Abstract
Quantitative, evolutionary models that incorporate within- and between-species variation are critical for interpreting the fossil record of human diversity, and for making taxonomic distinctions. However, small sample sizes, sexual dimorphism, temporal trends, geographic variation, and the limited number of relevant extant models have always made the consideration of variation difficult for paleoanthropologists. Here we provide a brief overview of current early hominin diversity. We then argue that for many species our limited understanding of within species variation hampers our ability to make taxonomic decisions with any level of statistical certainty. Perhaps more significantly, the underlying causes of between-species variation among early hominins are poorly studied. There have been few attempts to correlate aspects of the phenotype with meaningful evidence for niche differentiation, to demonstrate the selective advantage of traits, or to provide other evidence for macroevolutionary divergence. Moreover, current depictions of vast pattern (but not size) diversity are inconsistent with expectations derived from most other extant primate clades that have adaptively radiated. If indeed the early hominin record is highly speciose, the reasons for this remain unclear.
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Notes
The term ‘hominin’ is used to refer to all members of our lineage following the split from a common ancestor shared with the chimpanzee. ‘Early hominin’ is used here to refer to those members of our lineage that are not members of the genus Homo. We recognize that early members of the genus Homo and other early hominins overlapped temporally for in excess of 1 million years, rendering this terminology flawed, if convenient.
Hominoids are apes and humans, and their ancestors.
Effective sample size is an expression of known trait variability. Early hominin species with very limited trait variability at the time full species rank was proposed include: O. tugenensis (N = 1, except N = 2 for maxillary and mandibular third molars and a proximal femur); S. tchadensis (N = 1, except N = 2 for maxillary third molar); Ar. kadabba (N = 1, except N = 2 for a few dental dimensions); Ar. ramidus (N = 1, except N = 2 for humerus and a few teeth); A. anamensis (N = 1, except N = 2–4 for several posterior teeth); A. bahrelghazali (N = 1); K. platyops (N = 1, except N = 2 for some temporal bone features); A. garhi (N = 1); P. aethiopicus (N = 1). See discussion in Smith (2005).
There is reason for concern that these estimates are too high, as this level of diversity is unexpected for animals of a similar size—for a discussion of this issue in the genus Homo, see Conroy (2002).
Branching events do not, of course, preclude anagenetic change, and in fact there is good evidence for such an ancestor-descendent relationship from A. anamensis to A. afarensis (Kimbel et al., 2006). However, most interpretations of this diversity suggest that cladogenesis is also present (Begun, 2004).
In framing such research, we think it needs to be recognized that the literature on human evolution has tended to accept the possible implications of a very limited set of general processes that describe patterns of speciation (such as competitive exclusion) while essentially ignoring other important generalizations about species diversity (such as niche construction and self-organized similarity) (Laland & Sterelny, 2006; Scheffer & van Nes, 2006).
There is considerable debate surrounding baboon taxonomy, and whether the myriad forms are distinct at the subspecific or specific level. For a view representing the latter, see Grubb et al. (2003).
Given this, the cause of their subsequent demise is no longer clear (Wood & Strait, 2004).
Of course, there are other ways to detect niche differentiation as well. For example, because of the close relationship between absolute body size and diet across all primates (Fleagle, 1999), differences in body size among early hominins may themselves provide a signal of niche differentiation. In fact, size evolution in primates is a likely consequence of adaptation to fill empty dietary niches (Marroig & Cheverud, 2001, 2005). Unfortunately, reliable estimates of body weight are unavailable for the earliest hominin taxa, leaving researchers to compare other aspects of morphology—such as tooth size—as a surrogate for overall size differences. From about 4.2 million years, we have somewhat better size estimates (Jungers, 1988; McHenry, 1992), which indicate that these australopiths are generally comparable in body size. Similarly, correlations between different locomotor adaptations and environments would indicate that these hominins occupied a diverse range of habitats. However, substantial postcranial material is not available for a number of early hominin genera, making comparative studies difficult.
There are, of course, exceptions to this. For example, the primary contributors to facial integration in apes and humans are the zygomatic and oral regions, while studies of both Old and New World monkeys indicate integration in the oral region alone. Nevertheless, the overall pattern of covariation is similar.
Here too, cognitive biases affect our interpretations, as the placing of objects into categories (differentiated by degree, or kind) is not only a method of taxonomy and phylogenetic modeling, but a fundamental process by which all humans organize the world. As summarized by Murphy (2003: 514):
…people are far too willing to latch onto a possible category for objects and then to rely on it even when it is uncertain…there is a strong drive from early childhood to categorize entities and to assume that such categories reflect deep and important regularities…not only do we rely on categories when they are uncertain, simply asking about a category results in our using categorical information.
Humans are not just apes at a different size, so some reorganization of morphological patterns has occurred at some point. We acknowledge that there are exceptions to the primate rules, however, multiple exceptions within a highly-branched lineage are unlikely.
Another possible explanation for mosaicism that has received little attention is gene flow, an important shaper of diversity when one is dealing with small populations. Although a number of recent studies have suggested that hybridization is more common than previously appreciated in hominin evolution, these studies have overwhelmingly focused on the genus Homo (Brown et al., 2004; Reed, Smith, Hammond, Rogers, & Clayton, 2004; Stefansson et al., 2005; Swisher et al., 1996; Trinkaus, 2005; Zilhao & Trinkaus, 2002). Only one study has been concerned with earlier hominin evolution, and this focused on hybridization between chimpanzee ancestors and early hominins (Patterson, Richter, Gnerre, Lander, & Reich, 2006), rather than between early hominins. What does a tree look like if there is reticulation? This is not clear and needs to be tested, though assumedly hominin populations would diverge more slowly, and hybrid populations would display a wider range of phenotypic variation than you would see in the parental populations (Ackermann, Rogers, & Cheverud, 2006).
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Acknowledgements
We thank Benedikt Hallgrímsson for inviting us to write this review. Jim Cheverud, Charlie Lockwood, Matt Sponheimer, and three anonymous reviewers provided extremely helpful comments on previous versions of this manuscript.
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Ackermann, R.R., Smith, R.J. The Macroevolution of our Ancient Lineage: What We Know (or Think We Know) about Early Hominin Diversity. Evol. Biol. 34, 72–85 (2007). https://doi.org/10.1007/s11692-007-9002-7
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DOI: https://doi.org/10.1007/s11692-007-9002-7