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Ecosystem variability and early human habitats in eastern Africa

机译:东非的生态系统变化与人类早期栖息地

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The role of savannas during the course of early human evolution has been debated for nearly a century, in part because of difficulties in characterizing local ecosystems from fossil and sediment records. Here, we present high-resolution lipid biomarker and isotopic signatures for organic matter preserved in lake sediments at Olduvai Gorge during a key juncture in human evolution about 2.0 Ma-the emergence and dispersal of Homo erectus (sensu lato). Using published data for modern plants and soils, we construct a framework for ecological interpretations of stable carbon-isotope compositions (expressed as δ~(13)C values) of lipid biomarkers from ancient plants. Within this framework, δ~(13)C values for sedimentary leaf lipids and total organic carbon from Olduvai Gorge indicate recurrent ecosystem variations, where open C_4 grasslands abruptly transitioned to closed C_3 forests within several hundreds to thousands of years. Carbon-isotopic signatures correlate most strongly with Earth's orbital geometry (precession), and tropical sea-surface temperatures are significant secondary predictors in partial regression analyses. The scale and pace of repeated ecosystem variations at Olduvai Gorge contrast with long-held views of directional or stepwise ari-dification and grassland expansion in eastern Africa during the early Pleistocene and provide a local perspective on environmental hypotheses of human evolution.
机译:稀树草原在人类早期进化过程中的作用已争论了近一个世纪,部分原因是难以从化石和沉积物记录中表征当地生态系统。在这里,我们为人类进化约2.0 Ma的关键时刻-直立人(sensu lato)的出现和扩散提供了高分辨率的脂质生物标志物和同位素特征,用于有机沉积在Olduvai峡谷湖沉积物中的有机物。利用现代植物和土壤的公开数据,我们为古代植物的脂质生物标记物的稳定碳同位素组成(表示为δ〜(13)C值)构建了生态学解释的框架。在这个框架内,Olduvai峡谷的沉积叶脂和总有机碳的δ〜(13)C值表明了生态系统的反复变化,在几百年至几千年的时间里,开放的C_4草原突然转变为封闭的C_3森林。碳同位素特征与地球的轨道几何形状(岁差)密切相关,热带海洋表面温度是部分回归分析中重要的次要预测因子。奥杜瓦伊峡谷生态系统反复变化的规模和步伐与更新世早期在非洲东部长期定向或逐步干旱化和草原扩张的长期观点形成鲜明对比,并为人类进化的环境假说提供了局部视角。

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