Canine sexual dimorphism in<i>Ardipithecus ramidus</i>was nearly human-like

  • Gen Suwa
    The University Museum, The University of Tokyo, Tokyo 113-0033, Japan;
  • Tomohiko Sasaki
    The Kyoto University Museum, Kyoto University, Kyoto 606-8501, Japan;
  • Sileshi Semaw
    Centro Nacional de Investigación sobre la Evolución Humana, 09002 Burgos, Spain;
  • Michael J. Rogers
    Department of Anthropology, Southern Connecticut State University, New Haven, CT 06515;
  • Scott W. Simpson
    Department of Anatomy, Case Western Reserve University School of Medicine, Cleveland, OH 44106;
  • Yutaka Kunimatsu
    Department of Business Administration, Ryukoku University, Kyoto 612-8577, Japan;
  • Masato Nakatsukasa
    Laboratory of Physical Anthropology, Kyoto University, Kyoto 606-8502, Japan;
  • Reiko T. Kono
    Faculty of Letters, Keio University, Yokohama 223-8521, Japan;
  • Yingqi Zhang
    Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing 100044, China;
  • Yonas Beyene
    French Center for Ethiopian Studies, Addis Ababa, Ethiopia;
  • Berhane Asfaw
    Rift Valley Research Service, Addis Ababa, Ethiopia;
  • Tim D. White
    Centro Nacional de Investigación sobre la Evolución Humana, 09002 Burgos, Spain;

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<jats:title>Significance</jats:title><jats:p>Humans have the proportionately smallest male canines among all anthropoids and little canine sexual dimorphism. However, the evolutionary emergence of this defining condition remains unclear because until now we have lacked a reliable method of determining dimorphism in weakly dimorphic fossil species. Using a probability-based method we recently developed, we estimated canine size dimorphism in the ∼4.5 million-year-old<jats:italic>Ardipithecus ramidus</jats:italic>and found it to be weak and comparable to that of modern humans. Our analysis of >300 fossils spanning 6 million years shows that male canine size reduction occurred early in human evolution, broadly coincident with the adoption of bipedality. This suggests a profound and evolutionarily deep sociobehavioral shift that minimized male–male aggression, most likely mediated by female choice.</jats:p>

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