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Taxonomic Classification and the origin of Man Variability and Evolution, 2000, Vol. 8: 3138 31 WIOLETTA NOWACZEWSKA Department of Anthropology, University of Wroc³aw, Poland E-mail: [email protected] TAXONOMIC CLASSIFICATION AND THE ORIGIN OF MAN* NOWACZEWSKA W. 2000. Taxonomic Classification and the Origin of Man. Variability and Evolution, Vol. 8, 3138, Figs. 2, Adam Mickiewicz University, Faculty of Biology, Institute of Anthropology, Poznañ. Abstract: The chaos noticeable presently in the taxonomic classification of the hominids significantly increases the amount of different interpretations of phylogeny of our species. That chaos is considered to be caused by difficulties in research of osseous material and the lack of unanimity prevalent among researchers, as to what the term species actually means. General acceptance of basic rules related with taxonomic assessment of the accessible human remains and applying the Cracrafts Phylogenic Species Concept (while taking into consideration the assumption suggesting that a diagnosis of a species should be based on perceiving it as a specific combination of features both primitive and advanced) have become commonly recognized as factors, which could minimize significantly the confusion that arose in this field. Key words: chaos in taxonomic classification, definitions of a species, reconstruction of phylogeny Chaos to be found in the modern paleoanthropology in respect to taxonomic classification of osseous remains of fossil hominids generates the need for discussing that alarming fact. What is the reason for the process we observe, which in its extreme forms either excessively increases the number of new species or reduces it * Paper presented on Congress of Polish Anthropological Association in Rynia, 1417 September 1999. 32 W. Nowaczewska to a very small amount? Yet not more than thirty years ago, the majority of anthropologists accepted the opinion, that Homo erectus existed in Africa, Europe and Asia. In 1984 Andrew proposed the list of autapomorphies for the Asiatic form of Homo erectus, emphasizing that he treats the African forms as a separate species with plesiomorphic features. According to him, while evolving from anteceding Homo ergaster species, the Asiatic Homo erectus went through a process of speciation (after Turner and Chamberlain 1989). That opinion is not shared by Wolpoff (et al. 1994), who claims that features indicated by Andrews as autapomorphies occur also in other late-Pleistocene hominids. Basing on an estimation of plesiomorphic and apomorfic features of African and Asiatic Homo erectus forms, Turner and Chamberlain (1989) assert that these populations belong to one species. Rightmire (1986) believes the taxonomic values of features recognized by Andrews as apomorphies to be highly uncertain, as not all of them appear in individuals included in the Asiatic group. Kennedy (1991), Braüer and Mbua (1992) analyzed the occurrence of these features at different hominid groups and determined many of them to be present also at Homo habilis, and some of them even at archaic Homo sapiens and Australopithecus africanus, what convinced them that these features should not to be regarded as autapomorphies. There is no unanimity among paleoanthropologists as to the classification of human fossil forms preceding the classical Neanderthal in Europe. It still has not been established whether individuals from Mauer, Vértesszöllös, Arago, Petralona, Bilzingsleben represent late, advanced form of Homo erectus, or rather so called archaic Homo sapiens (Jones et al. 1992). Similar situation applies to the classification of hominids represented by earlier fossil record. The final settlement of hypodigm of Homo habilis and Homo rudolfensis species (Wood and Collard 1999) is therefore still debated over. Comparative analysis of the features variability range concerning the skulls of individuals classified to Homo habilis (sensu lato) (Wood, 1992) showed, that there are considerable differences between part of material originating in Koobi Fora and the rest of the sample. Even among paleoanthropologists who agree upon the existence of Homo rudolfensis there is still an argument as to whether the entire population or only some of the individuals from Koobi Fora should be classified to that species. Recently, there even appeared a problem of classifying each of hominids species into the appropriate genus. Wood and Collard (1999) suggest, that Homo habilis and Homo rudolfensis species should not belong to the Homo genus, but to the Australopithecus. Another sign of increasing looseness in interpretation of taxonomic classification is the hypothesis of evolutionary Homo sapiens species put forward by the Wolpoffs team, according to which all Pleistocene hominids apart from the representatives of Australopithecus genus and the Homo habilis species (sensu lato) are the representatives of the Homo sapiens species. While trying to establish the reasons of present chaos in taxonomic classification of hominids, we must remember the basic difficulties facing every paleoanthropologist examining fossil material and the rules he should comply with. And it is not to forget that, a paleoanthropologist can only use the morphological resemblance Taxonomic Classification and the origin of Man 33 criterion. Available collections of fossil remains usually cannot be considered as samples of one population and therefore evolutional affinities can only be established between populations and not the particular individuals. Comparative analysis of specific features of individuals should be preceded with verification of their sex and age, as these two factors often influence the degree of development of the compared feature. It is important to compare mature individuals, whose most morphological features are already developed. This condition may not always be possible to fulfill, because of limited number of specimen. In such situations the comparison should be devoid of features, development of which is related to age. However, the paleoanthropologists do not always comply with these basic rules. For example Bermúdez de Castro et al. (1997) proposed creation of a