2 Recent Evolution of the Human Foot

2 Recent Evolution of the Human Foot This chapter deals with the recent evolution of the human foot. Its scope is more limited than Chapter 1. Instead of spanning approximately 400 million years it cov- ers only 8 million years, and instead of dealing with the vertebrate branch of the Tree of Life it considers just one of the many small terminal twigs, or clades, that make up the primate part of the Tree of Life. We focus on a series of extinct mam- mals called hominins that through time, and in fits and starts, accumulate morphology more similar to modern humans and less like that of the other great apes. Researchers have differing views about how many branches the twig has, and they also disagree about how reliably we are able to reconstruct the branching pattern within the twig. We try to reflect these different views in our discussion. This chapter has three sections. The first provides an overview of the hominin fossil record. We use a relatively complex taxonomic hypothesis consistent with human evolution being interpreted as a series of closely related branches called adaptive radiations. A second simpler taxonomic hypothesis that interprets human evolution as a more ‘ladderlike’ progression is presented in summary form. The second section of the chapter reviews hypotheses about the evolution of hominin bipedal locomotion. This focuses on scenarios put forward to explain the origin of bipedalism, and on debates about whether bipedalism evolved once, or several times, within the hominin clade. The third section reviews each hominin taxon, focusing on what is known about the foot. Most of this information is gleaned from fossil foot bones that have, with varying degrees of certainty, been assigned to hominin taxa. However, other information has come from so-called ‘trace fossils,’ which are the impres- sions made by the feet of early hominins. Hominin Evolution: An Overview Terminology For reasons given below we treat modern humans as one of the ‘great apes’, the other three being the two African great apes, chimpanzee (Pan) and the gorilla (Gorilla), and the only Asian great ape, the orangutan (Pongo). Palaeoanthropologists have differed, and still do differ in the way they classify the higher primates. We have tried to avoid technical terms in this section, but some are necessary in order to understand the implications of the different classifications. Chapter co-written by Nicole L. Griffin. 28 The Human Foot Linnaean taxonomic categories immediately above the level of the genus, that is, the family and the tribe, have vernacular equivalents that end in ‘id’ and ‘in’, respectively. In the past Homo sapiens has been considered to be distinct enough to be placed in its own family, the Hominidae, with the other great apes grouped together in a separate family, the Pongidae. Thus, modern humans and their close fossil relatives were referred to as ‘hominids’, and the other great apes and their close fossil relatives were referred to as ‘pongids’ (Table 2.1A). However, this scheme is incon- sistent with morphological and genetic evidence suggesting modern humans are more closely related to chimpanzees than they are to either gorillas or orangutans. In response to the overwhelming evidence that modern humans and chimpanzees are more closely related to each other than either is to the gorilla or the orangutan, some researchers advocate combining modern humans and chimps in the same genus (e.g., [1]), which according to the rules of zoological nomenclature must be Homo. We adopt a less radical solution. We lump all the great apes into a single family, the Hominidae (Table 2.1B). Within the Hominidae we recognise three subfamilies, the Ponginae for the orangutans, the Gorillinae for the gorillas, and the Homininae for modern humans and chimpanzees. The latter subfamily is broken down into two tribes, Panini (or ‘panins’) for chimpanzees, and Hominini (or ‘hominins’) for modern humans. The latter is further broken down into two subtribes: one, Australopithecina, for the extinct primitive hominin genera, and the other, Hominina for the genus Homo, which includes the only living hominin taxon, Homo sapiens. Modern humans and all the fossil taxa judged to be more closely related to modern humans than to chimpanzees are called ‘hominins’; the chimpanzee equivalent is ‘panin’. Thus, modern humans are ‘hominids’ (family), ‘hominines’ (subfamily), and then ‘hominins’ (tribe). We use ‘australopith’ when we refer to taxa belonging to the subtribe Australopithecina. Defining Hominins Molecular biology has revolutionised our knowledge of the relationships within the great ape clade of the Tree of Life. Relationships between organisms can now be pursued at the level of the genome instead of having to rely on morphology (traditional hard and/or soft-tissue anatomy or the morphology of proteins) for information about relatedness. Comparisons of the DNA of organisms have been made using two methods. In DNA hybridisation all the DNA is compared, but at a relatively crude level. In