|Mrs. Ples, an Australopithecus africanus specimen|
R.A. Dart, 1925
Classically excluded but cladistically included:
Australopithecus (//, OS-trə-lə-PITH-i-kəs; from Latin australis 'southern', and Ancient Greek πίθηκος (pithekos) 'ape') is a genus of early hominins that existed in Africa during the Late Pliocene and Early Pleistocene. The genus Homo (which includes modern humans) emerged within Australopithecus, as sister to e.g. Australopithecus sediba. Also the genera Paranthropus and Kenyanthropus emerged within Australopithecus. 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.
The earliest known member of the genus, A. anamensis, existed in eastern Africa around 4.2 million years ago. Australopithecus fossils become more widely dispersed throughout eastern and southern Africa (the Chadian A. bahrelghazali indicates the genus was much more widespread than the fossil record suggests), before eventually becoming pseudo-extinct 1.9 million years ago (or 1.2 to 0.6 million years ago if Paranthropus is included). While none of the groups normally directly assigned to this group survived, Australopithecus gave rise to living descendants, as the genus Homo emerged from an Australopithecus species 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 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. In 1950, evolutionary biologist Ernst Walter Mayr said that all bipedal apes should be classified into the genus Homo, and considered renaming Australopithecus to Homo transvaalensis. However, the contrary view taken by Robinson in 1954, excluding australopiths from Homo, became the prevalent view. The first australopithecine fossil discovered in eastern Africa was an A. boisei skull excavated by Mary Leakey in 1959 in Olduvai Gorge, Tanzania. Since then, the Leakey family has continued to excavate the gorge, uncovering further evidence for australopithecines, as well as for Homo habilis and Homo erectus. The scientific community took 20 more years to widely accept Australopithecus as a member of the human family tree.
In 1997, an almost complete Australopithecus skeleton with skull was found in the Sterkfontein caves of Gauteng, South Africa. It is now called "Little Foot" and it is around 3.7 million years old. It was named Australopithecus prometheus 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. On the basis of craniodental evidence, Strait and Grine (2004) suggest that A. anamensis and A. garhi should be assigned to new genera. It is debated whether or not A. bahrelghazali should be considered simply a western variant of A. afarensis instead of a separate species.
Australopiths shared several traits with modern apes and humans, and were widespread throughout Eastern and Northern Africa by 3.5 million years ago (MYA). The earliest evidence of fundamentally bipedal hominins is a 3.6 MYA fossil trackway in Laetoli, Tanzania, which bears a remarkable similarity to those of modern humans. The footprints have generally been classified as australopith, as they are the only form of prehuman hominins known to have existed in that region at that time.
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.
The brains of most species of Australopithecus were roughly 35% of the size of a modern human brain with an endocranial volume average of 466 cc (28.4 cu in). Although this is more than the average endocranial volume of chimpanzee brains at 360 cc (22 cu in) the earliest australopiths (A. anamensis) appear to have been within the chimpanzee range, whereas some later australopith specimens have a larger endocranial volume than that of some early Homo fossils.
Most species of Australopithecus were diminutive and gracile, usually standing 1.2 to 1.4 m (3 ft 11 in to 4 ft 7 in) tall. It is possible that they exhibited a considerable degree of sexual dimorphism, males being larger than females. 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. One paper finds that A. afarensis had a level of dimorphism close to modern humans.
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 Australopithicus 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. However, australopiths generally evolved a larger postcanine dentition with thicker enamel. Australopiths in general had thick enamel, like Homo, while other great apes have markedly thinner enamel. Robust australopiths wore their molar surfaces down flat, unlike the more gracile species, who kept their crests.
