Department of Anatomy, Physiology & Pharmacology, College of Medicine, University of Saskatchewan, 107 Wiggins Road, Saskatoon, Saskatchewan S7N 5E5, Canada.
College of Dentistry, University of Saskatchewan, 105 Wiggins Road, Health Sciences Building, Saskatoon, SK, S7N 5E5, Canada.
J Hum Evol. 2021 Dec;161:103072. doi: 10.1016/j.jhevol.2021.103072. Epub 2021 Oct 7.
A tenet of mammalian, including primate dental evolution, is the Inhibitory Cascade Model, where first molar (M1) size predicts in a linear cline the size and onset time of the second (M2) and third (M3) molars: a larger M1 portends a progressively smaller and later-developing M2 and M3. In contemporary modern Homo sapiens, later-developing M3s are less likely to erupt properly. The Inhibitory Cascade Model is also used to predict molar sizes of extinct taxa, including fossil Homo. The extent to which Inhibitory Cascade Model predictions hold in contemporary H. sapiens molars is unclear, including whether this tenet informs about molar initiation, development, and eruption. We tested these questions here. In our radiographic sample of 323 oral quadrants and molar rows from contemporary humans based on mesiodistal crown lengths, we observed the distribution of molar proportions with a central tendency around parity (M1 = M2 = M3) that parsed into 13 distinct molar size ratio patterns. These patterns presented at different frequencies (e.g., M1 > M2 > M3 in about one-third of cases) that reflected whether the molar row was located in the maxilla or mandible and included both linear (e.g., M1 < M2 < M3) and nonlinear molar size ratio progressions (e.g., M1 > M2 < M3). Up to four patterns were found in the same subject's mouth. Lastly, M1 size alone does not predict M3 size, developmental timing, or eruption; rather, M2 size is integral to predicting M3 size. Our study indicates that human molar size is genetically 'softwired' and sensitive to factors local to the human upper jaw vs. lower jaw. The lack of a single stereotypical molar size ratio for contemporary H. sapiens suggests that predictions of fossil H. sapiens molar sizes using the Inhibitory Cascade Model must be made with caution.
哺乳动物，包括灵长类动物，牙齿进化的一个原则是抑制级联模型，其中第一磨牙 (M1) 的大小以线性梯度预测第二磨牙 (M2) 和第三磨牙 (M3) 的大小和出现时间：较大的 M1 预示着 M2 和 M3 逐渐变小和发育较晚。在现代人类中，发育较晚的 M3 不太可能正常萌出。抑制级联模型也用于预测已灭绝分类群的磨牙大小，包括化石人类。在现代人类磨牙中，抑制级联模型的预测程度尚不清楚，包括这一原则是否能说明磨牙的起始、发育和萌出。我们在这里检验了这些问题。在我们的 323 个口腔象限和磨牙列的放射学样本中，基于近远中冠长度，我们观察到磨牙比例的分布，其中心趋势为均等 (M1 = M2 = M3)，可分为 13 种不同的磨牙大小比例模式。这些模式以不同的频率出现（例如，三分之一的情况下 M1 > M2 > M3），反映了磨牙列位于上颌骨还是下颌骨，包括线性（例如，M1 < M2 < M3）和非线性磨牙大小比例进展（例如，M1 > M2 < M3）。在同一个受试者的口中发现了多达四种模式。最后，M1 的大小本身不能预测 M3 的大小、发育时间或萌出；相反，M2 的大小是预测 M3 大小的关键。我们的研究表明，人类磨牙大小在基因上是“软布线”的，并且对外上颌骨和下颌骨局部的因素敏感。现代人类没有单一的典型磨牙大小比例，这表明使用抑制级联模型预测化石人类的磨牙大小必须谨慎。
