人类到底是在哪里学会直立行走？是森林还是草原？最新的一项研究表明，热带草原在人类进化过程中所起的作用，比之前认为的要长久和明显得多。
上世纪20年代提出的“草原说”认为，人类从森林向草原的过渡迫使人们直立行走，同时脑容量增大，形成了人类的特性。该理论自提出以来不断遭到质疑，有科学家指出，东非原始人化石最多的地方其实覆盖着多片林地，因此有很多人认为人类是在森林里学会直立行走的。
不过，英国《自然》杂志刊登的新研究支持了“草原说”。美国犹他州立大学托尔·塞林教授领导的科研小组发现，每个发现有人类化石的地区，总有证据表明该地区当时存在类似草原的空旷区，这说明大约400万年前人类开始学行走的时候，空旷的草原环境即已具备，甚至对人类直立行走起到了主要的作用。(生物谷 Bioon.com)
doi:10.1038/nature10306
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Woody cover and hominin environments in the past 6[thinsp]million years
Thure E. Cerling; Jonathan G. Wynn; Samuel A. Andanje; Michael I. Bird; David Kimutai Korir; Naomi E. Levin; William Mace; Anthony N. Macharia; Jay Quade; Christopher H. Remien
The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). 13C/12C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than ~40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years.
