ScienceDaily (Mar. 20, 2010) — More than three million years ago, the ancestors of modern humans were still spending a considerable amount of their lives in trees, but something new was happening.
Images show aerial and lateral images of (top to bottom) (A) a human footprint made walking normally with an extended knee and hip gait; (B) a human footprint made walking with bent-knees and hips to mimic a chimpanzee; and (C) a scan of a cast from the Laetoli fossil trackway. Notice that the toe is much deeper than the heel in the middle print and that the Laetoli scan looks more like the human print. (Credit: Randy Haas, University of Arizona)
The results of their research appears in PLoS ONE, a journal from the Public Library of Science.
A trackway of fossil footprints preserved in volcanic ash deposited 3.6 million years ago was uncovered in Laetoli, Tanzania, more than 30 years ago. The significance of those prints for human evolution has been debated ever since. The most likely individuals to have produced these footprints, which show clear evidence of bipedalism, or walking on two legs, would have been members of the only bipedal species alive in the area at that time, Australopithecus afarensis. That species includes "Lucy," whose skeletal remains are the most complete of any individual A. afarensis found to date.
A number of features in the hips, legs, and back of this group indicate that they would have walked on two legs while on the ground. But the curved fingers and toes as well as an upward-oriented shoulder blade provide solid evidence that Lucy and other members of her species also would have spent significant time climbing in trees.
This morphology differs distinctly from our own genus, Homo, who abandoned arboreal life around 2 million years ago and irrevocably committed to human-like bipedalism. Since the Laetoli tracks were discovered, scientists have debated whether they indicate a modern human-like mode of striding bipedalism, or a less-efficient type of crouched bipedalism more characteristic of chimpanzees whose knees and hips are bent when walking on two legs.
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David A. Raichlen1*, Adam D. Gordon2, William E. H. Harcourt-Smith3,4, Adam D. Foster1, Wm. Randall Haas, Jr1
1 School of Anthropology, University of Arizona, Tucson, Arizona, United States of America, 2 Department of Anthropology, University at Albany–SUNY, Albany, New York, United States of America, 3 Department of Anthropology, Lehman College, Bronx, New York, United States of America, 4 Division of Vertebrate Paleontology, American Museum of Natural History, New York, New York, United States of America
Abstract
Background
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.
Conclusions
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.
Citation: Raichlen DA, Gordon AD, Harcourt-Smith WEH, Foster AD, Haas WR Jr (2010) Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics. PLoS ONE 5(3): e9769. doi:10.1371/journal.pone.0009769
Editor: Karen Rosenberg, University of Delaware, United States of America
Received: November 22, 2009; Accepted: February 28, 2010; Published: March 22, 2010
Copyright: © 2010 Raichlen et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: Funding was provided from the University of Arizona. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: raichlen@email.arizona.edu
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