|Reconstruction of the KNM-ER 1470 skull|
Homo rudolfensis (also Australopithecus rudolfensis) is an extinct species of the Hominini tribe, on the morphological boundary between the genera Homo and Australopithecus. Its oldest fossil is dated to 2.4 million years ago, at the very beginning of the Pleistocene, with the possible exception of the LD 350-1 representing the oldest fossil evidence of the emergence of archaic humans (the genus Homo) from their australopithecine ancestors.
H. rudolfensis is known only through a handful of representative fossils, the first of which was discovered by Bernard Ngeneo, a member of a team led by anthropologist Richard Leakey and zoologist Meave Leakey in 1972, at Koobi Fora on the east side of Lake Rudolf (now Lake Turkana) in Kenya.
It remains an open question whether the fossil evidence is sufficient for postulating a separate species, and if so whether this species should be classified within the genus Homo or genus Australopithecus, and if as Homo, whether it should be subsumed under Homo habilis, or even a morphologically diverse species Homo erectus.
The fossil KNM-ER 1470 was the center of much debate concerning its species. The skull was at first incorrectly dated at nearly three million years old, predating the Homo habilis species. Since then, the estimate has been corrected to 1.9 million years, but the differences in this skull, when compared to others of the Homo habilis species, are said to be too pronounced, leading to the presumption of a Homo rudolfensis species, contemporary with Homo habilis. It is not certain whether H. rudolfensis, H. habilis or some, as of yet undiscovered, third species was ancestral to the later Homo line.
In March 2007, a team led by Timothy Bromage, an anthropologist at New York University, reconstructed the skull of KNM-ER 1470. The new construction looked very ape-like (possibly due to an exaggerated rotation of the skull) and the cranial capacity based on the new construction was reported to be downsized from 752 cm³ to about 526 cm³, although this seemed to be a matter of some controversy. Bromage said his team's reconstruction included biological knowledge not known at the time of the skull's discovery, of the precise relationship between the sizes of eyes, ears, and mouth in mammals. A newer publication by Bromage has since increased the cranial capacity estimate back up, from 526 cm³ to 700 cm³.
2012 Koobi Fora finds
In August 2012, a team led by Meave Leakey published an academic paper in Nature announcing three additional H. rudolfensis fossils from northern Kenya had been found: two jawbones with teeth and a face. The face (fossil KNM-ER 62000) was of a juvenile, but had features in common with KNM-ER 1470, suggesting that the latter skull's uniqueness is due to being a separate species, rather than a large male H. habilis. Team member Fred Spoor described the face as "incredibly flat", with a straight line from the eye socket to the incisor tooth. The jawbones, which appeared to match KNM-ER 1470 and KNM-ER 62000, were also shorter and more rectangular than known H. habilis specimens.
The fossils were dated to about two million years ago, being contemporaneous with H. habilis. According to Leakey et al., "the new fossils confirm the presence of two contemporary species of early Homo [that is, habilis and rudolfensis], in addition to Homo erectus, in the early Pleistocene of eastern Africa". Lee Rogers Berger, however, called the argument "weak", and proposed the finds be compared to other possibilities, such as Australopithecus africanus and Australopithecus sediba. Tim D. White of the University of California also challenged the findings, asking, "How can practitioners in this field possibly expect to be able to accurately identify fossil species based upon a few teeth, jaws, and lower faces in light of what we know about the great variation found among different individuals in a single living species?" Leakey replied, "I would challenge Tim to find any primate in which you would see the same degrees of variation as those that we are seeing between our new fossils and KNM-ER 1802". KNM-ER 1802 is a lower-jaw fossil that is thought to be of a Homo rudolfensis. Given the difference between this fossil and the ones found in 2012, Leakey has proposed that the fossil is not of a H. rudolfensis, but possibly, of a H. sapiens. Bernard Wood considers that it is "perfectly possible" that there were interactions between these different species.
The scientific name Pithecanthropus rudolfensis was proposed in 1986 by V. P. Alekseyev; it was subsequently transferred to the genus Homo by Colin Groves in 1989, with the skull KNM-ER 1470 proposed as the lectotype by B. Wood in 1999.
Comparisons between fossil OH 24, recognized as a Homo habilis skull discovered by Peter Nzube, along with the female H. habilis fossil KNM-ER 1813, and KNM-ER 1470 (male) have brought much controversy as to whether H. rudolfensis and H. habilis should be classified as two separate species or lumped together into H. habilis.
When compared to other older H. habilis fossils like OH 24, the mandible and jaw of ER 1470 do not fit within the limits of variation of H. habilis. KNM-ER 1470 displays less prognathism and a rounder brain case. After much debate, but no clear settlement, fossil KNM-ER 1813 was found in 1973 by Kamoya Kimeu, which helped settle some disputes regarding the H. habilis and H. rudolfensis species. When compared to ER 1813, ER 1470 manifests a larger braincase ranging from 750-800ml.
Even if sexual dimorphism were considered, the size difference in the mandible and teeth would be too great compared to KNM-ER 1813. Fossil KNM-ER 1470, a male H. rudolfensis, has massive teeth in comparison to the female H. habilis fossil KNM-ER 1813 and portrays a much larger brain case than KNM-ER 1813. When KNM-ER 1813 and KNM-ER 1470 are compared to OH 24, similarities between OH 24 and KNM-ER 1813 are more widely evident than with KNM-ER 1470. These similarities include smaller orbits, the projection of the mid-face below the nose and a smaller skull size over all. The assumed ages of these three fossils, the places where they were found and some of the anatomical similarities between them leads many scholars to assume that the two species – H. habilis and H. rudolfensis – co-existed somewhere in the East African area sometime between 2.0 and 1.5 million years ago along with Homo erectus and Paranthropus boisei.
Discoveries at Dmanisi, Georgia, which had diverse physical traits and differences in tooth wear, suggest that all the contemporary groups of early Homo in Africa, including Homo ergaster, Homo habilis, and Homo rudolfensis are of the same species and should be assigned to Homo erectus.
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