陆地上最早的四足动物或许并非是勇敢的探险者，它们有可能仅仅是依沼泽而生的食腐动物。它们并非未经“思考”便冒险踏进干旱的沙漠，而后才开始随环境而进化的。一块来自波兰东南部古泻湖中的、3.95亿年前的、长约8英尺的四足动物化石表明，人类远古的祖先，有可能并不是那些勇敢的、需要从一个池塘“跃”向另一个池塘的鱼类，而是生活在林区的具四附肢生物，因为它们不需要“冒险”。
最近，发表在The Journal of Geology杂志上的一项研究"Woodland Hypothesis for Devonian Tetrapod Evolution"显示，鱼类与两栖类的过渡形式存在于布满树的洪泛平原里。该研究的主要科研人员俄勒岗大学的Gregory Retallack博士说，来自马里兰、纽约和宾夕法尼来的诸多化石都显示它们并非生存于池塘或沙漠中，而是与潮湿的林地土壤相伴生，池塘或沙漠土壤中能够找到许多鱼类化石，但都不是我们的远古祖先。
基于化石的研究，鱼类与两栖类的过渡形式存在于布满树的洪泛平原里。Retallack博士继续说，我们的远古祖先并不笨，它们是典型的机会主义者，它们利用了充满树木的洪泛平原的优势得以繁衍生息。附肢能够帮助绕开诸多树木的阻碍，而韧性的脖子则能使它们在浅水中取食。因此，蝾螈（salamanders）与鱼类最主要的区别——附肢与脖子，进化出来的目的并非是为了在沙漠中“探险”，而是在潮湿洪泛林地中的适应性特征。
Retallack博士说，对过渡形式的化石产出地周围的古土壤与沉积物的研究，是理解鱼类何时何地在何种状态下开始行走的关键。许多汽车上的“达尔文鱼”（Darwin fish，一条具腿的漫画鱼）装饰纹其实代表了生命进化的一个重要阶段。这一研究还挑战了之前四足动物生活于浅海中的理论。（生物谷Bioon.com）
doi:10.1086/659144
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Woodland Hypothesis for Devonian Tetrapod Evolution
Gregory J. Retallack
AbstractThe rarity of Devonian tetrapods and the absence of tetrapods during the first 14 million years of the Mississippian (Romer’s Gap) have inspired hypotheses of fish-tetrapod evolutionary transition as an escape from difficult habitats such as deserts or stagnant waters. These hypotheses and Romer’s Gap are tested here using depth to calcic horizon in paleosols of the northern Appalachians as a proxy for precipitation and vegetation changes during the Devonian and Mississippian. All tetrapod bones and trackways in this region, as well as fossil tree remains, were found at times of high precipitation (indicated by deep calcic paleosols), when subhumid woodlands expanded at the expense of semiarid shrublands in alluvial lowlands. No tetrapods or tree fossils were found in abundant paleosols with the shallow calcic horizons of aridland soils. The mean annual precipitation requirement of Devonian-Mississippian tetrapods and trees was at least mm. Global distribution of prototetrapods and aquatic tetrapods also shows that they lived earliest (Eifelian) in coastal lagoon or estuary margin soils of humid regions rather than in aridlands or intertidal flats. Tetrapods later spread to a variety of habitats, but continued preference for noncalcareous soils of woodlands and forests may explain Romer’s Gap as a preservational artifact. A woodland hypothesis of tetrapod evolution is presented here: limbs and necks were selected for by scavenging and hunting in shallow-flooded woodlands and oxbow lakes during a unique period in Earth history, after evolution of flood-ponding trees and before effective terrestrial predator resistance.
