Australopithecus anamensis is a hominin species that lived roughly between 4.3 and 3.8 million years ago, and is the oldest known Australopithecus species.

Nearly 100 fossil specimens of A. anamensis are known from Kenya and Ethiopia, representing over 20 individuals. The first fossils of A. anamensis discovered are dated to around 3.8 and 4.2 million years ago and were found in Kanapoi and Allia Bay in northern Kenya.

A. afarensis is normally accepted to have emerged within this lineage. However, A. anamensis and A. afarensis appear to have lived side-by-side for at least some period of time, and whether the lineage that led to extant humans emerged in A. afarensis, or directly in A. anamensis is not fully settled.

Fossil evidence determines that Australopithecus anamensis is the earliest hominin species in the Turkana Basin, but likely co-existed with afarensis towards the end of its existence. A. anamensis and A. afarensis may be treated as a single grouping.

Preliminary analysis of the sole upper cranial fossil indicates A.&nbsp;anamensis had a smaller cranial capacity (estimated 365–370 cm<sup>3</sup>) than A.&nbsp;afarensis. Patterson and colleagues subsequently revised their estimation of the specimen's age to 4.0–4.5 mya based on faunal correlation data.

In 1994, London-born Kenyan paleoanthropologist Meave Leakey and archaeologist Alan Walker excavated the Allia Bay site and uncovered several additional fragments of the hominid, including one complete lower jaw bone which closely resembles that of a common chimpanzee (Pan troglodytes), but whose teeth bear a greater resemblance to those of a human. Based on the limited postcranial evidence available, A.&nbsp;anamensis appears to have been habitually bipedal, although it retained some primitive features of its upper limbs.

In 1995, Meave Leakey and her associates, taking note of differences between Australopithecus afarensis and the new finds, assigned them to a new species, A.&nbsp;anamensis, deriving its name from the Turkana word anam, meaning "lake". with findings in the 2000s from stratigraphic sequences dating to about 4.1–4.2 million years ago. These new fossils, sampled from a woodland context, include the largest hominid canine tooth yet recovered and the earliest Australopithecus femur.

In 2010, journal articles were published by Yohannes Haile-Selassie and others describing the discovery of around 90 fossil specimens in the time period 3.6 to 3.8 million years ago (mya), in the Afar area of Ethiopia, filling in the time gap between A.&nbsp;anamensis and Australopithecus afarensis and showing a number of features of both. This supported the idea (proposed for instance by Kimbel et al. in 2006 The skull itself was found by Afar herder Ali Bereino in 2016. Other scientists (e.g. Alemseged, Kimbel, Ward, White) cautioned that one forehead bone fossil, which they viewed as not conclusively A.&nbsp;afarensis, should not be taken as disproving the possibility of anagenesis yet. The skull itself was found by Afar herder Ali Bereino in 2016.

Environment

Australopithecus anamensis was found in Kenya, specifically at Allia Bay, East Turkana. Through analysis of stable isotope data, their environment is believed to have been more closed woodland canopies surrounding Lake Turkana than are present today. The greatest density of woodlands at Allia Bay was along the ancestral Omo River. More open savanna was thought to exist in the basin margins or uplands. Similarly at Allia Bay, the environment was suggested as much wetter. While not definitive, nut or seed-bearing trees could have been present at Allia Bay, but more research is needed.

Diet

Studies of the occlusal dental microwear on A.&nbsp;anamensis molar fossils show a pattern of long striations. This pattern is similar to the microwear on the molars of gorillas, suggesting that A.&nbsp;anamensis had a similar diet to that of the modern gorilla. The microwear patterns are consistent on all A.&nbsp;anamensis molar fossils regardless of location or time. This shows that their diet largely remained the same no matter what their environment. The signal derived from A.&nbsp;anamensis buccal microwear is consistent with feeding on hard and brittle foodstuffs.

The earliest dietary isotope evidence in Turkana Basin hominin species comes from the A. anamensis. This evidence suggests that their diet consisted primarily of C3 resources, possibly with a small amount of C4-derived resources. Within the next 1.99- to 1.67-million-year period, at least two distinctive hominin taxa shifted to a higher level of C4-resource consumption. At this point, no cause for this shift in diet is known. This research does not by itself indicate a plant-based diet, because the isotopes can be ingested by eating animals and insects that fed on C3 and C4 resources. The thicker tooth enamel of A.&nbsp;anamensis suggests that it consumed harder foods than the earlier Ar.&nbsp;ramidus, which may have been an omnivore similar to chimpanzees.

All Australopithecus species were bipedal and small-brained, and had large teeth. These similarities include thick tooth enamel, which is a shared derived trait of all Australopithecus species, and shared with most Miocene hominoids. Although considered to be the more primitive of the australopiths, A.&nbsp;anamensis had parts of the knee, tibia, and elbow that were different from apes, which indicates bipedalism as the species' form of locomotion.

In addition to the modified body parts that indicate bipedalism, A.&nbsp;anamensis fossils show evidence of tree climbing. Archeological finds indicate that A.&nbsp;anamensis had long forearms, as well as modified features of the wrist bone. Forearm bones belonging to A.&nbsp;anamensis have been found to be 265 to 277&nbsp;mm in length.

Fossil evidence reveals that A.&nbsp;anamensis had a somewhat wide jaw joint that was flat from front to back, which resembles a curvature similar to those seen in great apes. Furthermore, the ear canals of A.&nbsp;anamensis fossils are narrow in diameter. The ear canal most resembles that of chimpanzees and is contrasting to the wide ear canals of both later Australopithecus and Homo.

See also

  • List of human evolution fossils

References

  • Human Timeline (Interactive) – Smithsonian, National Museum of Natural History (August 2016).