A Look at Modern Human Origins

Kenyanthropus platyops (Updated 8/25/03)

Introduction

KNM-WT 40000Photograph of KNM-WT 40000.(Taken from Nature.)
2001 was a big year in paleoanthropology with the announcements of "Millenium Man" Orrorin tugenensis and the later announcement of Kenyanthropus platyops by Meave Leakey et al. This eastern African hominid was excavated in 1998 and 1999 from Lomekwi in the Nachukui Formation, west of Lake Turkana, during fieldwork aiming to expand the knowledge of the 3-4 mya period of this region.

The holotype specimen is KNM-WT 40000, a fairly complete cranium missing the cranial base and part of the maxilla. The Leakey team designated a new genus (Kenyanthropus) for the specimen, creating a type species platyops. The species name means approximately "flat-faced" in Greek, in reference to the smaller degree of subnasal prognathism that is one of the characteristics setting the specimen apart from Australopithecus species. A partial left maxilla (KNM-WT 38350) was designated as a paratype of the species, and several other specimens (including a temporal, another partial maxilla, and several isolated teeth) were excavated along with the cranium and included in the description. Two mandibles and isolated molars previously escavated in the 1980s from the same location were also designated to K. platyops.

Leakey et al. claimed that while the overall size and general shape fits the australopithecine form present in contemporaneous Australopithecus afarensis and Australopithecus africanus, the cranium has a suite of features that differ, making it an outlier with several derived characters not shared with the australopithecines (the reason the material was given a new genus rather than just a new species designation). Leakey et al. claim platyops is distinct from the australopithecines, sharing features with KNM-ER 1470 (Homo rudolfensis) that may indicate that it is the ancestor of Homo, while the australopithecines are a side-branch that is not ancestral to Homo.

However, as with all newer finds, there is controversy surrounding the claims. The cranium is distorted from post-mortem diploic expansion and compression inferoposteriorly, making many of the measurements more approximations than exact measures. Tim White of the University of California, Berkeley, has argued that many of the features of platyops are an artifact of this distortion. He argues that it is more likely that this material is simply another variant on A. afarensis, which is widespread during this time period and in this part of Africa. Various researchers have come down on both sides of the debate, some thinking distortion caused some of the presumed characteristics, and that this is just another australopithecine variant, and others that believe that Leakey et al. were well aware of the distortions, and so they likely would have recognized if this was the case.

Diagnostic Features

Very little has been published regarding this material, and as very little material is associated with the species and its close similarity to afarensis, there is not many very explicit qualitative differences, being more a suite of characters that are quantitatively different. The descriptive characters of the platyops, and the distinctive features that distinguish it from other taxa are directly taken from the intial announcement by Leakey et al. The discussion on the relative validity of the distinctions and platyops phylogentic relationships are also taken directly from Leakey et al., with my comments as well.

The holotype specimen is KNM-WT 40000, a partial cranium consisting of two major parts: a neurocranium with the superior and lateral orbital margins, but with most of the cranial base missing, and a splanchnocranium missing the premolar and anterior tooth crowns and the right incisor roots. Most of the cranial vault is heavily distorted through diploic expansion (geologic materials entering the microscopic holes in the bone and expanding the bone) and compression forces inferoposteriorly. The splanchnocranium portion is better preserved, but still shows lateral skewing in the nasal area, anterior displacement of the right canine, and expansion of the alveolar and zygomatic processes.

Some of its diagnostic characteristics include:

