School of Anthropology, University of Arizona, Tucson, Arizona, United States of America.
PLoS One. 2010 Mar 22;5(3):e9769. doi: 10.1371/journal.pone.0009769.
Debates over the evolution of hominin bipedalism, a defining human characteristic, revolve around whether early bipeds walked more like humans, with energetically efficient extended hind limbs, or more like apes with flexed hind limbs. The 3.6 million year old hominin footprints at Laetoli, Tanzania represent the earliest direct evidence of hominin bipedalism. Determining the kinematics of Laetoli hominins will allow us to understand whether selection acted to decrease energy costs of bipedalism by 3.6 Ma.
METHODOLOGY/PRINCIPAL FINDINGS: Using an experimental design, we show that the Laetoli hominins walked with weight transfer most similar to the economical extended limb bipedalism of humans. Humans walked through a sand trackway using both extended limb bipedalism, and more flexed limb bipedalism. Footprint morphology from extended limb trials matches weight distribution patterns found in the Laetoli footprints.
These results provide us with the earliest direct evidence of kinematically human-like bipedalism currently known, and show that extended limb bipedalism evolved long before the appearance of the genus Homo. Since extended-limb bipedalism is more energetically economical than ape-like bipedalism, energy expenditure was likely an important selection pressure on hominin bipeds by 3.6 Ma.
关于人类直立行走这一标志性特征的起源，一直存在争议，焦点在于早期两足动物的行走方式更像人类——拥有能量效率更高的伸展后肢，还是更像猿类——弯曲后肢。坦桑尼亚莱托利的 360 万年前的人类足迹代表了最早的人类直立行走的直接证据。确定莱托利人类的运动学特征，将使我们了解到 360 万年前，是否通过选择降低了直立行走的能量成本。
方法/主要发现：通过实验设计，我们发现莱托利人行走时的负重转移最类似于人类经济的伸展后肢直立行走方式。人类在沙质小道上行走时，既采用伸展后肢的直立行走方式，也采用更为弯曲后肢的直立行走方式。从伸展后肢试验中获得的足迹形态与在莱托利足迹中发现的体重分布模式相匹配。
这些结果为我们提供了目前已知最早的、运动学上类似人类的直立行走的直接证据，并表明伸展后肢的直立行走方式比人属的出现要早得多。由于伸展后肢的直立行走方式比类似猿类的直立行走方式更节省能量，因此能量消耗很可能是 360 万年前对人类两足动物的一个重要选择压力。
