Body and canine size dimorphism in fossils inform sociobehavioral hypotheses on human evolution and have been of interest since Darwin’s famous reflections on the subject. Here, we assemble a large dataset of fossil canines of the human clade, including all available Ardipithecus ramidus fossils recovered from the Middle Awash and Gona research areas in Ethiopia, and systematically examine canine dimorphism through evolutionary time. In particular, we apply a Bayesian probabilistic method that reduces bias when estimating weak and moderate levels of dimorphism. Our results show that Ar. ramidus canine dimorphism was significantly weaker than in the bonobo, the least dimorphic and behaviorally least aggressive among extant great apes. Average male-to-female size ratios of the canine in Ar. ramidus are estimated as 1.06 and 1.13 in the upper and lower canines, respectively, within modern human population ranges of variation. The slightly greater magnitude of canine size dimorphism in the lower than in the upper canines of Ar. ramidus appears to be shared with early Australopithecus, suggesting that male canine reduction was initially more advanced in the behaviorally important upper canine. The available fossil evidence suggests a drastic size reduction of the male canine prior to Ar. ramidus and the earliest known members of the human clade, with little change in canine dimorphism levels thereafter. This evolutionary pattern indicates a profound behavioral shift associated with comparatively weak levels of male aggression early in human evolution, a pattern that was subsequently shared by Australopithecus and Homo.

化石中的身体和犬类大小的二态性为人类进化的社会行为假设提供了依据，并且自从达尔文对该主题的着名思考以来一直引起人们的兴趣。在这里，我们汇集了人类进化枝化石犬科动物的大型数据集，包括从埃塞俄比亚中阿瓦什和戈纳研究区发现的所有可用的Ardipithecus ramidus化石，并通过进化时间系统地检查犬科动物的二态性。特别是，我们应用贝叶斯概率方法，在估计弱和中等二态性水平时减少偏差。我们的结果表明，Ar。枝条犬类的二态性明显弱于倭黑猩猩，倭黑猩猩是现存类人猿中二态性和行为上最不具攻击性的。Ar中犬的平均雄性与雌性尺寸比。在现代人类种群的变异范围内，上犬齿和下犬齿的ramidus估计分别为 1.06 和 1.13。Ar的下犬齿比上犬齿的犬齿尺寸二态性的幅度略大。ramidus似乎与早期的南方古猿共享，这表明雄性犬齿的减少最初在行为上重要的上犬齿中更为先进。现有的化石证据表明，在Ar之前，雄性犬的体型急剧缩小。枝条和已知最早的人类进化枝成员，此后犬类二态性水平几乎没有变化。这种进化模式表明了一种深刻的行为转变，与人类进化早期相对较弱的男性攻击水平相关，这种模式随后被南方古猿和人共享。