  • Particularly small M2 crown size, falling below the known ranges of other early hominids.
  • Molar enamel thickness comparable to that of A. anamensis and A. afarensis.
  • Canine roots are smaller relative to M2 crown size than in A. ardipithecus and A. anamensis, larger than in A. boisei, and about equivalent in size to A. afarensis, A. africanus, and H. habilis.
  • I1 and I2 roots are straight and equivalent in size.
  • Cross-sectional area of I2 at the alveolar margin is ~90% of I1, compared to the 50-70% in other known hominids.
  • Incisor alveoli are aligned coronally, just anterior to the bicanine line, and the overlying nasoalveolar clivus is flat sagittally and transversely.
  • Lack of canine jugum on the preserved left side, reflecting the small canine root size.
  • Moderate subnasal prognathism with a very vertically oriented clivus.
  • A small and narrow nasal aperture.
  • Tall malar region with a low and curved zygomaticoalveolar crest.
  • Anterior surface of the zygomatic process of the maxilla is positioned between P3 and P4.
  • Australopithecus-like supraorbital region, distinct from the robust australopithecines and H. habilis, but similar to H. rudolfensis.
  • Postorbital constriction similar to Australopithecus, H. habilis, and H. rudolfensis, and less than the robust australopithecines.
  • Double temporal lines on the posterior parietals that contribute to indistinct compound temporal/nuchal lines.
  • Narrow external auditory meatus with a small aperture, similar to A. ramidus and A. anamensis.
  • Mandibular fossa similar to A. africanus and A. afarensis.
  • Preserved posterior half of foramen magnum indicates it was oval shaped.
  • Occipital endocranium lacks any indication of the occipital/marginal venous sinus system characteristic of the australopithecines.
  • Bilateral sulci suggest that the transverse/sigmoid sinus system was well developed.
  • Cranial capacity cannot be determined accurately, but is likely within the range of the australopithecines.

The sex of KNM-WT 40000 is indeterminate, as there is not a broad enough range of material designated to platyops to distinguish sexually dimorphic characters. While the small M2 crown size may indicate female, the close proximity and raised aspect of the temporal lines on the posterior aspect of the parietals is not seen in known female hominid crania, which may indicate it is a male.

Leakey et al. conclude that the cranium is most similar to KNM-ER 1470, when compared to the gracile and robust australopithecines, earlier hominids like Ardipithecus, and later members of Homo. The flat orthognathic-looking face compares favorably with both the robust australopithecines and ER 1470, but lacks most of the derived features of the robust australopithecines, and its facial characteristics differ from them in the same ways described for ER 1470. In both KNM-ER 1470 and KNM-WT 40000 it is the flat and orthognathic nasoalveolar clivus that aligns with the plane of the nasal aperture, with the anteriorly set tall malar region more vertically oriented. However, KNM-WT 40000 lacks the derived short nasal bones and everted nasal margin, and is less orthognathic in the midfacial region than is seen in KNM-ER 1470.

The paratype for K. platyops is KNM-WT 38350, a partial left maxilla. This specimen shares some features with KNM-WT 40000, including the anterior surface of the zygomatic process positioned between P3 and P4, and also its small molar size with its M1 crown area approaching the minimum values of A. anamensis, A. afarensis, and H. habilis. It also has three clear P3 roots as in KNM-WT 40000, and while the P4 only shows 2 roots clearly, the deeply grooved buccal root may split apically. This is a variable trait in Australopithecus.

Other fragmented material recovered from Lumekwi and may belong to platyops includes KNM-WT 38343A, a right maxilla fragment with three well-separated P3 roots and a damaged canine that seems relatively low-crowned relative to A. afarensis, and KNM-WT 40001, a right temporal bone that is well-preserved, but lacking the squama and petrous apex. KNM-WT 40001 has a rounded projecting mastoid process with an anteriorly positioned tip, a well-developed digastric fossa forming a narrow deep groove running posteriolaterally from the stylomastoid foramen that separates the mastoid process from the adjacent nuchal plane. Similarly to KNM-WT 40000, its tympanic element lacks a petrous crest with an extremely small external auditory meatus. However, they cannot be clearly associated to platyops and Leakey et al. has left them unassigned to a species. Leakey et al. posit several specimens previously attributed to afarensis as potentially platyops, including KNM-WT 8556 and KNM-WT 16006.

In general, the Lumekwi material is distinguished as distinct from Ardipithecus, gracile Australopithecus, robust Australopithecus, and indisputable Homo (as opposed to H. habilis and H. rudolfensis) by a variety of features that creates the justification of creating a new genus. Kenyanthropus is distinguished from Ardipithecus by:

  • Buccolingually narrow M2.
  • Thick molar enamel.
  • A more cylindrical articular eminence and deeper mandibular fossa on the temporal.

