One of a rash of new extinct hominid genera discovered and described during the turn of the 21st century, Kenyanthropus immediately garnered substantial press coverage and an onslaught of criticism after its naming in 2001. Characterized by its describers as a distinct genus that suggested a more complex early evolutionary history of the human lineage, this taxon was later dismissed by other researchers as either belonging to Australopithecus or Homo. Regardless of their taxonomic placement, the specimens described as Kenyanthropus exhibit a mosaic of primitive and derived characters that may help elucidate the characteristics, habits, and environments of human ancestors.
Kenyanthropus was described in 2001 by Meave G. Leakey, Fred Spoor, Frank H. Brown, Patrick N. Gathogo, Christopher Kiarie, Louise N. Leakey, and Ian McDougall. Fossils belonging to the type species Kenyanthropus platyops were discovered at the Lomekwi Site in Kenya, on the western side of Lake Turkana, during a series of expeditions from 1998— 1999. Over 30 hominid fossils were uncovered during the field sessions, two of which were described as K. platyops. These specimens were found in the Pliocene Nachukui Formation, indirectly radiometrically dated as 3.5 million years old. The holotype, discovered by Justus Erus in August 1999 and denoted as KNM-WT 40000, is a largely complete, yet heavily distorted cranium lacking most of the cranial base and the premolar and anterior tooth crowns. The paratype, KNM-WT 38350, is a partial left maxilla found by B. Onyango in August 1998.
Leakey and co-authors recognized that the overall size of the holotype fell within the size range of Australopithecus afarensis and A. africanus, but erected a new genus based on the cranium’s mosaic of primitive and derived characters. These derived characters include a flat face, a condition known as orthognathy; a tall cheek region; and small molars. Of these characters, the orthognathic facial morphology of Kenyanthropus is most unique. While other extinct hominids, including Paranthropus, possess such a morphology, only in Kenyanthropus is an orthognathic face associated with small molars. In addition, Kenyanthropus shows the earliest evidence of orthognathy in the hominid fossil record. Despite these advanced features, Kenyanthropus also displays many primitive traits shared with australopithecines, including flat nasal margins and a small brain that compares in size with those of chimpanzees. Interestingly, Kenyanthropus shares many characters with Homo rudolfensis, including the lack of a depression behind the brow ridge and a flat plane beneath the nose bone. These characters may indicate that Kenyanthropus is a close relative, possibly an ancestor, of modern humans, and has led to the suggestion that H. rudolfensis be transferred to the genus Kenyanthropus (as K. rudolfensis).
Aside from providing important anatomical information, the discovery of Kenyanthropus added to the diversity of the eastern African hominid record of 3—4 million years ago, which had previously been represented solely by A. afarensis and A. anamensis. Additionally, the unique combination of derived and primitive characteristics indicated that Kenyanthropus had evolved a specific diet, and pointed to a diet-driven radiation early in the history of the human lineage. Based on this evidence, Leakey and her coauthors argued that human evolution didn’t follow a well-defined path, with a continuum of species leading to Homo sapiens, but instead took the form of a “bush,” with many unique species branching off at various points. Coupled with the discovery of other genera announced at roughly the same time, such as Orrorin, Sahelanthropus, and Ardipithecus, this interpretation of Kenyanthropus and its implications for human evolution diverged sharply from traditional views that advocated a “straight-line” path to modern humans.
However, other researchers have disagreed with the views of Leakey and her co-workers. Paleoanthropologist Tim White argued that the holotype cranium of Kenyanthropus was too distorted to allow proper comparison to other hominid fossils. He noted that a geological process called Expanding Matrix Distortion (EMD) heavily damaged the cranium and resulted in its breaking into some 4,000 separate pieces. Since EMD doesn’t enlarge or distort all dimensions equally, it commonly leads to an indecipherable complex of deformation that is often impossible to correct for, thus making precise identification tenuous. Since comparison to other fossils is made difficult by EMD, White argued that it is impossible to differentiate Kenyanthropus from the contemporaneous A. afarensis and A. anamensis, especially considering the known cranial variation in modern apes and humans. As a result, he opted for a conservative taxonomy that sunk Kenyanthropus into Australopithecus.
Camilo J. Cela-Conde and Francisco J. Ayala have suggested a different placement for Kenyanthropus. Unlike White, they recognized the distinct features of Kenyanthropus as real, and not a product of deformation. However, they contended that the similarities between Kenyanthropus and Homo rudolfensis, including smaller molars and thinner tooth enamel, were indicative of a close phylogenetic relationship. Therefore, they advocated that Kenyanthropus be included in the genus Homo, likely as its earliest known species. Their proposal placed the appearance of Homo at 3.5 million years ago.
Regardless of the uncertain taxonomic placement of Kenyanthropus, the discovery of the holotype and paratype, along with representatives of their associated flora and fauna, are revealing new insights regarding the environments and life strategies of human forebearers. Geological evidence preserved at the Lomekwi Site indicates that the area was wet and heavily vegetated during the time of Kenyanthropus. The discovery of certain bovid fossils suggests a mosaic of woodland and forest habitats, which argues against the common hypothesis that much of human evolution was driven by a sudden environmental shift between forests and savannas. Additionally, the paleoenvironment of the Lomekwi Site is similar to the few known hominid sites of similar age, including Laetoli in Tanzania, Hadar in Ethiopia, and Bahr el Ghazal in Chad.
While only a preliminary description of its bones has been published, Kenyanthropus is regarded as being among the most intriguing and informative of a handful of new hominid genera described during the close of the 20th century and the dawn of the 21st.
Although some arguments remain over the validity of these new taxa, their discoveries are proof that much about human evolution remains to be discovered.
References:
- Cela-Conde, C. J., & Ayala, F. J. (2003). Genera of the human lineage. Proceedings of the National Academy of Sciences, 100, 7684-7689.
- Leakey, M. G., Spoor, F., Brown, F. H., Gathogo, P. N., Kiarie, C., Leakey, L. N., & McDougall, I. (2001). New Hominin genus from Eastern Africa shows diverse middle pliocene lineages. Nature, 310, 433-440.
- White, T. (2003). Early hominids—diversity or distortion? Science, 299,1994-1997.