DNA sequencing the base sequences of comparable sections of DNA are determined and then compared. The results of both hybridisation (e.g., [2]) and sequencing studies of both nuclear and mtDNA (e.g., [1,3,4–6]) are virtually unanimous in suggesting that modern humans and the two African apes are more closely related to each other than any of them is to the orangutan. They also suggest that modern humans and modern chimpanzees (belonging to the genus Pan) are more closely related to each other than either is to the gorilla. Recent Evolution of the Human Foot 29 Table 1 ‘Old’ and ‘New’ Taxonomies (A) A traditional ‘premolecular’ taxonomy of higher primates. Extinct taxa are in bold. Superfamily Hominoidea (hominoids) Family Hylobatidae (hylobatids) Genus Hylobates Family Pongidae (pongids) Genus Pongo Genus Gorilla Genus Pan Family Hominidae (hominids) Subfamily Australopithecinae (australopithecines) Genus Ardipithecus Genus Australopithecus Genus Kenyanthropus Genus Orrorin Genus Paranthropus Genus Sahelanthropus Subfamily Homininae (hominines) Genus Homo (B) A taxonomy of higher primates that recognizes the close genetic links between Pan and Homo. Extinct taxa are in bold type. Superfamily Hominoidea (hominoids) Family Hylobatidae (hylobatids) Genus Hylobates Family Hominidae (hominids) Subfamily Ponginae Genus Pongo (pongines) Subfamily Gorillinae Genus Gorilla (gorillines) Subfamily Homininae (hominines) Tribe Panini Genus Pan (panins) Tribe Hominini (hominins) Subtribe Australopithecina (australopiths) Genus Ardipithecus Genus Australopithecus Genus Kenyanthropus Genus Orrorin Genus Paranthropus Genus Sahelanthropus Subtribe Hominina (hominans) Genus Homo Most attempts to predict the date of the Pan/Homo dichotomy suggest that the hypothetical ancestor of modern humans and chimpanzees lived between about 5 and 8 Mya (e.g., [7]), but some researchers (e.g., [8]) favour a substantially earlier date (10–14 Mya) for the divergence of the Pan and Homo lineages. 30 The Human Foot Thus, if we accept that the hominin twig of the Tree of Life may extend back in time to ca. 8 Mya, and that the earliest unambiguous hominin is probably Australopithecus anamensis (see below), then between 8 and 4 Mya we would expect to find primitive hominin and primitive panin taxa, and close to 8 Mya we should expect to see evidence of the common ancestor of panins and hominins. Not all of these taxa are direct ancestors of modern humans and chimpanzees; many will belong to extinct panin and hominin subclades. Some of these taxa could also belong to extinct clades that have yet to be recognised in the fossil record. Organising the Hominin Fossil Record The classification of the hominin fossil evidence is controversial. Some researchers favour recognising relatively few taxa, whereas others think that more species and genera are needed to accommodate the observed morphological diversity. We use a relatively speciose (or ‘splitting’) taxonomy, but we also provide an example of a less speciose (or ‘lumping’) taxonomy so that readers can appreciate how the evidence for human evolution would look if it were interpreted in a different way. The taxa included in the two taxonomies are listed in Table 2.2. Some of the taxon names are used in different senses in the speciose and less-speciose taxonomies. When we refer in the text to the hypodigm (the fossil evidence referred to that taxon) of one of these taxa in the speciose taxonomy we use the taxon name followed by sensu stricto (e.g., Au. afarensis sensu stricto or its abbreviation, Au. afarensis s. s.). This signifies that we are using the taxon name in its ‘strict’ sense. When we refer to the same taxon in the more inclusive taxonomy (i.e., the hypodigm is larger) the Linnaean bino- mial is followed by sensu lato (e.g., Au. afarensis sensu lato or Au. afarensis s. l.). This indicates we are using the taxon name in a ‘looser’ sense. To save repeti- tion, and because in most cases we are referring to the taxa in their strict sense, readers should assume we mean sensu stricto unless we specifically refer to sensu lato, or use s. l. The temporal spans of the taxa in the speciose taxonomy are illustrated in Table 2.2 and Figure 2.1. The ages of the first and last appearances of any taxon in the fossil record (called the ‘first appearance datum’ or FAD, and ‘last appearance datum’ or LAD, respectively) almost certainly underestimate the temporal range of the taxa. Nonetheless, FADs and LADs provide an approximate temporal sequence for the hominin taxa. The heights of the columns of those taxa with good, well-dated, fossil records (e.g., Australopithecus afarensis and Paranthropus boisei) are a reasonable estimate of the temporal range of those taxa, but the heights of the columns for the taxa marked with an asterisk (e.g., Sahelanthropus tchadensis and Australopithecus bahrelghazali) reflect uncertainties about the age Table 2 Splitting and lumping hominin taxonomies and skeletal represention1 of splitting hominin taxa Informal A.

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