new species of Homo antecessor basing on features of a partial face of one juvenile individual. It is also essential for a paleoanthropologist to determine the ranges of variability of the examined features in the analyzed group of individuals. By comparing these ranges with the variability observed at presently living species closely related to primates, one obtains informations as to which differences may be considered variations within one species and which are to be thought of as dissimilarities between two species. Incorrect estimation of variability of features in a given group may cause individuals of the same species to be classified into two different ones (or on the contrary to classify individuals of two different species into one). The problems presented above suggest, that a researcher classifying fossil specimen has to overcome numerous difficulties, what often results in different interpretations of the same findings. These differences could be minimized by identifying a universally accepted definition of a species. Every concept of a species influences the shape of the genealogical tree of man in certain way. Thus, one of the basic reasons for the existence of many various models of origin of our species is the lack of unanimity in settling of what actually the species is. We can differentiate the currently prevailing ideas of species into two groups: one not being connected with the dimension of time and the other taking this factor into account. The first group includes such concepts of species as biological, ecological and phenetic, but since it does not take into account the dimension of time, it cannot be employed by paleoanthropologists. The other group contains concepts useful for paleoanthropologists, two of which are worth mentioning in detail; and these are: Evolutionary Species Concept modified by Wiley (1978) and Phylogenetic Species Concept by Cracraft (1983). Wiley uses modified Simpsons evolutional definition of a species that describes species as (...) a lineage of ancestral-descendant populations which maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fate (Wiley 1978). Wiley emphasizes that species is a line, which is not to be divided into sections (called chronospecies), and which in order to preserve the distinctions of its evolutional tendency has to be reproductively isolated. The process of distinguishing chronospecies 34 W. Nowaczewska is too subjective according to him (Wiley 1978). The problem here concerns the difficulties of defining, how distant in time the groups of individuals should be from one other in order to regard them as reproductively isolated. Designating such a boundary in the form of certain point on the examined line is practically devoid of sense, because if populations create phylogenetic lineage of dependencies ancestordescendant, then the lineage can intersect with one another at any point. Therefore, albeit following one another in a lineage, (as parents, children, grand-children, etc.) the individuals create one species, it does not preclude that if the time distance between two groups of individuals belonging to that same lineage (the same species) would be large enough, they would be unable to cross with one another, even if they were brought together, because of substantial morphological changes that had occurred during that time distance. According to Wiley (1978), a species stretches between two processes of speciation (cladogenesis). In other words, the time frame of a species can only be established based on the branching of evolutional line; the ancestor species can still exist after the speciation if it preserves its own historic distinction and its development tendencies. Evolutionary Species Concept of Wiley has been used by a team of scientists under Wolpoffs leadership to proof the polycentric origin of man and to construct a hypothesis of the evolutionary Homo sapiens species. That hypothesis assumes, that from the moment Homo erectus (sensu lato) appeared, until the beginning of our species (through the archaic Homo sapiens form including the Neanderthal) there occurred no speciation at all. According to that assumption, such species as: Homo erectus (classical), Homo heidelbergensis, Homo neanderthalensis and Homo sapiens belong to one species of Homo sapiens. While interpreting hominid osseous fossil, Wolpoff et al. (1994) assumed that a polytypic species of Homo erectus evolved gradually through different forms (characterized by a mosaic of features) into our species simultaneously in Africa, Europe, Asia and Indonesia. Thus, he acknowledged, that the representatives of Homo erectus and Homo sapiens constitute an evolutionary lineage of antecedent and descendant populations, so according to Wileys definition they belong to one species (see Fig. 1). One should stress, however, that according to scientists of Wolpoffs team, in the course of evolving from Homo habilis into Homo erectus there occurred a cladogenesis, that explains not only the morphological differences between these two groups of hominids, but also (and even more importantly) the fact that Homo habilis survived in an unchanged form, while coexisting with Homo erectus for the period of several hundreds of thousands of years. Accepting these assumptions as the basic criterion for the occurrence of cladogenesis allows us to claim, that in the course of origin of Homo sapiens, there occurred the process of speciation, because there exist evidences of hypothetical coexistence of Indonesian form of Homo erectus with representatives of our species on Java (Swisher et al. 1996), and of coexistence of archaic Homo sapiens with Homo erectus in Eastern Asia (Tiemei et al. 1994). 