Science Literature

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."

The Laetoli trackways (Credit: John Reader/Photo Researchers Inc., source here)

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.

"We compared footprints made by subjects walking with a normal, extended limb gait, and with a bent-knee, bent-hip (BKBH) ape-like gait at their preferred speeds with sand water content of 6-8%. These substrate conditions match those of Laetoli, which are described as similar to damp, fine to medium grained sand. We also examined the effects of increased speed and increased substrate moisture (10-12% water) on footprint morphology. We tested the hypothesis that a BKBH gait alters body weight transfer and produces significantly different footprint morphology than an extended limb gait."

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.

"[W]e experimentally generate footprints and quantify a selection of relevant subject-dependent and kinesiological variables that might influence the topology of a footprint, left behind in a granular substrate. Analysis will provide a better insight into the information that can (or cannot) be deduced from footprints."

This is a cautious and critical approach, and justifiably so. Their analyses confirmed the complex nature of footprint generation, and they found no unique variable that "determines" the morphology of a footprint. Mistakes have been made.

"For instance, the "obvious" transfer of weight along the lateral margin of the foot is taken as an argument for well-established bipedalism, but it is also observed in chimpanzees and bonobos, although a medial transfer of pressure along the metatarsal heads is not often evident in apes. When focusing on footprints alone (without detailed knowledge of foot anatomy, as is often the case in paleoanthropology) even the footprints of these apes may be falsely interpreted as made by bipeds with a well-developed longitudinal arch."

Having said this, the research did document cross-correlations between the different kinesiological factors investigated. This is where valid and useful information can be expected to be extracted from fossil trackways. This is the kind of information that was obtained in the US research.

"With these precautions in mind, we argue that different zones of a footprint do contain information on several kinesiological aspects of gait. Future research into footprint formation, soil mechanics and hominin foot morphology and locomotion should further increase insight in the causal relationship between these factors."

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 (for more, go here). 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.

Laetoli Footprints Preserve Earliest Direct Evidence of Human-Like Bipedal Biomechanics
Raichlen DA, Gordon AD, Harcourt-Smith WEH, Foster AD, Haas WR Jr
PLoS ONE, March 2010, 5(3): e9769. doi:10.1371/journal.pone.0009769

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.
[. . .] 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.

Experimentally generated footprints in sand: Analysis and consequences for the interpretation of fossil and forensic footprints
K. D'Aout, L. Meert, B. Van Gheluwe, D. De Clercq, P. Aerts
American Journal of Physical Anthropology, 2010, 141(4), 515-525.

Fossilized footprints contain information about the dynamics of gait, but their interpretation is difficult, as they are the combined result of foot anatomy, gait dynamics, and substrate properties. We explore how footprints are generated in modern humans. Sixteen healthy subjects walked on a solid surface and in a layer of fine-grained sand. In each condition, 3D kinematics of the leg and foot were analyzed for three trials at preferred speed, using an infrared camera system. Additionally, calibrated plantar pressures were recorded. After each trial in sand, the depth of the imprint was measured under specific sites. When walking in sand, subjects showed greater toe clearance during swing and a 7 degrees higher knee yield during stance. Maximal pressure was the most influential factor for footprint depth under the heel. For other foot zones, a combination of factors correlates with imprint depth, with pressure impulse (the pressure-time integral) gaining importance distally, at the metatarsal heads and the hallux. We conclude that footprint topology cannot be related to a single variable, but that different zones of the footprint reflect different aspects of the kinesiology of walking. Therefore, an integrated approach, combining anatomical, kinesiological, and substrate-mechanical insights, is necessary for a correct interpretation.