Another Perspective on Hominid Diversity

In his Perspective “Early hominids—diversity or distortion?” (28 March, p. 1994), Tim White criticizes fellow paleoanthropologists for too often identifying as separate species, or even genera, specimens that in reality had belonged to the same species. He presents two bonobo skulls (differing in supraorbital toral thickness, midfacial robusticity, subnasal elongation, and orbital shape) and argues that, if they were hominid specimens, some paleoanthropologists would place them in separate taxa. Perhaps. But variability aside, these bonobos are essentially identical in toral and midfacial morphological detail. One has a more exaggerated subnasal curvature and subovoid orbits, while the other's orbits are more rectangular. Thus, orbital shape notwithstanding, these specimens vary in ways expected of members of the same species, i.e., in degree of expression, not kind, of morphology. Nevertheless, White's distrust of claims of hominid diversity is partly understandable. Often, type specimens lack morphological detail and taxonomic judgements are based on nonbiological criteria, e.g., time and/or geography. Unfortunately, once published, hominid taxa, and usually only the describers' interpretations of them, are perpetuated through media hype and marketing pressures for textbooks to be “up to date.” How might one propose a new taxon? Following the International Code of Zoological Nomenclature, first compare your specimens with the type specimens (holotypes) of recognized taxa to determine if they represent one of them. If not, conservatively suggest that the specimens constitute a morph (a group of specimens united by uniquely shared morphologies) or propose a new species and/or genus. In contrast to this taxonomic procedure, systematic analysis yields hierarchies of hypotheses. For example, first hypothesize morphs on the basis of morphologically sound specimens. With further study, you might combine some morphs and/or subdivide others. Then, theorize patterns of relationship between morphs. This may lead to taxonomic decisions (e.g., morphs as species), but you can also start addressing questions of diversity. Of course, one systematist's morph or species might be another's species or genus, but this conceit is far less important than elucidating patterns of relationship. Thus, suppose I delineate groups A, B, and C and identify them as different species, or even genera, but A and B actually belonged to the same species. If I conclude that A and B are more closely related to each other than either is to C, my conclusion remains viable. For example, if I refer White's bonobos to different taxa, specific morphologies of “being bonobo” should still lead me to conclude that they shared a common ancestor not shared with specimens of other taxa. # RESPONSE {#article-title-2} Schwartz raises the issue of methods by which fossil species are recognized. He provides a surprising view into the methods he uses to recognize early hominid species diversity [which presumably led to his suggestion of three contemporary Homo taxa from a single Dmanisi stratum and locality ([1][1])]. Schwartz's approach is overtly typological. He contends that within-species variation is “in degree of expression, not kind, of morphology.” Biology abandoned the Platonic concept of “essences” long ago. Even in paleoanthropology, the typological approach has had rare overt application since its popularity peaked during the first half of the 1900s. Operationally, Schwartz suggests “first compare your specimens with the type specimens (holotypes) of recognized taxa to determine if they represent one of them.” As Mayr noted in 1969, “Species consist of variable populations, and no single specimen can represent this variability.” ([2][2], p. 369). In a comment directly relevant to the question of Kenyanthropus , Simpson noted 42 years ago that the “[f]inal decision as to conspecific status depends, however, not on nearness to any one specimen, type or other, but on falling within or outside of ranges of variation inferred for the whole taxon.” [[3][3], p. 184), his italics]. In modern paleontology, comparison with the hypodigm (the available fossil record of a taxon) is the criterion for taxonomic recognition. Schwartz's typology represents a different approach. Hominoid fossils are rare, and decades of research have shown how easy it is for trivial, normal individual variation to be confounded with a real taxonomic signal—particularly when typologists use individual fossils as the units of comparison. Compounding the problem today is the fact that limited sets of morphological data are often subjectively employed in cladistic and phenetic analyses. Here, the subjective dichotomous categorization of continuous skeletal variation combines with the lack of appreciation for intragroup variation to produce erroneous phylogenetic and taxonomic conclusions. The two bonobo crania I illustrated, beyond those features Schwartz notes, show variation in zygomatic root position and lower facial projection, “characters” used to support the purportedly new genus and species Kenyanthropus platyops . The apparent lack of appreciation for such degrees of normal within-species variation, compounded by geological distortion of the fossil, led to the erection of the questionable taxon, the subject of my Perspective. Schwartz does not provide any evidence to support the hypothesis that this fossil is a species separate from Australopithecus afarensis . 1. 1.[↵][4]1. J. H. Schwartz , Science 289, 55 (2000). [OpenUrl][5][CrossRef][6][PubMed][7] 2. 2.[↵][8]1. E. Mayr , Principles of Systematic Zoology (McGraw-Hill, New York, 1969). 3. 3.[↵][9]1. G. G. 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