Written by C. David Kreger
Introduction

The species designation of Homo rudolfensis is a much debated topic, over both whether it is a separate species, and if it is an australopithecine rather than a member of the genus Homo. The type specimen of the species is KNM-ER 1470. This specimen was discovered by Richard Leakey's team in 1972, east of Lake Turkana at Koobi Fora in northern Kenya. This discovery was of a fairly complete cranium without any remaining teeth. Due to uncertainties created by its large brain size and its early initial dates, Leakey did not attribute the specimen to a species, but simply as a member of the genus Homo.

In 1986, Russian anthropologist Valerii Alexeev applied the species name of Pithecanthropus rudolfensis to ER 1470. Despite the incompleteness of the description, and Alexeev's unfamiliarity with the original specimen, the name stuck. The genus name of Pithecanthropus has been dropped by those who see rudolfensis as a valid species and replaced with the genus Homo. Recently, there have been attributions of the rudolfensis specimens as belonging to Australopithecus, and thus another genus change may be in the future of this species if it retains validity as a separate species.

One of the main problems with the rudolfensis species is that there are no postcranial remains that are associated with cranial remains. The rudolfensis specimens have large brains in conjunction with megadont postcanines, and without postcranial evidence it is unknown whether these features are due to a larger body size than contemporary habilis specimens. Due to this problem, competing ideas abound regarding the validity of rudolfensis and its proper place in hominid phylogeny. Some researchers see the larger brain and tooth size as indicative of allometric changes due to increased body size, with rudolfensis and habilis constituting the same species, with the former the males and the later the females. Some see rudolfensis as the ancestor of habilis with a decrease in brain size occurring, and others see the two on completely different evolutionary lines.

This debate is long from over, and all the scenarios have one problem or another. Perhaps with future discoveries that are attributed to the rudolfensis, a clearer picture will appear as to the relationship of these early Homo.

Diagnostic Features

The type specimen of Homo rudolfensis is KNM-ER 1470, discovered by Bernard Ngeneo at Koobi Fora, Kenya, in 1972. The research team involved (led by Richard Leakey) attributed the toothless cranium to the genus Homo with the species indeterminate due to the large brain size and questionable morphological association with known hominids. The cranium was reconstructed by Meave Leakey into two main pieces that give an estimated brain size of 775cc. The specimen was originally thought to be around 2.9 myr old, due to an inaccurate dating of 2.6 myr for the KBS volcanic tuff located above it. This inaccuracy was caused by contamination of older material, and the tuff is now know to be much younger. The specimen is now thought to date to approximately 1.8 myr (Leakey et al. may have been more willing to attribute the specimen to habilis had they known the real antiquity of the specimen from the beginning). Though this date is now generally accepted for the specimen, the geologists who orignally dated the KBS tuff continue to argue for a later date for the specimen. While they admit the dating of the volcanic tuff was inaccurate, F. Fitch and colleagues claim that the depth of the specimen beneath the tuff shows a much earlier age, daing to around 2.4 myr.

Compared to a smaller Koobi Fora cranium, KNM-ER 1813, ER 1470 shows many differences that distinguish the specimen from habilis. These features include:

• A slight supraorbital torus across the forehead with no sulcus or depression behind it.
• A much longer face, with the upper part narrower than the middle.
• The maxilla is squared off rather than rounded, with a very short, shallow palate.
• Evidence of much more megadont postcanines.
  

Other features that comprise ER 1470 (but which are not necessarily distinct to the species) include:

• A lack of crests and heavy muscle markings that characterize australopithecine crania.
• A marked constriction of the braincase behind the orbitals (but less that occurs in robust australopithecines. )
• A bulging frontal bone that rises steeply to meet the square parietal bone that form the thin-walled sides of the brain case.
• An occipital bone that is smoothly rounded rather than flexed as in Homo erectus.

Other cranial remains that have been attributed to rudolfensis include KNM-ER 1590, KNM-ER 3732, KNM-ER 1801, and KNM-ER 1802. Alexeev originally attributed ER 1470 as a male rudolfensis and ER 1813 as a female, but ER 1813 is generally considered to be habilis. There is no directly associated postcrania for rudolfensis (which makes the species problematic), though some specimens have been attributed to the species. Limbs such as the ER 1472 and ER 1481 femora have been attributed to rudolfensis, suggesting a dramatic allometric difference, but there is much questionable about associating postcrania and cranial material together, when there is no objective sample to compare them to (an associated cranium and postcranial material).

Another specimen that may be attributable to rudolfensis is OMO 75-14, a mandible and cranial fragments that have been dated to around 2.0 myr. In spite of very large postcanine size (the M2 size exceeds all other non-robust specimens and its shape resembles the boisei condition), its premolar morphology is not like any robust australopithecine, according to G. Suwa et al. The teeth are larger than any habilis specimen, and seem to show definite Homo-like patterns. However, isolated mandibles and fragments are hard enough to attribute, and even worse when trying to attribute them to a species many researchers do not even consider a valid taxon. B. Wood has given a diagnosis of mandibles that should be placed within rudolfensis which include:

• Broader postcanine tooth crowns; they are narrower in habilis.
• A large P4 talonid (an extra area on the back of the tooth that acts to make it squarer in profile).
• Double-rooted P4s and sometimes P3s (these are single-rooted in habilis).
  

The dating of this species is significant, in that a date earlier than habilis makes this species the first habiline, and with its very large brain, a candidate for being a direct human ancestor. The initial date of 2.9 myr for ER 1470 has been modified, but as mentioned above, there is some support for a date of approximately 2.4 myr for the specimen. However, habilis itself has had its origins pushed back to approximately 2.3 myr (A.L. 666-1), making the designation of the "first" habiline difficult even if ER 1470 is older than is now generally accepted. Also important when considering the habilines (and rudolfensis in particular) is the idea that they are not Homo at all, but rather Australopithecus.

Analyses by D. Lieberman and B. Wood seem to indicate that the habilines share more features with the australopithecines than with Homo. Evidence for this position include:

• A. Walker's initial impression of ER 1470 was of an expanded but very similar specimen to STS 5, an africanus specimen.
• D. Lieberman et al. analysis which showed that rudolfensis shows more affinities to the australopithecines rather than with the other species of Homo.
• It would help account for the robust australopithecine features found in rudolfensis, because africanus closely resembles the robusts in many ways.
  

The large degree of variation in africanus shows that the degree of difference between rudolfensis and earlier africanus is such that attribution to differing species is not required.

Conclusions

As stated, the attribution of the species rudolfensis to any specimen is somewheat controversial, since many paleoanthropologists do not see rudolfensis as a valid species. Its dating (whether the early dates proposed by some or the contemporaneous dates to habilis) makes its brain size an issue, and raises questions about current standard phylogeny of the human line. Homo rudolfensis may be the first member of the genus Homo on a path to modern humans, or it may be a more Homo-like australopithecine with no direct bearing on the evolution of H. sapiens. Nothing can be stated for sure at this point, except that there will be much more future debate on the issue.


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