴模式有所不同。从近中臼齿到远侧臼齿的渐变形状变化和较低的共生体分辨率与“关键”牙齿的概念一致。在现存的非人类类人动物中观察到的常见的同分异构模式表明，在猿进化过程中，基本的同分异构变异的发育模式在很大程度上是保守的。