About 3.6 million years ago, a couple of early hominins (group ancestral to humans) walked over some fresh volcanic ash and left footprints. The footprints got fossilized and were discovered millions of years later, in 1976, by Mary Leakey at Laetoli site in modern Tanzania. The footprints were found at a site that is rich in fossils of early hominins, near the Olduvai Gorge. Comparison of footprints to skeletal remains suggests that the species that left them was Australopithecus afarensis.
What is curious about the footprints is that there were no knuckle-prints associated with them, suggesting that these early humans walked upright. But, there are two ways a primate can walk upright – with bent hips and knees the way modern apes walk upright, and with straight legs the way modern humans walk.
This week, PLoS ONE published an article in which the Laetoli footprints were put to a test. Human subjects were asked to walk either completely upright, with straight legs, or emulating great apes, with bent hips and knees. They walked through the sand and their footprints were measured, visualized and analyzed in comparison to the Laetoli footprints. The distribution of weight between the heel and the ball of the foot of the Laetoli footprint matches much more closely the distribution of weight in human subjects who walked completely upright. This suggests that the Australopithecus afarensis also walked fully upright, pushing the origin of upright posture back to 3.6. million years ago – an important shift back into the past for the discussions of human evolution.
The image is from the article Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics by David A. Raichlen, Adam D. Gordon, William E. H. Harcourt-Smith, Adam D. Foster, and Wm. Randall Haas, Jr. From the Abstract:
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.
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.