Sarita A. Morse

Human-like external function of the foot, and fully upright gait, confirmed in the 3.66 million year old Laetoli hominin footprints by topographic statistics, experimental footprint-formation and computer simulation

It is commonly held that the major functional features of the human foot (e.g. a functional longitudinal medial arch, lateral to medial force transfer and hallucal (big-toe) push-off) appear only in the last 2 Myr, but functional interpretations of footbones and footprints of early human ancestors (hominins) prior to 2 million years ago (Mya) remain contradictory. Pixel-wise topographical statistical analysis of Laetoli footprint morphology, compared with results from experimental studies of footprint formation; foot-pressure measurements in bipedalism of humans and non-human great apes; and computer simulation techniques, indicate that most of these functional features were already present, albeit less strongly expressed than in ourselves, in the maker of the Laetoli G-1 footprint trail, 3.66 Mya. This finding provides strong support to those previous studies which have interpreted the G-1 prints as generally modern in aspect.

Does footprint depth correlate with foot motion and pressure

Footprints are the most direct source of evidence about locomotor biomechanics in extinct vertebrates. One of the principal suppositions underpinning biomechanical inferences is that footprint geometry correlates with dynamic foot pressure, which, in turn, is linked with overall limb motion of the trackmaker. In this study, we perform the first quantitative test of this long-standing assumption, using topological statistical analysis of plantar pressures and experimental and computer-simulated footprints. In computer-simulated footprints, the relative distribution of depth differed from the distribution of both peak and pressure impulse in all simulations. Analysis of footprint samples with common loading inputs and similar depths reveals that only shallow footprints lack significant topological differences between depth and pressure distributions. Topological comparison of plantar pressures and experimental beach footprints demonstrates that geometry is highly dependent on overall print depth; deeper footprints are characterized by greater relative forefoot, and particularly toe, depth than shallow footprints. The highlighted difference between ‘shallow’ and ‘deep’ footprints clearly emphasizes the need to understand variation in foot mechanics across different degrees of substrate compliance. Overall, our results indicate that extreme caution is required when applying the ‘depth equals pressure’ paradigm to hominin footprints, and by extension, those of other extant and extinct tetrapods.

Human Footprints: Fossilised Locomotion?

In this fi rst chapter we provide a broad overview of human trace fossils (ichnology) and outline the contents of and rationale for this book. The potential for human tracks to tell us about how our ancestors may have walked is discussed as is the contribution that human tracks can make in other areas of archaeology and forensic science. Key defi nitions are introduced, as is a simple model of human track formation.

Laetoli's lost tracks: 3D generated mean shape and missing footprints.

The Laetoli site (Tanzania) contains the oldest known hominin footprints, and their interpretation remains open to debate, despite over 35 years of research. The two hominin trackways present are parallel to one another, one of which is a composite formed by at least two individuals walking in single file. Most researchers have focused on the single, clearly discernible G1 trackway while the G2/3 trackway has been largely dismissed due to its composite nature. Here we report the use of a new technique that allows us to decouple the G2 and G3 tracks for the first time. In so doing we are able to quantify the mean footprint topology of the G3 trackway and render it useable for subsequent data analyses. By restoring the effectively ‘lost’ G3 track, we have doubled the available data on some of the rarest traces directly associated with our Pliocene ancestors.

Preserving the Impossible: Conservation of Soft-Sediment Hominin Footprint Sites and Strategies for Three-Dimensional Digital Data Capture

