KeywordsCranial Capacity Early Homo Postcranial Bone Wide Palate Intellectual Milieu
Homo rudolfensis is a species of early Homo that is known from ~1.8 million years ago in East Africa. Despite more than 40 years of research and analyses, it is still unclear where this hominin sits on the human evolutionary tree.
Fossil hominin bones found in Kenya in 1973 remain enigmatic despite 43 years of discussion and analysis. The bones comprise a partial cranium and face and leg bones. The cranium and face have the Kenya National Museum name KNM-ER 1470. Just where they fit on the human evolutionary tree has long occupied researchers, but a resolution has not been forthcoming, and it is unclear as to what other fossils might belong in the same species as KNM-ER 1470.
History of Discovery and Taxonomic Challenges
In 1973 Mr. Bernard Ngeneo discovered a relatively intact cranium in the East Rudolf area of Kenya during fieldwork led by Mr. Richard Leakey. The cranium is known by its museum reference KNM-ER 1470 (Kenya National Museums-East Rudolf). Possibly associated postcranial bones were also found – a right femur (KNM-ER 1472), discovered by Dr. J. Harris; part of a second right femur (KNM-ER 1475; discovered by Mr. Kamoya Kimeu); and skeletal material (KNM-ER 1481; Dr. J. Harris) (Leakey 1973).
The brain case and partial facial skeleton were found in deposits under a volcanic tuff that was at that time dated to 2.6 mya. We now know that the skull is from deposits dated to 1.8 million years ago (Mcdougall et al. 2012) and are thus contemporaneous with cranium KNM-ER 1813 H. habilis (East Africa) that was at the time the earliest member of our genus.
Since its discovery, opinions have differed about the species into which this cranium should be placed. In a joint publication Alan Walker and Richard Leakey “agreed to disagree,” Walker preferring to place KNM-ER 1470 in Australopithecus and Leakey would place it to Homo (Walker and Leakey 1978, p. 54). The challenge facing us is the unexpected mix of characters in the specimen. It has a domed calvarium with steeply sloping sides that have parietal eminences; the frontal (forehead) rises relatively steeply (but not as greatly as modern humans), which are characteristics of Homo. But the size and flatness of the face and the shallow, broad, and short palate are reminiscent of Australopithecus, although the wide palate in relation to its length differs from the shape of the Australopthecine palates (Leakey 1973). Its cranial capacity is 752 cc while Australopithecines’ cranial volume range from 387 to 560 (Holloway et al. 2004).
Because KNM-ER 1470 existed at the same time and in the same general region as H. habilis it became customary to include KNM-ER 1470 in that species, although Groves (1985, p. 264) notes that there was never any attempt to justify this assumption. The differences in the cranial capacities and general morphology of KNM-ER 1470 and crania attributed to H. habilis, however, led some researchers to suggest that the fossils might better be assigned to two species, while others attributed these differences to sexual dimorphism within H. habilis.
The first to separate the species into two was Groves and Mazak (1975) who attributed a mandible KNM-ER 992, at the time included H. habilis, to a new species, H. ergaster. Wood (1985) showed that the KNM-ER 1470 parietal bone is within the H. erectus range in some aspects but different in others and that the base of KNM-ER 1470 is metrically similar to H. erectus (OH 9, KNM-ER 3733 and 3883). He, therefore, asks, in relation to KNM-ER 1470 and H. habilis KNM-ER 1813 “Are we dealing with one taxon, or two?” (Wood 1985, p. 211). Using statistical methods applied to expected cranial capacities in a population, he finds that the chance of the two crania being from the one population is <5% and that KNM-ER 1470 and KNM-ER 1813 should not be uncritically regarded as being sampled from the same taxon (Wood 1985). He concludes that even more tantalizing than the differences between KNM-ER 1470 and KNM-ER 1813 is the possibility that not two but three species existed in the East Africa in the Early Pleistocene: H. habilis, H. erectus, and a taxon that included KNM-ER 1470.
