最近刊登在美國(guó)《國(guó)家科學(xué)院院刊》上的一項(xiàng)研究報(bào)告稱,，非洲奧杜威山谷周?chē)h(huán)境在干旱遼闊的草原和封閉潮濕的森林之間發(fā)生了迅速變化,，這很可能影響了早期人類(lèi)的進(jìn)化。并且研究人員表示,，水和其影響的植物在人類(lèi)進(jìn)化中產(chǎn)生了重要的作用,。
一直以來(lái)，地方氣候和植物為理解現(xiàn)代人類(lèi)是如何進(jìn)化的提供了背景信息,，但是迄今為止基于陸地環(huán)境替代的記錄太少,，因此無(wú)法揭示出地方生態(tài)系統(tǒng)的定義性特征。并且,，陸地環(huán)境變化記錄的缺失和水有效性的間接替代性數(shù)據(jù)也會(huì)限制對(duì)水,、植物和早期人類(lèi)之間關(guān)系的理解。
美國(guó)賓夕法尼亞州立大學(xué)地球科學(xué)系的Katherine Freeman及其同事通過(guò)研究保存在奧杜威山谷的湖沉積物中有機(jī)物碳同位素特征,，找出了大約200萬(wàn)年前哪些植物支配著當(dāng)?shù)丨h(huán)境,，而就在那個(gè)時(shí)候,，直立人出現(xiàn)并開(kāi)始擴(kuò)散。
同時(shí),，在另一篇相關(guān)論文中,，科學(xué)家根據(jù)附近湖沉積物中的植物蠟和海藻脂的氫同位素比率，構(gòu)造出了奧杜威山谷生態(tài)系統(tǒng)的湖水蒸發(fā)和降水可利用度的高分辨率記錄,。這個(gè)記錄表明該地區(qū)曾遇到了干旱明顯波動(dòng)的情況,，在干旱時(shí)期草原是主流，而濕潤(rùn)時(shí)期的特征是樹(shù)木覆蓋的擴(kuò)展,。
研究人員發(fā)現(xiàn),，植物生命歷史揭示出，僅僅在幾百年到幾千年時(shí)間里,，開(kāi)放草原與封閉森林之間出現(xiàn)了突然的轉(zhuǎn)變,。而這些發(fā)現(xiàn)與人們長(zhǎng)期所持的一種觀念相沖突，原來(lái)的觀念認(rèn)為,，在很長(zhǎng)時(shí)間里,，干旱逐漸增加，促進(jìn)了非洲大陸草原的擴(kuò)展,，而這反過(guò)來(lái)又驅(qū)動(dòng)著人類(lèi)的進(jìn)化,。Freeman研究小組還總結(jié)道，“淡水的可用性在東非生態(tài)系統(tǒng)快速氣候變化時(shí)期的早期人類(lèi)繁衍方面起到了重要影響”,。
這些研究表明,，保存在湖沉積物中的有機(jī)物可以帶來(lái)地方生態(tài)變化的詳細(xì)歷史，并且揭示出過(guò)去的氣候變化是如何影響人類(lèi)進(jìn)化的,。（(生物谷Bioon.com)
DOI:10.1073/pnas.1209405109
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PMID:
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.
