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.

[10][11][12] Fossil evidence determines that Australopithecus anamensis is the earliest hominin species in the Turkana Basin,[13] but likely co-existed with afarensis towards the end of its existence.

[10] The first fossilized specimen of the species, although not recognized as such at the time, was a single fragment of humerus (arm bone) found in Pliocene strata in the Kanapoi region of West Lake Turkana by a Harvard University research team in 1965.

[18][16] In 1994, the 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. anamensis appears to have been habitually bipedal, although it retained some primitive features of its upper limbs.

[19] 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. anamensis, deriving its name from the Turkana word anam, meaning "lake".

[5] Although the excavation team did not find hips, feet or legs, Meave Leakey believes that Australopithecus anamensis often climbed trees.

Tree climbing was one behavior retained by early hominins until the appearance of the first Homo species about 2.5 million years ago.

[7] Specimens have been found between two layers of volcanic ash, dated to 4.17 and 4.12 million years, coincidentally when A. afarensis appears in the fossil record.

[21] These new fossils, sampled from a woodland context, include the largest hominid canine tooth yet recovered and the earliest Australopithecus femur.

[24] Other scientists (e.g. Alemseged, Kimbel, Ward, White) cautioned that one forehead bone fossil, which they viewed as not conclusively A. afarensis, should not be taken as disproving the possibility of anagenesis yet.

[11][23] In August 2019, scientists announced the discovery of MRD-VP-1/1, a nearly intact skull, for the first time, and dated to 3.8 million years ago, of A. anamensis in Ethiopia.

Through analysis of stable isotope data, it is believed that their environment had more closed woodland canopies surrounding Lake Turkana than are present today.

[30] The palate, rows of teeth, and other characteristics of A. anamensis dentition suggests that they were omnivores and their diets were composed heavily on fruit, similar to chimpanzees.

[33] Although considered to be the more primitive of the australopiths, A. anamensis had parts of the knee, tibia, and elbow that were different from apes, which indicates bipedalism as the species' form of locomotion.

[34] In addition to the modified body parts that indicate bipedalism, A. anamensis fossils show evidence of tree climbing.

[34] Both the forearms and finger bones of A. anamensis indicate a potential of utilizing the upper limbs as support when operating in trees or on the ground.

[35] The curved proximal hand phalanx of A. anamensis in the fossil record that contains strong ridges is indicative of its potential ability to climb.

[36] Fossil evidence reveals that A. 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.