最近刊登在美国《国家科学院院刊》上的一项研究报告称，非洲奥杜威山谷周围环境在干旱辽阔的草原和封闭潮湿的森林之间发生了迅速变化，这很可能影响了早期人类的进化。并且研究人员表示，水和其影响的植物在人类进化中产生了重要的作用。
一直以来，地方气候和植物为理解现代人类是如何进化的提供了背景信息，但是迄今为止基于陆地环境替代的记录太少，因此无法揭示出地方生态系统的定义性特征。并且，陆地环境变化记录的缺失和水有效性的间接替代性数据也会限制对水、植物和早期人类之间关系的理解。
美国宾夕法尼亚州立大学地球科学系的Katherine Freeman及其同事通过研究保存在奥杜威山谷的湖沉积物中有机物碳同位素特征，找出了大约200万年前哪些植物支配着当地环境，而就在那个时候，直立人出现并开始扩散。
同时，在另一篇相关论文中，科学家根据附近湖沉积物中的植物蜡和海藻脂的氢同位素比率，构造出了奥杜威山谷生态系统的湖水蒸发和降水可利用度的高分辨率记录。这个记录表明该地区曾遇到了干旱明显波动的情况，在干旱时期草原是主流，而湿润时期的特征是树木覆盖的扩展。
研究人员发现，植物生命历史揭示出，仅仅在几百年到几千年时间里，开放草原与封闭森林之间出现了突然的转变。而这些发现与人们长期所持的一种观念相冲突，原来的观念认为，在很长时间里，干旱逐渐增加，促进了非洲大陆草原的扩展，而这反过来又驱动着人类的进化。Freeman研究小组还总结道，“淡水的可用性在东非生态系统快速气候变化时期的早期人类繁衍方面起到了重要影响”。
这些研究表明，保存在湖沉积物中的有机物可以带来地方生态变化的详细历史，并且揭示出过去的气候变化是如何影响人类进化的。（(生物谷Bioon.com)
DOI:10.1073/pnas.1209405109
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Water, plants, and early human habitats in eastern Africa
Clayton R. Magill, Gail M. Ashley, and Katherine H. Freeman
Water and its influence on plants likely exerted strong adaptive pressures in human evolution. Understanding relationships among water, plants, and early humans is limited both by incomplete terrestrial records of environmental change and by indirect proxy data for water availability. Here we present a continuous record of stable hydrogen-isotope compositions (expressed as δD values) for lipid biomarkers preserved in lake sediments from an early Pleistocene archaeological site in eastern Africa—Olduvai Gorge. We convert sedimentary leaf- and algal-lipid δD values into estimates for ancient source-water δD values by accounting for biochemical, physiological, and environmental influences on isotopic fractionation via published water–lipid enrichment factors for living plants, algae, and recent sediments. Reconstructed precipitation and lake-water δD values, respectively, are consistent with modern isotopic hydrology and reveal that dramatic fluctuations in water availability accompanied ecosystem changes. Drier conditions, indicated by less negative δD values, occur in association with stable carbon-isotopic evidence for open, C4-dominated grassland ecosystems. Wetter conditions, indicated by lower δD values, are associated with expanded woody cover across the ancient landscape. Estimates for ancient precipitation amounts, based on reconstructed precipitation δD values, range between approximately 250 and 700 mm·y−1 and are consistent with modern precipitation data for eastern Africa. We conclude that freshwater availability exerted a substantial influence on eastern African ecosystems and, by extension, was central to early human proliferation during periods of rapid climate change.