Australopithecus species are thought to have eaten mainly fruit, vegetables, and tubers, and perhaps easy-to-catch animals such as small lizards. Much research has focused on a comparison between the South African species A. africanus and Paranthropus robustus. Early analyses of dental microwear in these two species showed, compared to P. robustus, A. africanus had fewer microwear features and more scratches as opposed to pits on its molar wear facets. 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 1992, trace-element studies of the strontium/calcium ratios in robust australopith fossils suggested the possibility of animal consumption, as they did in 1994 using stable carbon isotopic analysis. 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. However, such divergence in chewing adaptations may instead have been a response to fallback food availability. In leaner times, robust and gracile australopithecines may have turned to different low-quality foods (fibrous plants for the former, and hard food for the latter), but in more bountiful times, they had more variable and overlapping diets. In a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that robust australopiths ate predominantly fruit (frugivory).
It was once thought that Australopithecus could not produce tools like Homo, but the discovery of A. garhi associated with large mammal bones bearing evidence of processing by stone tools showed this to not have been the case. Discovered in 1994, this was the oldest evidence of manufacturing at the time until the 2010 discovery of cut marks dating to 3.4 mya attributed to A. afarensis, and the 2015 discovery of the Lomekwi culture from Lake Turkana dating to 3.3 mya possibly attributed to Kenyanthropus. More stone tools dating to about 2.6 mya in Ledi-Geraru in the Afar Region were found in 2019, though these may be attributed to Homo.
In January 2009, four fossils were discovered through the efforts of the Dikika Research Project working in the Lower Awash Valley in north central Ethiopia. Two of the fossils displayed traces of cut marks (DIK-55-2, a right rib fragment of a large ungulate & DIK-55-3, a femur shaft fragment of a young bovid). Research team members stated that fossilization dated after the formation of cut marks, evidence proved by the Secondary electron imaging (SEI) and energy dispersive X-ray (EDX) spectrometry data. Through the optical and environmental scanning electron microscopy (ESEM) observations, the research team also claimed that they could limit the possibility of trampling and biochemical causes, and attributed the cut marks to the effect of cutting and percussion, which indicating the use of stone tools. Considering the contemporaneity and the discovery of Australopithecus afarensis specimen on the same site (e.g. DIK-1-1), most of the paleoanthropologists attributed this behavior to Australopithecus afarensis.
In 2015, another research team published a paper stating that the ability of tool use of Australopithecus afarensis could be confirmed by its trabecular bone pattern. This research team argued that there had been too much dependence on the examination of external morphology in the study of paleoanthropology and that sometimes evidence from external morphology could be ambiguous. Some traits could be ancestral and no longer serve a purpose. In the case of the trabecular bone, it changes through life according to mechanical loading, therefore it could be a good hand morphology reference for Australopithecus afarensis on the matter of appropriate hand postures for tool use. After the comparison of Hylobatids, Pongo, Gorilla, Pan, recent H. sapiens and early H. sapiens, this team concluded that a complete suite of derived later Homo-like hand morphology was not necessary for using tools, and the trabecular orientation of Australopithecus afarensis metacarpals was clearly shaped by the use of tools.
There are debates, however, regarding whether the Australopithecus afarensis was simply a tool user or a craftsman. One research team in 2017 claimed that the Australopithecus afarensis did not have a forceful and precise grip that was adequate for tool making. This team focused their study on the 5th ray of Australopithecus afarensis, a trait which is important for the ability of hand to grip. The morphology of hamate-metacarpal V joint (CMC-V) could determine the flexibility of the 5th ray and the formation of a powerful grip, through the simulation of musculoskeletal models (with the presence of certain technical and informational limitations), evidence showed the pulp of the 5th ray did not provide the necessary conditions for the maintenance of a large-sized object, thus making the hypothesis of tool maker Australopithecus afarensis questionable.
The spot where the first Australopithecus boisei was discovered in Tanzania.
Original skull of Mrs. Ples, a female A. africanus
Taung Child by Cicero Moraes, Arc-Team, Antrocom NPO, Museum of the University of Padua.
Forms such as Ardipithecus, Sahelanthropus, and Orrorin have also been admitted to the pantheon, though this has clearly been facilitated by their great age. And in a nod to history, the venerable genus Paranthropus has been grandfathered in for use by those who think it useful. But except for the widely dismissed revival of Praeanthropus, there has been little real rethinking of the hugely minimalist hominid taxonomy, generic as well as specific, that Mayr foisted on us all those years ago...