Human footprints provide some of the most publically emotive and tangible evidence of our ancestors. To the scientific community they provide evidence of stature, presence, behaviour and in the case of early hominins potential evidence with respect to the evolution of gait. While rare in the geological record the number of footprint sites has increased in recent years along with the analytical tools available for their study. Many of these sites are at risk from rapid erosion, including the Ileret footprints in northern Kenya which are second only in age to those at Laetoli (Tanzania). Unlithified, soft-sediment footprint sites such these pose a significant geoconservation challenge. In the first part of this paper conservation and preservation options are explored leading to the conclusion that to ‘record and digitally rescue’ provides the only viable approach. Key to such strategies is the increasing availability of three-dimensional data capture either via optical laser scanning and/or digital photogrammetry. Within the discipline there is a developing schism between those that favour one approach over the other and a requirement from geoconservationists and the scientific community for some form of objective appraisal of these alternatives is necessary. Consequently in the second part of this paper we evaluate these alternative approaches and the role they can play in a ‘record and digitally rescue’ conservation strategy. Using modern footprint data, digital models created via optical laser scanning are compared to those generated by state-of-the-art photogrammetry. Both methods give comparable although subtly different results. This data is evaluated alongside a review of field deployment issues to provide guidance to the community with respect to the factors which need to be considered in digital conservation of human/hominin footprints.

Inferences from Human Tracks

What can, and perhaps more importantly cannot, be inferred from a series of human tracks? In this chapter we explore this question by first looking at the relationships that exist between various foot dimensions and such things as stature and body mass. We explore the population specific nature of these empirical relationships and demonstrate their limitations with respect to the interpretation of tracks in the geological record. In the latter part of the chapter we explore what can deduced about the speed of a track-maker and the way in which variations in that speed may be reflected in the topology of the tracks themselves.

Methods of Data Capture and Analysis

The sophistication and quality of field data obtained from human tracksites has increased dramatically during the last decade from the largely descriptive papers of Holocene tracksites common before the late 1990s to the more sophisticated data-rich papers of recent years. There are exceptions of course to this generalisation largely around the tracks at Laetoli which drove early innovation in methods. In this chapter we review the methods and approaches that can be adopted at human tracksites and equip the interested researcher with the knowledge necessary to execute such investigations themselves given suitable excavation permits and permissions. We recognise four broad stages to the process each of which is considered in turn: (1) geo-prospection and excavation; (2) recognition of human tracks and their dating; (3) methods of digital data capture; and (4) methods of analysis.

Geoconservation of Human Tracks

In the previous chapter we have seen how there is a wide diversity of human tracksites each with a different depositional history and mechanism of track preservation. The challenge for the geoarchaeologist is not only to document such sites but also to advise with respect to their long-term conservation. At many human tracksites this is challenging due to the nature of the soft, erodible substrate and the geomorphological environments in which the tracks are now exposed. In this chapter we explore some of these challenges and suggest some solutions.

World Review of Human Track Sites

Human tracks have now been recorded at a number of sites across the globe. Lockley et al. (Ichnos 15:106–125, 2008) provides a definitive review of many of these sites and our aim here is to focus on a few important examples which are either in the authors’ judgement particularly significant or feature within this book. Sites can be grouped on many different criteria such as by: (1) geographical regions; (2) geological facies in which they are preserved; (3) their age and therefore potential species of track-maker; or (4) by their archaeological or palaeoanthropological significance. While there is a natural tendency to focus on the unusual, biggest, or oldest, in reality footprint sites tend to separate into those which pre-date Homo sapiens and those that don’t. Those that do are limited in number but have the potential to offer information about the evolution of gait between hominin species and as such they accord a level of significance far greater than other footprint sites. Such sites are few in number however and while Holocene sites may not have the glamour of older localities, they have the potential to offer important laboratories in which to explore the interaction of a track-maker’s gait with such things as substrate. For ease we have chosen to divide this chapter into those examples that potentially pre-date Homo sapiens (Pliocene to Early/Middle Pleistocene) and those that don’t (Late Pleistocene to Holocene).

The Role of Substrate in Track Formation and Topology

Before any inferences about a human track-maker can be made from the tracks they leave we first need to understand how substrate properties (consistency) mediate and record the interaction between the foot and the ground. A substrate has a profound influence in a range of different ways: it controls the way in which a track-maker walks; it moderates and cushions the pressure interaction between the sole and the substrate and therefore the depth record that results; it determines how the strain is accommodated and the type of deformation that occurs; and it determines the immediate survival and preservation of a track as well as its longer term taphonomy. We argue that geology matters in the study of human tracks and in this chapter we explore this first by looking at the role of substrate as a variable in track formation and secondly by looking at the taphonomic process which bear on track formation and preservation in the geological record.