35 Taxonomic Classification and the origin of Man Homo sapiens Archaic Homo sapiens (Homo neanderthalensis Homo heidelbergensis) Homo erectus Fig. 1. Hominids Phylogenetic relationships The problem that appears while analyzing these facts rises the question as to how long the coexistence of related to one another (and morphologically different) species should last in order for us to determine, that speciation took place. The Phylogenetic Species Concept by Cracraft (1983) that is based upon the concept of Hennig defines species as an irreducible (basal) cluster of organisms, diagnosably distinct from other such clusters, and within which there is and parental pattern of ancestry and descent. According to this definition, a species is the smallest monophyletic group, which may be identified with a single line stretching between two processes of speciation (cladogenesis). Because the main goal of cladistic analysis is to create monophyletic taxa basing on their advanced evolutionary features, it was suggested that species should be recognized on the basis of their autapomorphies. This way of determining the diagnostic features of species was, according to Cracraft (1983), the main reason for wide spreading of Phylogenetic Species Concept in paleoanthropology. Cracraft stresses that through this concept, he wishes to define a species, as both an evolutional and a reproductive unit. Consequently, there emerges a question, whether defining a species through its autapomorphies really reflects the two above mentioned functions. According to the cladists, a species is understood as a mosaic of specialized and primitive features inherited from ancestors with the exception of autapomorphies, which may be acquired (and may not) during its existence. A parental species cannot possess features characteristic only of itself, because of being an ancestor for other species to which these characteristics are transferred. This fact is also depicted by the rule saying, that the nearest common ancestor of any two taxa has to possess all apomorphies of these species. Analyzing a sample of fossil material (while not knowing, that it contains specimen of ancestor species) only on the basis of autapomorphic features may lead to not detecting all the species represented in the sample. That leads to the conclusion, that by using this type of 36 W. Nowaczewska Homo sapiens Homo neanderthalensis Homo heidelbergensis Homo erectus Homo ergaster Homo rudolfensis X Homo species Fig. 2. Phylogenetic Tree of Human Evolution (The Part Only) (after Wood 1994) features in taxonomical analysis we understate the number of species represented in a sample (Kimbel 1991). Some discrepancy can be perceived here, as we may detect only one phylogenetic species where in reality exist two basic evolutionary units instead of one. Among anthropologists there are however advocates of this way of recognizing species. One of them is Tattersall (1986, 1992), according to whom even one autapomorphia suffices for a new species to be identified. He explains his theorem with the results of research works over nearly related species of primates (mostly the lemurs), according to which in sisterly species only a single feature can be used for distinguishing them, as the ranges of variability for most of analyzed features correspond completely or in a large degree. However, Tattersall does not give any information about this of type research led on species of chimpanzees or of gorillas. According to him, the paleoanthropologists who focus their attention on intraspecious variabilities make a mistake that results in an understated number of species. Meanwhile, Groves (1989) turns our attention to the fact, that from the moment cladistic analysis found its use in paleoanthropology, the number of new taxa has suddenly escalated (see Fig. 2). The number of taxa is overstated when the criterion of autapomorphia is used in the analysis of polytypic species. Turner turns our attention to this very problem (1985, Turner & Chamberlain, 1989) and claims, that species identifications in the fossil record based on the distribution of apomorfic states of skeletal characters are likely to be misleading, since such features may very well have evolved within species rather than concomitant with speciation. What we observe here is certain discrepancy between a cladistic species identified through autapomorphies and a species understood as a reproductive community. According to the criterion of reproductive isolation we may recognize only one species, where the cladists would distinguish several of them. The problem results Taxonomic Classification and the origin of Man 37 from using methods of cladistic analysis for grouping organisms into species, what should not be the case, as it leads to either overstating or understating the number of species. In my opinion, the only reasonable proposal, but surely not fully solving the problem, was mentioned by Cracraft (1983) saying that species should be recognized basing on the specific combination of their advanced and primitive features. The problem of identification of species in osseous material is a very difficult task, as the analysis of genetic and morphological variability of many animal species shows, that there is no correlation among these features and the development of reproductive isolation (Ridley 1993). It has been proved, that numerous species differing only with several genes very seldom create hybrids, while those that are genetically and morphologically more different do it considerably more often. One can therefore suppose, that the most essential in forming of a new species are changes in genes coding sexual features of an organism and its sexual preferences. One should remember, that basing on a very large number of skull features of apes and Homo sapiens, it had been determined, that chimpanzee is related in a greater degree to gorilla than to a man. Yet later analysis of genetic material showed, that the DNA structure of genus Pan is more similar to that of Homo sapiens than to that of gorilla. Analogous situation can be applied to the fossil material, which after all is analyzed only on the basis of morphological features. Therefore, we are not in a position to answer the question as to whether (even in case of theoretical recognition of genetic material of the examined individuals) similar discrepancies (between morphological and DNA analysis) as in the above mentioned case of chimpanzee would not be observed. Hence, the question we need to address is, whether it is worth to undertake the attempt of reconstructing the phylogenesis of our species. And that question, must be answered by each paleoanthropologist to himself. 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