Two Interpretations of Human Evolution: Essentialism and Darwinism
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ANTHROPOLOGICAL REVIEW • Vol. 72, 66–80 (2009) Two interpretations of human evolution: Essentialism and Darwinism Maciej Henneberg Biological Anthropology and Comparative Anatomy Research Unit, University of Adelaide, Adelaide 5005, and Department of Archaeology, Flinders University Australia; E-mail: [email protected] ABSTRACT Despite intensive studies of a large number of fossils discovered during the 20th century there is no consensus as to the interpretation of the process of hominin evolution. Some authors see as many as six genera and some 17 species, while others argue for a single lineage from Plio/Pleistocene until today. Such diversity of interpretations of the same facts indicates lack of a uniform theoretical basis underlying studies of human evolution. Debates can be resolved using basic principles of scientific inquiry – parsimony and falsification of null hypotheses. Hypothesis testing is now possible with respect to the evolution of basic hominin characteristics such as brain size, body size and the size of the dentition that have sample sizes of a few hundred individual data points each. These characters display a continuous change with time. Analyses of variance do not falsify the null hypothesis of the existence of only one species at any time – variances around regression lines on time do not differ from the variance observed in the single species of Homo sapiens – distributions of residuals are normal. Thus, splitting of the hominin lineage into coeval species can only be based on descriptive character- istics that are liable to errors of subjective judgment. KEY WORDS: hominin, hominid, Australopithecus, brain, stature Understanding of the evolutionary pro- how our major traits – such as mental ca- cess that produced modern humans is of pacities and erect bipedal locomotion – crucial importance for the way people emerged are still ongoing [Carruthers and perceive themselves and their lives. It in- Chamberlain 2000, Cela-Conde and Ayala forms ideologies underlying political and 2007]. The number of validly named hu- economic decisions. Thus it is important man species by the end of the 20th century to have this process documented and inter- exceeded 50 [Meikle and Parker 1994, preted in accordance with the best rules of Henneberg 1997], while it has been pro- scientific practice. Despite over 150 years posed that an even larger number should of studies of human evolution, debates on be identified [Tattersall 1987] and actually Received 10 June; accepted 8 December 2009 DOI: 10.2478/v10044-008-0016-2 © 2009 Polish Anthropological Society Two interpretations of human evolution 67 has recently [Brown et al. 2004]. It does the ideal pattern of the world of living not add, though, to the clarity of the pic- things, the “pattern of creation”. Similari- ture of our origins. ties among individuals were used to distil Over the last half-century several au- essences of ideal types of their respective thors proposed that the parsimony and fal- kinds and those types were then organized sification of hypotheses should be applied into nested hierarchies of species, genera, to studies of human evolution postulating families, classes, orders, phyla, etc. [Lin- a “single species hypothesis” [Brace 1967; naeus 1758]. Thus, conceptually, the small- Wolpoff 1968, 1971]. According to Hunt est unit of life was a species, not an indi- [2003], Frank Livingstone proclaimed vidual. An individual was but an example himself to be the proponent of single spe- of an ideal species type. Individual varia- cies hypothesis still at the beginning of the tion was consciously ignored for the sake 21st century. Kevin Hunt himself argues of clarity. Once described, taxa became strongly in support of the single species hy- units of study for biologists. Discussions pothesis using data on taxonomic diversity about the complexity of life in a particular of large-bodied genera of mammals. Re- region or the entire world were based on the cently Holliday [2003] proposed an inter- number of species and higher taxa present mediate interpretation of hominid diversity. in various environments, the interactions He used the notion of syngameon which is still being studied as those between par- a network of separate cohesive taxa that ex- ticular taxa, while descriptions of evolution change genes among themselves. He sup- consist of biblical lists of who is a descend- ported this interpretation with observations ant of whom and what the family tree looks of gene flow among several species of the like [Henry and Wood 2007, Stanford et al. two genera of baboons – Theropithecus 2009]. and Papio – [Jolly 1993, 2001; Jolly et al. As an example, following a suggestion 1997]. Quintyn [2009] summarized current by Robert Eckhardt: discussions concerning hominin diversity And unto Enoch was born Irad: and Irad be- and concluded that a temporary ban on cre- gat Mehujael: and Mehujael begat Methush- ating new hominin taxa should be imposed ael; and Methushael begat Lamech [Genesis until a clearer understanding of hominin 4:18, 1611]. variation can be reached. Homo erctus is descended from Homo habi- lis, which in turn descended from Australo- pithecus garhi … [Larsen 2008]. Taxonomy, essentialism and the concept of species Figure 1 is an example of the classic rep- resentation of a hominin family tree. Such To make sense of the variety of forms representation may be as well reflecting that surround us we must be able to reduce a pattern of creation. No explanations of it to a manageable number of generalizing how and why particular branches arose are categories that can then be used in various given. No uncertainty as to possible mix- thought processes. Thus, the human ten- ing of categories is indicated, nor is there dency to categorize: From the time platonic uncertainty indicated in separating some idealism arose in classical antiquity to the categories from others. early enlightenment, the task of natural- Taxa are perceived as real entities and ists was perceived as that of discovering they can only have two states: existence or 68 M. Henneberg Australopithecus afarensis Australopithecus aethiopicus Australopithecus boisei Australopithecus robustus Australopithecus africanus Homo habilis Homo erectus “Archaic” H. sapiens Neandertals Homo sapiens sapiens Fig. 1. Scheme originally published on page xxix of the Encyclopedia of Human Evolution and Prehisto- ry [Tattersall et al. 1988] and repeated with modification in the second edition [Delsonet al. 2000], author’s own drawing. This is not evolution, this is systematics. It may as well be a “pattern of creation”. extinction; categorically yes or no. Such [Lee 2003] in the face of the overwhelming an approach, though producing voluminous understanding that life is variable and no descriptions, cannot provide explanations of stable categories actually exist. Following the process of evolution since evolutionary Lee [2003], current concepts of species can mechanisms are based on individual varia- be subdivided into (1) similarity concepts tion and transmission of this variation from using phenetic resemblance of organisms as one generation of individuals to the next. the sole criterion, (2) cohesion concepts in It can be argued that before we can analyze which organisms are purported to share fea- anything and search for processual explana- tures allowing them to remain coherent from tions, we must first provide a solid descrip- generation to generation, (3) monophyly tion. Such description, however, must be in concepts stressing membership of a single terms that lend themselves to further pro- nonreticulating lineage, and (4) interbreed- ductive analysis. In biology, the use of ar- ing concepts in which members of a species bitrarily and sharply delimited categories exchange genes among themselves while is not conducive to productive analysis. It being reproductively isolated from others. leads rather to “catastrophic” explanations Concepts belonging to the first three groups in the style of George Cuvier [Brace 1981]. are valid for any taxonomic rank, not be- Taxonomic descriptions may be useful in ing specific to species. Depending on how certain applications, such as ecology, but strictly one defines phenetic similarity, level the fact that they ignore individuals and the of cohesion or monophyletic inclusiveness, flow of time from generation to generation one can define supraspecific taxa (like ge- is always a limiting factor. nus) or infraspecific groupings (like races). Currently there exist some 23 definitions The interbreeding concepts emphasize that of species [Mayden 1997, Quintyn 2009]. actual exchange of genetic material oc- They all attempt to “salvage a Linnean rank” curs only within a species [Mayr 1969, Two interpretations of human evolution 69 1995], a phenomenon which is unique to whether a Neandertal man and a modern this taxonomic rank and making it the only woman would produce fertile offspring. “real” supraindividual biological unit. For a test falsifying a hypothesis of their This is the unit of evolution because it is conspecificity we must rely on secondary in the process of differential reproduction inferences derived from comparisons of of populations isolated genetically from morphology and partly reconstructed DNA other populations that the adaptive change sequences. in gene pool can occur. The phylogenetic species concept stresses the same phenom- Individual variation and enon of reproductive continuity, but from Darwinian view of life a point of view of change through