Kenyanthropus is distinguished from A. anamensis and A. garhi by the derived morphology of the lower face, including:

  • Moderate subnasal prognathism.
  • Sagittally and transversely flat nasoalveolar clivus.
  • Anteriorly positioned maxillary zygomatic process.
  • Similarly sized I1 and I2 roots.
  • Small M1 and M2 crown area.

is distinguished from A. africanus by:

  • Moderate subnasal prognathism.
  • Sagittally and transversely flat nasoalveolar clivus.
  • Anteriorly positioned maxillary zygomatic process.
  • Similarly sized I1 and I2 roots.
  • Small M1 and M2 crown area.
  • A tall malar region.
  • A low and curved zygomaticoalveolar crest.
  • A narrow nasal aperture.
  • Absence of anterior facial pillars.
  • A tubular, long and crestless tympanic element.
  • A small external auditory meatus.

is distinguished from A. afarensis by:

  • Moderate subnasal prognathism.
  • Sagittally and transversely flat nasoalveolar clivus.
  • Anteriorly positioned maxillary zygomatic process.
  • Similarly sized I1 and I2 roots.
  • Small M1 and M2 crown area.
  • A transversely flat midface.
  • A small external auditory meatus.
  • Absence of an occipital/marginal venous sinus system.

Kenyanthropus is distinguished from the robust australopithecines and from Homo (exluding H. habilis and H. rudolfensis) by its lack of derived cranial features.

KNM-WT 40000 was recovered near the contact of the Nachukui Formation with Miocene volcanic rocks in the northern tributary of Lomekwi (Nabetili). It was collected 12 m above the Lokochot Tuff and 8 m below the ß-Tulu Bor Tuff. The Lohochot Tuff is dated to 3.57 myr, and the ß-Tulu Bor Tuff is dated to 3.40 myr, giving an estimated date of 3.5 myr for KNM-WT 40000.

The mudstone that contained KNM-WT 40000 was deposited along the northern margin of a shallow lake that extended to Kataboi and further. Other specimens between the burrowed bed and the Tulu Bor Tuff were also preserved in lake-margin envrionments, as was the case with several other specimens found in other strata. The faunal assemblages from the Lomekwi sites indicate paleoenvironments that were relatively well watered and well vegetated. The proportions of bovids found indicate a mosaic of habitats but primarily woodland and forest-edge species dominating. Other indications of closed-forest environments include the common presence of Theropithecus brumpti (a baboon species). The fauna and geology indicate that these hominids inhabited a closed-forest or forest-edge environment in the presence of a stream-fed lake with seasonal floodplains.

Conclusions

In the past 3 years there have been several new species designated, and all of them have had some degree of controversy and doubt expressed by various professionals. Hopefully the next few years will see clarification and justification that will either lead to the strengthening of some of these species or their destruction. Kenyanthropus may be a new variant of Australopithecus, possibly a sister taxon to afarensis, it may be subsumed under the variation seen in afarensis, or it may become strengthened as a separate genus, perhaps ancestral to H. rudolfensis and leaving the australopithecines out in the cold in terms of being ancestral to modern humans.

However, it is still early, and with such sketchy material, and no associated post-crania, it is doubtful that the question will be settled soon unless either more material is uncovered, or the material is shown to be either distorted and simply a variant of a known species, or shown to fall within the range of variation of a known species as is.


Bibliography

This bibliography contains the sources of the information cited above, as well as any sources that could provide any other information on the subject. If you know of any other sources that are pertinent to K. platyops, please e-mail me the citation in the format used below, and I will add it to the list. Any problems with information I presented above can be sent to me here. I don't want to provide disinformation, and any corrections are gladly accepted (with proper documentation of what is wrong and why, with sources). Thanks!

Cela-Conde, C.J., and F.J. Ayala. 2003. "Genera of the human lineage." In Proceedings of the National Academy of Sciences, vol. 100, pp. 7684-7689.

Leakey, M.G., F. Spoor, F.H. Brown, P.N. Gathogo, C. Klarle, L.N. Leakey, and I. McDougall. 2001. "New hominin genus from eastern Africa shows diverse middle Pliocene lineages." In Nature, vol. 410, pp. 433-440.

Lieberman, D.E. 2001. "Another face in our family tree." In Nature, vol. 410, pp. 419-420.

Schwartz, J.H., and T.D. White. 2003. "Another perspective on hominid diversity." In Science, vol. 301, pp. 763-764.

White, T. 2003. "Early hominids - Diversity or distortion?" In Science, vol. 299, pp. 1994-1997.