The Later Stone Age Calvaria from Iwo Eleru, Nigeria: Morphology and Chronology

BackgroundIn recent years the Later Stone Age has been redated to a much deeper time depth than previously thought. At the same time, human remains from this time period are scarce in Africa, and even rarer in West Africa. The Iwo Eleru burial is one of the few human skeletal remains associated with Later Stone Age artifacts in that region with a proposed Pleistocene date. We undertook a morphometric reanalysis of this cranium in order to better assess its affinities. We also conducted Uranium-series dating to re-evaluate its chronology. Methodology/Principal FindingsA 3-D geometric morphometric analysis of cranial landmarks and semilandmarks was conducted using a large comparative fossil and modern human sample. The measurements were collected in the form of three dimensional coordinates and processed using Generalized Procrustes Analysis. Principal components, canonical variates, Mahalanobis D2 and Procrustes distance analyses were performed. The results were further visualized by comparing specimen and mean configurations. Results point to a morphological similarity with late archaic African specimens dating to the Late Pleistocene. A long bone cortical fragment was made available for U-series analysis in order to re-date the specimen. The results (∼11.7–16.3 ka) support a terminal Pleistocene chronology for the Iwo Eleru burial as was also suggested by the original radiocarbon dating results and by stratigraphic evidence. Conclusions/SignificanceOur findings are in accordance with suggestions of deep population substructure in Africa and a complex evolutionary process for the origin of modern humans. They further highlight the dearth of hominin finds from West Africa, and underscore our real lack of knowledge of human evolution in that region.

研究背景
近年来，晚期石器时代（Later Stone Age）的测年结果被修正至远超此前认知的时间深度。与此同时，该时期的人类遗骸在非洲地区十分稀少，西非地区更是罕见。伊沃埃勒鲁（Iwo Eleru）墓葬是该地区少数几件关联晚期石器时代人工制品、且被认为属于更新世（Pleistocene）时期的人类骨骼遗骸之一。本研究对该颅骨开展形态测量学重新分析，以更好地评估其亲缘关系；同时进行铀系测年，重新评估其年代学框架。

方法与主要结果
本研究依托大型对比化石及现代人类样本集，对颅骨标志点与半标志点实施三维几何形态测量学分析。测量数据以三维坐标形式采集，并通过广义Procrustes分析（Generalized Procrustes Analysis）进行处理。研究开展了主成分分析、典型变量分析、马氏距离（Mahalanobis D²）分析以及Procrustes距离分析。通过对比样本与平均形态构型，进一步可视化呈现分析结果。
分析结果显示，该颅骨与年代为晚更新世的晚期古非洲人类标本形态相似。研究获取一段长骨皮质片段用于铀系分析，以重新测定该标本的年代。测年结果（约11.7~16.3 ka）支持伊沃埃勒鲁墓葬属于晚更新世末期的年代框架，这与最初的放射性碳测年结果及地层学证据的推断一致。

结论与意义
本研究结果与此前提出的非洲地区存在深度群体亚结构、以及现代人类起源伴随复杂演化过程的假说相符。本研究进一步凸显了西非地区古人类化石发现的匮乏，也强调了我们对该区域人类演化历程认知的严重不足。