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Genus of hominin ancestral to modern humans
the collective tribe

Australopithecus (,/ɒ//s//t//r//ə//l//ə/'/p//ɪ//θ//ᵻ//k//ə//s//, // //l//oʊ// /, OStrələPITHikəs, loh ; ) is a genus of early hominins that existed in Africa during the Pliocene and Early Pleistocene. The genera Homo (which includes modern humans), Paranthropus, and Kenyanthropus evolved from some Australopithecus species. Australopithecus is a member of the subtribe Australopithecina, which sometimes also includes Ardipithecus, though the term "australopithecine" is sometimes used to refer only to members of Australopithecus. Species include A. garhi, A. africanus, A. sediba, A. afarensis, A. anamensis, A. bahrelghazali and A. deyiremeda. Debate exists as to whether some Australopithecus species should be reclassified into new genera, or if Paranthropus and Kenyanthropus are synonymous with Australopithecus, in part because of the taxonomic inconsistency. at some time between 3 and 2 million years ago. Australopithecus possessed two of three duplicated genes derived from SRGAP2 roughly 3.4 and 2.4 million years ago (SRGAP2B and SRGAP2C), the second of which contributed to the increase in number and migration of neurons in the human brain. Significant changes to the hand first appear in the fossil record of later A. afarensis about 3 million years ago (fingers shortened relative to thumb and changes to the joints between the index finger and the trapezium and capitate).

The first Australopithecus specimen, the type specimen, was discovered in 1924 in a lime quarry by workers at Taung, South Africa. The specimen was studied by the Australian anatomist Raymond Dart, who was then working at the University of the Witwatersrand in Johannesburg. The fossil skull was from a three year old bipedal primate (nicknamed Taung Child) that he named Australopithecus africanus. The first report was published in Nature in February 1925. Dart realised that the fossil contained a number of humanoid features, and so he came to the conclusion that this was an early human ancestor. Later, Scottish paleontologist Robert Broom and Dart set out to search for more early hominin specimens, and several more A. africanus remains from various sites. Initially, anthropologists were largely hostile to the idea that these discoveries were anything but apes, though this changed during the late 1940s. However, the contrary view taken by Robinson in 1954, excluding australopiths from Homo, became the prevalent view. which has since been placed within A. africanus. Other fossil remains found in the same cave in 2008 were named Australopithecus sediba, which lived 1.9 million years ago. A. africanus probably evolved into A. sediba, which some scientists think may have evolved into H. erectus, though this is heavily disputed. In 2003, Spanish writer Camilo José Cela Conde and evolutionary biologist Francisco J. Ayala proposed resurrecting the genus Praeanthropus to house Orrorin, A. afarensis, A. anamensis, A. bahrelghazali, and A. garhi, but this genus has been largely dismissed.

With the apparent emergence of the genera Homo, Kenyanthropus, and Paranthropus in the genus Australopithecus, taxonomy runs into some difficulty, as the name of species incorporates their genus. According to cladistics, groups should not be left paraphyletic, where it is kept not consisting of a common ancestor and all of its descendants. Resolving this problem would cause major ramifications in the nomenclature of all descendent species. Possibilities suggested have been to rename Homo sapiens to Australopithecus sapiens (or even Pan sapiens), or to move some Australopithecus species into new genera. In 2002 and again in 2007, Cele Conde et al. suggested that A. africanus be moved to Paranthropus. It is debated whether or not A. bahrelghazali should be considered simply a western variant of A. afarensis instead of a separate species.

