Kenyanthropus Platyops

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 ofKenyanthropus 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.

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