Why so many species of Homo?

Have you ever been confused by the number of species that many paleoanthropologists recognize within the genus Homo? This is one of the shorter lists: H. habilis s.s.H. rudolfensisH. ergasterH. erectus s.s.H. florensisH. antecessorH. heidelbergensisH. rhodesiensis, H. neanderthalensisH. sapiens s.s. According to studies of different mammalian taxa (see Holliday 2006), at least 1,4 million years is needed for two close populations to become reproductively isolated. Then, why are there so many species of hominines within some 2,5 million years (or less, if you  as me, do not see Homo habilis as a part of Homo)? Does genus Homo represent special case of some hyperspeciation rate within class of mammals? I don’t think so.

First, it seems unlikely that humans, as top ecological generalists, were more inclined to speciation processes than other mammalian groups. On the contrary, one would expect smaller number of species in humans, in relation to other groups of mammals.

Second, paleoanthropological taxonomy and phylogeny is based on cladistic method. According to cladistics, only unique derived characters (autapomorphies) can be used for species distinction. That means that ALL of the representatives of species, and ONLY THEM, MUST posses those characters. The sad truth is that even for the best known and defined fossil groups of hominins (such as Neanderthals, for example) we do not have clearly defined autapomorphies. It seems that autapomorphy concept is useless in paleoanthropology. So we end up with pheneticaly (and descriptively) defined paleospecies, that are viewed as cladistic groups, and are used as such in cladistic-based phylogenetic scenarios. This is wrong on so many levels. Also, hardly any human fossil group demonstrates a clear example of cladistic speciation event in the way that popular punctuated equilibria theory predicts; the most important changes, such as rise in cranial capacity, could be also observed as trends in accordance with anagenetic tempo of evolution. Late existence of east Asian Homo erectus, has been viewed as proof of cladistic speciation event; but, re-dating of Zhoukoudian and Ngadong hominins (Gibbons 1996, Shea et al. 2009), has shown that these late dates are not correct.

Third, paleoanthropology is a an elitist branch of science, in which people make their names by discovering fossils and making almost each fossil a new species. It’s a kind of marketing in science, and has nothing to do with biological truths.

Now, lets just look at a few examples. Homo ergaster (Groves, Mazak 1975), is defined as African species distinct from Asian Homo erectus (s.s.) on the basis of geography and the special set of cranial features. Species is now accepted to be the direct ancestor of later hominids such as Homo heidelbergensisHomo sapiens, and Homo neanderthalensis rather than Asian Homo erectus.  It is one of the earliest members of the genus Homo, possibly descended from, or sharing a common ancestor with, Homo erectus.  It all fits well if we exclude OH 9 fossil from the story. Fossil possesses a typically Asian Homo erectus set of features (compare it to Sangrian 17 morphology). Did this skull belong to the individual that traveled all the way from Java to Olduvai Gorge? Most logical answer is that so called “Homo ergaster” was in fact the part of large polymorphic species of Homo erectus sensu lato.

Homo antecessor has been (Bermúdez de Castro et al. 1997) defined as a uniquely European species from the early Pleistocene, with “special” set of cranial and dental features. Authors view H. antecessor as the last common ancestor of Neanderthals and anatomically modern Homo sapiens. But, every cranial or dental trait of H. antecessor is also found in Asian Homo erectus (see Carbonell et al. 2005, Etler 1990). Also, the most important H. antecessor fossil (АТD6-69) belonged to the juvenile individual, which makes it inappropriate for species definition. For me, “H. antecessor” is just the local variant of Homo erectus.

Classical Neanderthals are best described paleospecies of humans. Although many cranial and dental traits have been proposed as autapomorphies, none of them stood the rigorous tests of science. Story of “Neanderthals: the sad extinct cousins of modern humans” was promoted by the proponents of “the African Eve” theory based on mtDNA analysis. All the other aspects of Neanderthal biology and behavior were interpreted in accordance to that genetic theory. Neanderthal sensu stricto extinction was accepted as scientific fact, and majority of works hypostasized about causes of that extinction. Works that questioned the paradigm of Neanderthals as failed experiment of nature, that proposed that they reproduced with anatomically modern human were strongly opposed or just simply ignored. However, in recent years, advances of ancient nucleic DNA extraction techniques and analysis changed this paradigm. Green et al. (2010) have shown that modern humans from Eurasia share from 1 to 4 % of nucleic DNA regions with Neanderthals. Simply put, Neanderthals did reproduce with anatomically modern humans, and they left their genes in modern populations. So, the most logical solution is that Neanderthals were part of our species; they can be viewed as variant, subspecies or distinct group within the species Homo sapiens.

So, to me answer is pretty clear: there were no more than a couple of Homo species, and between them we cannot see any clear-cuts. Although there is a great morphological variation in Pleistocene, we CAN distinguish different groups with different sets of morphologies - but we do not need to recognize each one of them at the species level. Howell’s (1999) concept of paleodemas or Jolly’s (2001) concept of allotaxa seem to work well in defining these groups, and their complex relations more so then cladistic species concept.

References:

Bermúdez de Castro, J. M. , et al. 1997. A Hominid from the Lower Pleistocene of Atapuerca, Spain: Possible Ancestor to Neandertals and Modern Humans.Science  276: 1392-1395

Carbonell, E., et al. 2005. An Early Pleistocene hominin mandible from Atapuerca-TD6, Spain, in Proceedings of the National Academy of Sciences102-16: 5674–5678

Etler, D. 1990. Homo erectus in East Asia: Human Ancestor or Evolutionary Dead End? Athena Review 4-1: 37−47

Green, R. E. 2010.  A Draft Sequence of the Neandertal Genome. Science 328: 710–722

Groves, C.P. and Mazak, V. 1975. An approach to the taxonomy of the Hominidae: gracile Villafranchian hominids of Africa. Casopis pro Nineralogii Geologii 20, 225–247.

Holliday, T.W. 2006. Neanderthals and modern humans: an example of a mammalian syngameon? pp. 281-298 in Neanderthals Revisited: New Approaches and Perspectives, eds.

Howell, F. C.1999. Paleo-demes, species, clades and extinctions in the Pleistocene homini record. Journal of Anthropolgical Research 55: 191–243

Jolly , C. J. (2001)A Proper Study for Mankind: Analogies From the Papionin Monkeys and Their Implications for Human Evolution . pp. 177–204 inYearbook of Physical Anthropology 44

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