Questioning the coexistence of more than one species of Homo in the human evolutionary journey occurred in an intellectual milieu that viewed the evolution of Homo as unilineal. That is, until the question of KNM-ER 1470 was raised it was thought that H. sapiens evolved from H. erectus that evolved from H. habilis. To maintain this paradigm, any variation around any of these species was considered a “grade.” In this context, Groves (1989) tested the unilineal concept of human evolution by applying cladistic analysis to the Australopithecines and Homo. His cladograms consistently returned results in which KNM-ER 1470 was on a separate branch to other species in the analyses, H. habilis and H. erectus. He attributed KNM-ER 1470 to a new species, Homo rudolfensis (Alexeev 1986). This species name is a derivative from Alexeev (1986), who, 3 years earlier, had attributed KNM-ER 1470 to Pithecanthropus rudolfensis. Under the rules of taxonomic nomenclature the name first published in accordance with the rules is the correct name for a taxon so, in this case, rudolfensis, is retained, but the genus name is amended, and the originator of the name is appended. The unique characters that distinguish H. rudolfensis are the flat form of the zygomatic root, lack of internal buttressing of the mandible, and some morphologies of the premolars (Groves 1989: 264).
Those that would retain KNM-ER 1470 in H. habilis regard the species as being highly sexually dimorphic, regarding the larger KNM-ER 1470 as a male of the species and the smaller crania as females (such as KNM-ER 1813 that has a cranial capacity of 509 cc; Holloway et al. (2004)). In this case, the species name H. habilis senso lato is used in the literature, and H. habilis senso stricto is applied if the researcher includes only the smaller crania in H. habilis.
A further challenge within this somewhat complicated situation is the lack of agreement as to the composition of H. rudolfensis. For example, William Kimbel and colleagues include KNM-ER 1470, KNM-ER 1590, KNM-ER 3732 H. rudolfensis; David Strait and colleagues’ sample comprised 9 fossils; Lee Berger and colleagues included 11 fossils in their compilation of H. rudolfensis; David Lordkipanidze and colleagues include KNM-ER 1470, KNM-ER 62000, and OH65 in the species; while other studies limit the species to KNM-ER 1470.
H. rudolfensis, then, is somewhat of an enigma. Where does it fit on the human evolutionary tree? What is it most closely related to? In 2001, Leakey et al. (2001) announced a new hominin genus and species discovered in East Africa, Kenyanthropus platyops sp. nov. The species comprises a largely complete cranium KNM-WT 40000 and part of an upper left jaw and face KNM-WT 38350. The former has an estimated age of 3.5 mya; the latter has an estimated age of 3.3 mya. Leakey et al. (2001) draw attention to the similarity of KNM-WT 40000 to KNM-ER 1470, H. rudolfensis. Both have a flat face, a vertically oriented cheek region, lack of a supraorbital sulcus, and an anterior origin for the root of the zygomatic arch. The main differences between the KNM-ER 1470 and K. platytops cranium KNM-WT 40000 relates to the more primitive nasal characteristics of KNM-WT 40000 and the higher cranial vault (larger braincase) of KNM-ER 1470 (Leakey et al. 2001). Cameron (2003) sees similarities among Kenyanthropus and H. rudolfensis in the pattern of growth stages in brain morphology and cranial and dental development that differed from Paranthropus. He suggested Kenyanthropus and H. rudolfensis may form a sister group relationship to the exclusion of Paranthropus. This led him to propose that K. platytops was the ancestor of H. rudolfensis and the species should be renamed Kenyanthropus rudolfensis. So it seemed that at last that the ancestor of H. rudolfensis had been identified, but others have questioned this and a possible phylogenetic relationship between Kenyanthropus and H. rudolfensis needs further clarification.
Just what H. rudolfensis is, where it sits on the human evolutionary tree, and even the tantalizing prospect that its ancestors may have been discovered are unresolved questions despite more than 40 years of comprehensive research. Is there hope for a resolution? Yes, probably. In the last 15 years, a remarkable suite of discoveries of new hominins has been made, so there is more comparative material with which to compare H. rudolfensis, and, judging by the recent increase in the rate of new fossil discoveries, there is every chance that more H. rudolfensis fossils will be found that will fill out our understanding of its morphology so that we can better assess its place on the human evolutionary tree.
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