School of Biological Sciences, Monash University, Victoria 3800, Australia.
Geosciences, Museum Victoria, Victoria 3001, Australia.
Nature. 2016 Feb 25;530(7591):477-80. doi: 10.1038/nature16972.
The variation in molar tooth size in humans and our closest relatives (hominins) has strongly influenced our view of human evolution. The reduction in overall size and disproportionate decrease in third molar size have been noted for over a century, and have been attributed to reduced selection for large dentitions owing to changes in diet or the acquisition of cooking. The systematic pattern of size variation along the tooth row has been described as a 'morphogenetic gradient' in mammal, and more specifically hominin, teeth since Butler and Dahlberg. However, the underlying controls of tooth size have not been well understood, with hypotheses ranging from morphogenetic fields to the clone theory. In this study we address the following question: are there rules that govern how hominin tooth size evolves? Here we propose that the inhibitory cascade, an activator-inhibitor mechanism that affects relative tooth size in mammals, produces the default pattern of tooth sizes for all lower primary postcanine teeth (deciduous premolars and permanent molars) in hominins. This configuration is also equivalent to a morphogenetic gradient, finally pointing to a mechanism that can generate this gradient. The pattern of tooth size remains constant with absolute size in australopiths (including Ardipithecus, Australopithecus and Paranthropus). However, in species of Homo, including modern humans, there is a tight link between tooth proportions and absolute size such that a single developmental parameter can explain both the relative and absolute sizes of primary postcanine teeth. On the basis of the relationship of inhibitory cascade patterning with size, we can use the size at one tooth position to predict the sizes of the remaining four primary postcanine teeth in the row for hominins. Our study provides a development-based expectation to examine the evolution of the unique proportions of human teeth.
人类和我们最亲近的亲属（人科动物）的磨牙大小变化强烈影响了我们对人类进化的看法。一个多世纪以来，人们注意到整体尺寸减小，第三磨牙尺寸不成比例减小，并将其归因于饮食变化或烹饪技能获得导致大牙列选择减少。牙齿沿牙列的大小变化的系统模式自 Butler 和 Dahlberg 以来一直被描述为哺乳动物，更具体地说是人科动物牙齿的“形态发生梯度”。然而，牙齿大小的潜在控制因素尚未得到很好的理解，假说范围从形态发生场到克隆理论。在这项研究中，我们提出了以下问题：是否存在控制人类牙齿大小进化的规则？在这里，我们提出抑制级联，这是一种影响哺乳动物相对牙齿大小的激活抑制剂机制，为所有人类下初级后牙（乳前磨牙和恒磨牙）产生默认的牙齿大小模式。这种配置也相当于形态发生梯度，最终指向可以产生这种梯度的机制。在南方古猿（包括 Ardipithecus、Australopithecus 和 Paranthropus）中，牙齿大小的模式保持不变，与绝对大小一致。然而，在包括现代人在内的 Homo 物种中，牙齿比例与绝对大小之间存在紧密联系，使得单个发育参数可以解释初级后牙的相对和绝对大小。基于抑制级联模式与大小的关系，我们可以使用一个牙齿位置的大小来预测牙列中其余四颗初级后牙的大小。我们的研究为检查人类牙齿独特比例的进化提供了基于发育的预期。
