The Laetoli trackways from Tanzania were first reported in 1979 and immediately attracted attention because they provided evidence of bipedalism. The tracks were preserved in volcanic ash dated at 3.6 million years. Many at the time thought they looked exactly like human footprints, but few of the researchers were willing to adopt this interpretation. The debate has been extensive and inconclusive, but some positive leads have recently been published. Evidence is now available to answer the question: did the makers of the trackways walk like humans or like apes?
“In particular, debates over the origins and evolution of bipedalism revolve around whether early bipeds walked with energetically economical human-like extended limb biomechanics, or with more costly ape-like bent-knee, bent-hip (BKBH) kinematics. If early hominins used a BKBH gait, then we must account for the persistence of an energetically costly form of bipedal walking until the evolution of the genus Homo.”
Research in the US involved human subjects walking with different gaits over damp sandy ground. The BKBH gait changes the way the body weight is carried by the feet and this, in turn, affects footprint morphology. Each footprint was recorded with a 3D scanner and quantitative comparisons were made. [. . .]
The findings were unambiguous. There are significant differences between true bipedal tracks and BKBH tracks – and the Laetoli data is definitely bipedal.
“The relative toe depths of the Laetoli prints show that, by 3.6 Ma, fully extended limb bipedal gait had evolved. Thus, our results provide the earliest unequivocal evidence of human-like bipedalism in the fossil record.”
Meanwhile, research in Belgium has been concerned with the more fundamental question as to whether any useful information can be gained from human-like trackways. The researchers were aware of many variables affecting trackway morphologies: the shape of the foot, the mechanical properties of the foot, its kinematics, its kinetics, and the mechanical properties of the substrate. [. . .]
So how does this impact our understanding of the Laetoli footprints? It allows us to conclude, with greater confidence than before, that the trackways were generated by bipeds. At very least, the pendulum has swung so that the burden of proof now rests on those who would question the bipedal interpretation. This creates a problem for those who are trying to construct an evolutionary pathway for Homo sapiens – because bipedalism has to occur early in the story. Australopithecus afarensis is the primary candidate for making the tracks, but evidence is accumulating that bipedalism was not a characteristic of this ape. The US researchers acknowledge an uncertainty problem: “While our results show that Laetoli hominins walked with human-like kinematics, we still cannot be sure of which hominin taxon made the footprints.” They concentrate their thoughts on adaptation: supposedly driven by the reduced energy requirements of a fully bipedal primate.
“Hypotheses for the origins of bipedalism often focus on selection for energy economy in early hominins. Energetic hypotheses are based on the reduced locomotor costs of humans compared to apes walking with BKBH gaits, and therefore, compared to ape-like pre-hominin ancestors.”
This adaptationist approach falls foul of the integrated nature of the changes that are needed for an ape to walk upright. These changes affect the feet bones, the leg length, the pelvis, the way the spine attaches to the pelvis and particularly the skull, the vision system, the balance system, and more. This cannot be a single-factor transformation under selection pressure. The adaptive landscape has a steep-sided mountain separating apes and man – and there is no tunnel through the mountain! The implication is that humans walked at Laetoli, and that the textbook stories of human evolution will have to be re-written.
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