A new four-million-year-old hominid species from Kenya
Nine hominid dental, cranial and postcranial specimens from Kanapoi, Kenya, and 12 specimens from Allia Bay, Kenya, were described as a new species of Australopithecus in the August 17, 1995 issue of Nature. The fossils date from between about 3.9 million and 4.2 million years ago. A mosaic of primitive and derived features infer a possible ancestor to Australopithecus afarensis. A tibia from KNM-KP 2983 established bipedality 0.5 million years earlier than prior evidence.
Ardipithecus ramidis extended the temporal distribution of hominids to 4.4 Mya. The relationship between Australopithecus afarensis and Ardipithecus ramidis, a potential ancestral species for all Hominidae, can now be assessed with more temporally intermediate evidence.
The name 'anam' means 'lake' in the Turkana language. All specimens were found near Lake Turkana in sediments associated with the precursor lake. The type was found in a Pliocene strata at Kanapoi in a strata dating to about 4.1 mya. Fossils from Allia Bay lie below or within the Moiti Tuff in the Koobi Fora Formation, and date to about 3.9 mya.
Australopithecus anamensis is distinguished by a small external acoustic meatus, long axes of mandibular bodies and tooth rows nearly parallel and close together, mental region of the mandible not strongly convex, long axis of symphysis slopes markedly posteroinferiorly, canines with very long, robust roots, trigons of upper molars much wider than talons, and distal humerus with thick cortex enclosing small medullary cavity.
Australopithecus anamensis is distinguished from A. afarensis by the upper canine root, other dental characteristics and the associated facial skeleton. It can be distinguished from Ardipithecus ramidis by the absolutely and relatively thicker tooth enamel and other dental characteristics.
The Australopithecus anamensis tibia indicates bipedalism. These features were described by the authors as follows,
The authors concluded, "that bipedal locomotion had evolved at least
half a million years before the previous earliest evidence (the footprints
at Laetoli) suggests." Some similarity was noted with the Laetoli A.
afarensis, especially the large canines.
The Kanapoi sedimentary sequence includes conglomerates, sandstones and claystones, and molluscan packstones capped by basalt. The hominids from Allia Bay are from beneath or within the Moiti Tuff. Both sites provided tephra-bearing horizons suitable for single crystal laser fusion 40Ar-39AR dating. Paleoecology evidence indicates fauna associated with a river and gallery forest at Allia Bay. The Kanapoi fossils include fish and aquatic reptiles.
Australopithecus anamensis fossils were initially discovered in 1965, when the distal end of a humerus (KNM-KP 271) was recovered at Kanapoi. Fieldwork was conducted at the site nearly 30 years later, by Meave Leakey et. al., authors of the Nature article and the species.
Leakey, Meave G., Craig S. Feibel, Ian McDougall and Alan Walker. 1995. New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya. Nature 376:565-571.