A. anamensis may have descended from or was closely related to Ardipithecus ramidus. A. anamensis shows some similarities to both Ar. ramidus and Sahelanthropus. According to the Chimpanzee Genome Project, the human–chimpanzee last common ancestor existed about five to six million years ago, assuming a constant rate of mutation. However, hominin species dated to earlier than the date could call this into question. Sahelanthropus tchadensis, commonly called "Toumai", is about seven million years old and Orrorin tugenensis lived at least six million years ago. Since little is known of them, they remain controversial among scientists since the molecular clock in humans has determined that humans and chimpanzees had a genetic split at least a million years later. One theory suggests that the human and chimpanzee lineages diverged somewhat at first, then some populations interbred around one million years after diverging. with an endocranial volume average of In modern populations, males are on average a mere 15% larger than females, while in Australopithecus, males could be up to 50% larger than females by some estimates. However, the degree of sexual dimorphism is debated due to the fragmentary nature of australopith remains. According to A. Zihlman, Australopithecus body proportions closely resemble those of bonobos (Pan paniscus), leading evolutionary biologist Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus. Furthermore, thermoregulatory models suggest that australopiths were fully hair covered, more like chimpanzees and bonobos, and unlike humans. The fossil record seems to indicate that Australopithecus is ancestral to Homo and modern humans. It was once assumed that large brain size had been a precursor to bipedalism, but the discovery of Australopithecus with a small brain but developed bipedality upset this theory. Nonetheless, it remains a matter of controversy as to how bipedalism first emerged. The advantages of bipedalism were that it left the hands free to grasp objects (e. g., carry food and young), and allowed the eyes to look over tall grasses for possible food sources or predators, but it is also argued that these advantages were not significant enough to cause the emergence of bipedalism. Earlier fossils, such as Orrorin tugenensis, indicate bipedalism around six million years ago, around the time of the split between humans and chimpanzees indicated by genetic studies. This suggests that erect, straight legged walking originated as an adaptation to tree dwelling. Major changes to the pelvis and feet had already taken place before Australopithecus. It was once thought that humans descended from a knuckle walking ancestor, but this is not well supported. Australopithecines have thirty two teeth, like modern humans. Their molars were parallel, like those of great apes, and they had a slight pre canine gap (diastema). Their canines were smaller, like modern humans, and with the teeth less interlocked than in previous hominins. In fact, in some australopithecines, the canines are shaped more like incisors. The molars of Australopithecus fit together in much the same way those of humans do, with low crowns and four low, rounded cusps used for crushing. They have cutting edges on the crests. Australopiths in general had thick enamel, like Homo, while other great apes have markedly thinner enamel. Microwear patterns on the cheek teeth of A. afarensis and A. anamensis indicate that A. afarensis predominantly ate fruits and leaves, whereas A. anamensis included grasses and seeds (in addition to fruits and leaves). The thickening of enamel in australopiths may have been a response to eating more ground bound foods such as tubers, nuts, and cereal grains with gritty dirt and other small particulates which would wear away enamel. Gracile australopiths had larger incisors, which indicates tearing food was important, perhaps eating scavenged meat. Nonetheless, the wearing patterns on the teeth support a largely herbivorous diet. In 2005, fossil animal bones with butchery marks dating to 2.6 million years old were found at the site of Gona, Ethiopia. This implies meat consumption by at least one of three species of hominins occurring around that time: A. africanus, A. garhi, and/or P. aethiopicus. In 2010, fossils of butchered animal bones dated 3.4 million years old were found in Ethiopia, close to regions where australopith fossils were found. Robust australopithecines (Paranthropus) had larger cheek teeth than gracile australopiths, possibly because robust australopithecines had more tough, fibrous plant material in their diets, whereas gracile australopiths ate more hard and brittle foods. In a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that robust australopiths ate predominantly fruit (frugivory). A study in 2018 found non carious cervical lesions, caused by acid erosion, on the teeth of A. africanus, probably caused by consumption of acidic fruit.

It is debated if the Australopithecus hand was anatomically capable of producing stone tools. A. garhi was associated with large mammal bones bearing evidence of processing by stone tools may indicate australopithecine tool production. Stone tools dating to roughly the same time as A. garhi (about 2.6 mya) were later discovered at the nearby Gona and Ledi Geraru sites, but the appearance of Homo at Ledi Geraru (LD 350 1) casts doubt on australopithecine authorship. In 2010, cut marks dating to 3.4 mya on a bovid leg were found at the Dikaka site, which were at first attributed to butchery by A. afarensis, but because the fossil came from a sandstone unit (and were modified by abrasive sand and gravel particles during the fossilisation process), the attribution to butchery is dubious. In 2015, the Lomekwi culture was discovered at Lake Turkana dating to 3.3 mya, possibly attributable to Kenyanthropus or A. deyiremeda.