Remains of Kenyanthropus platyops, which means “flat-faced man from Kenya,” have been recovered from sediments at Lomekwi, a site west of Lake Turkana in Kenya. The volcanic sediments in which specimens attributed to this species have been found are radioisotopically dated to between 3.3 and 3.5 million years ago. Kenyanthropus platyops is important to the study of human evolution because its anatomy and time range strongly suggest that at least two distinct types of bipedal hominins lived in East Africa between 3.0 and 4.0 million years ago.
Kenyanthropus platyops is known only from cranial remains (remains of the skull minus the lower jaw)—specifically a crushed and distorted cranium and a partial maxilla (the bone comprising the upper jaw and the majority of the face). Although other fossils have been found at Lomekwi, they have not been officially assigned to K. platyops. The features in these cranial fossils underline the distinctiveness of K. platyops vis-à-vis Australopithecus afarensis, with which it is contemporary. In particular, K. platyops exhibits a mixture of primitive (ancestral, in this case, resembling earlier, more ape-like hominin species) and derived (specialized, in this case, resembling later hominin species) features that are not found in combination in any other hominin species. Some of the features found in K. platyops, however, are similar to those found in Au. afarensis. The dental enamel in K. platyops, for example, like that in Au. afarensis, is thicker than in apes, but thinner than in the robust australopiths. The brain sizes of K. platyops and Au. afarensis are also very similar. The primitive features of this species derive largely from the dental remains. For instance, the upper first and second molars are smaller than those of any species in the genus Australopithecus and are similar in size (or slightly smaller) than those of Ardipithecus ramidus. In addition to the small size of the upper molars, the external acoustic meatus (outer earhole) in K. platyops is smaller than that of Au. afarensis and similar in size to that of Australopithecus anamensis, Ar. ramidus, and living chimpanzees. Markings on the base of the skull also indicate differences between K. platyops, on one hand, and Au. afarensis and the robust australopiths, on the other, in the way the blood emptied from the cranium (although some of the earlier remains of Au. afarensis share the blood drainage route exhibited by K. platyops). Like these dental features and aspects of the base of the skull, the morphology (shape and size) of the facial skeleton (the bones that comprise the face) is different from that of Au. afarensis, but the facial morphology in K. platyops does not resemble any earlier, more primitive, species. For instance, the maxilla and zygomatic bones, which make up the cheek and provide the site of attachment for the masseter muscle (a chewing muscle that closes the mouth), are positioned much more toward the front of the face than in Au. afarensis or any other species in the genera (plural of genus) Australopithecus and Ardipithecus. The part of the maxilla below the nose is likewise distinct from Au. afarensis and earlier species—i.e., it is flat from side to side and from top to bottom.
Due to the unique mixture of primitive and derived features found in K. platyops, it is difficult to precisely determine how this species was evolutionarily related to other hominin species. It is clear that the fossils assigned to K. platyops cannot be easily included in Au. afarensis; however, it is important to note that some scientists suggest that the complete K. platyops cranium is so distorted that the possibility that “flat-faced man” is simply a variant of Au. afarensis cannot be ruled out. Links between K. platyops and the robust australopiths have also been drawn, but the former lacks the extremely large teeth and very thick dental enamel exhibited by the latter. Some scientists also highlight similarities between K. platyops and species in the genus Homo. The flatness of the face below the nose, in particular, has been used to connect K. platyops to Homo rudolfensis, a more recent, flat-faced hominin species, but the dearth of relevant fossils during the roughly 1.5-million-year interval that presently separates these two species makes it difficult to test this idea.
To the scientists who discovered them, the distinctive mixture of primitive and derived traits exhibited by the K. platyops remains warrants naming them not only to a new species but also to a new genus. Thus, in both taxonomic (the naming of species) and phylogenetic (the evolutionary relationships among species) senses, K. platyops indicates the hominins living between 3.0 and 4.0 million years ago were diverse. Some scientists have suggested that this diversity is good evidence of a hominin adaptive radiation (a relatively rapid increase in the number of species) during this time, positing consequently that some of the species living at this time did not contribute to the lineage that eventually led to Homo sapiens.
Based on the nonhominin faunal remains associated with the K. platyops specimens, the habitat of this species is reconstructed as having been wetter and more closed than habitats associated with Au. afarensis at Hadar. Other lines of evidence indicate that K. platyops used a wide range of dietary resources.