Download TAXONOMIC CLASSIFICATION AND THE ORIGIN OF MAN*

Taxonomic Classification and the origin of Man
Variability and Evolution, 2000, Vol. 8: 31–38
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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, 31–38, 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 Cracraft’s 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, 14–17
September 1999.
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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 Wolpoff’s 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
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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 Simpson’s 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
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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 Wolpoff’s 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 Wiley’s definition they belong to one species (see Fig. 1). One
should stress, however, that according to scientists of Wolpoff’s 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).
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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
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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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