Department of Evolutionary Anthropology, Duke University, Durham, NC, 27705, USA; Division of Anthropology, American Museum of Natural History, New York, NY, 10024, USA.
Department of Integrative Anatomical Sciences, Keck School of Medicine, University of Southern California, Los Angeles, CA, 90033, USA; Human and Evolutionary Biology Section, Department of Biological Sciences, University of Southern California, Los Angeles, CA, 90089, USA.
J Hum Evol. 2021 Dec;161:103078. doi: 10.1016/j.jhevol.2021.103078. Epub 2021 Nov 6.
In 2000, a complete fourth metatarsal (Mt4) of the ∼3- to 4-Million-year-old hominin Australopithecus afarensis was recovered in Hadar, Ethiopia. This metatarsal presented a mostly human-like morphology, suggesting that a rigid lateral foot may have evolved as early as ∼3.2 Ma. The lateral foot is integral in providing stability during the push off phase of gait and is key in understanding the transition to upright, striding bipedalism. Previous comparisons of this fossil were limited to Pan troglodytes, Gorilla gorilla, and modern humans. This study builds on previous studies by contextualizing the Mt4 morphology of A. afarensis (A.L. 333-160) within a diverse comparative sample of nonhuman hominoids (n = 144) and cercopithecids (n = 138) and incorporates other early hominins (n = 3) and fossil hominoids that precede the Pan-Homo split (n = 4) to better assess the polarity of changes in lateral foot morphology surrounding this divergence. We investigate seven morphological features argued to be functionally linked to human-like bipedalism. Our results show that some human-like characters used to assess midfoot and lateral foot stiffness in the hominin fossil record are present in our Miocene ape sample as well as in living cercopithecids. Furthermore, modern nonhuman hominoids can be generally distinguished from other species in most metrics. These results suggest that the possession of a rigid foot in hominins could represent a conserved trait, whereas the specialized pedal grasping mechanics of extant apes may be more derived, in which case some traits often used to infer bipedal locomotion in early hominins may, instead, reflect a lower reliance on pedal grasping. Another possibility is that early hominins reverted from modern ape Mt4 morphology into a more plesiomorphic condition when terrestrial bipedality became a dominant behavior. More fossils dating around the Pan-Homo divergence time are necessary to test these competing hypotheses.
2000 年，在埃塞俄比亚哈达尔出土了一具约 300 万至 400 万年前的人科南方古猿阿法种的完整第四跖骨（Mt4）。这块跖骨呈现出与人类相似的形态，表明刚性的外侧足可能早在 320 万年前就已经进化出来了。外侧足在步态的蹬离阶段提供稳定性，对于理解向直立、跨步的两足行走的过渡至关重要。之前对这块化石的比较仅限于黑猩猩、大猩猩和现代人。本研究在之前研究的基础上，将 A. afarensis（A.L. 333-160）的 Mt4 形态置于一个多样化的非人类人科动物（n=144）和长尾猴科（n=138）的比较样本中，并纳入了其他早期人属（n=3）和化石人科动物（n=4），以更好地评估在这个分歧周围的外侧足形态变化的极性。我们研究了七个形态特征，这些特征被认为与人类似的两足行走功能相关。我们的结果表明，一些用于评估人属化石记录中中足和外侧足刚度的人类似特征，也存在于我们的中新世猿类样本以及现生长尾猴科动物中。此外，现代非人类人科动物通常可以在大多数指标上与其他物种区分开来。这些结果表明，在人属中拥有刚性的足部可能是一种保守的特征，而现生猿类特有的足部抓握力学可能更为衍生，如果是这样，那么一些通常用于推断早期人属两足行走的特征可能反映的是对足部抓握的依赖程度较低。另一种可能性是，当陆地两足行走成为主要行为时，早期人属从现代猿类的 Mt4 形态退回到更为原始的状态。需要更多的化石来检验这些相互竞争的假说。
