The University Museum, The University of Tokyo, Tokyo 113-0033, Japan;
The Kyoto University Museum, Kyoto University, Kyoto 606-8501, Japan.
Proc Natl Acad Sci U S A. 2021 Dec 7;118(49). doi: 10.1073/pnas.2116630118.
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 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 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 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 appears to be shared with early , 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 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 and .
体尺和犬齿大小二型性在化石中为人类进化的社会行为假说提供了信息，自达尔文（Darwin）就这一主题发表著名论述以来，一直备受关注。在这里，我们汇集了大量的人科化石犬齿数据集，包括在埃塞俄比亚中阿瓦什和贡纳研究区域中发现的所有可用化石，并通过进化时间系统地检查犬齿二型性。特别是，我们应用了一种贝叶斯概率方法，该方法可减少在估计弱和中度二型性时的偏差。我们的研究结果表明，犬齿二型性明显弱于倭黑猩猩（bonobo），在现存的大猿类中，倭黑猩猩的二型性和行为攻击性最小。的上下犬齿的雄性与雌性大小比的平均值分别估计为 1.06 和 1.13，处于现代人类种群变异范围之内。与早期人类相比，的下犬齿的犬齿大小二型性的幅度略大，这表明在上犬齿中，雄性犬齿的减少最初更为先进，行为上更为重要。现有的化石证据表明，在和最早已知的人科成员之前，雄性犬齿的大小发生了急剧减少，此后犬齿二型性水平几乎没有变化。这种进化模式表明，在人类进化的早期，与相对较弱的雄性攻击性水平相关的行为发生了深刻的转变，而这种模式随后也被和所共享。
