Australopithecus

***Australopithecus*** (pronounced ***AW**-struh-loh-**PITH**-i-kuhs*; from Latin *australis* "southern", Greek πίθηκος *pithekos* "ape") is a genus of hominins that existed in Africa from around 4.2 to 1.9 million years ago and from which the *Homo* genus, including modern humans, is considered to be descended. *Australopithecus* is a member of the subtribe Australopithecina, which includes *Paranthropus*, *Kenyanthropus*, *Ardipithecus* and *Praeanthropus*, though the term "australopithecine" is sometimes used to refer only to members of *Australopithecus*.

From palaeontological and archaeological evidence, *Australopithecus* apparently evolved in eastern Africa around 4.2 million years ago before spreading throughout the continent and eventually becoming extinct 1.9 million years ago (or 1.2 million years ago if *Paranthropus* is included). While none of the groups normally directly assigned to this group survived, *Australopithecus* does not appear to be literally extinct (in the sense of having no living descendants), as the *Homo* genus probably emerged from late *Australopithecus* species such as *Australopithecus garhi*, *Australopithecus africanus* and/or *Australopithecus sediba*. During that time, a number of australopithecine species emerged, including the aforementioned three species as well as *Australopithecus afarensis, Australopithecus anamensis, Australopithecus bahrelghazali* and *Australopithecus deyiremeda* (proposed).

Some debate exists as to whether other hominid species of this time, such as *Paranthropus robustus*, *Paranthropus boisei* and *Paranthropus aethiopicus* (considered 'robust australopiths'), belong to a separate genus or *Australopithecus* (considered 'gracile australopiths)'. The genus name *Paranthropus* was proposed by South African palaeontologist Robert Broom upon the discovery of *P. robustus* in 1938. Occasional suggestions have been made (by Cele-Conde *et al.* 2002 and 2007) that *A. africanus* should also be moved to *Paranthropus*. On the basis of craniodental evidence, Strait and Grine (2004) suggest that *Australopithecus* is paraphyletic and that *A. anamensis* and *A. garhi* should be assigned to new genera. Within *Australopithecus*, numerous researchers such as Leakey (2001), Ward (2001), White (2002), Alemseged (2006) and Kimbel (2009) suggest that *A. bahrelghazali* is simply a western version of *A. afarensis* and not a separate species.

*Australopithecus* species played a significant part in human evolution, with most scientists in the field believing *Homo* was derived from *Australopithecus* 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). One of the australopith species evolved into the genus *Homo* in Africa (perhaps *Homo habilis* – for instance, in January 2019, scientists reported that *A. sediba* is distinct from but shares anatomical similarities to both the older *A. africanus* and the younger *H. habilis*) and from early *Homo* species eventually into modern humans, *H. sapiens sapiens*.

## Evolution

Australopiths (A. anamensis) are probably descended from or closely related to Ardipithecus ramidus.. Some features of A. anamensis show similarities to features of both Ardipithecus ramidus (wide diastema, post-orbital constriction) and Sahelanthropus tchadensis (post-orbital constriction, shape of its mid-face and neurocranium), but also some dissimilarities.

Gracile australopiths shared several traits with modern apes and humans, and were widespread throughout Eastern and Northern Africa around 3.5 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania. This site contains hominid footprints that are remarkably similar to those of modern humans and have been dated to as old as 3.6 million years. 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.

*Australopithecus anamensis*, *A. afarensis*, and *A. africanus* are among the most famous of the extinct hominins. *A. africanus* was once considered to be ancestral to the genus *Homo* (in particular *Homo erectus*). However, fossils assigned to the genus *Homo* have been found that are older than *A. africanus*. Thus, the genus *Homo* either split off from the genus *Australopithecus* at an earlier date (the latest common ancestor being either *A. afarensis* or an even earlier form, possibly *Kenyanthropus*), or both developed from a yet possibly unknown common ancestor independently.

According to the Chimpanzee Genome Project, the human (*Ardipithecus*, *Australopithecus* and *Homo*) and chimpanzee (*Pan troglodytes* and *Pan paniscus*) lineages diverged from a common ancestor about five to six million years ago, assuming a constant rate of evolution. It is theoretically more likely for evolution to happen more slowly, as opposed to more quickly, from the date suggested by a gene clock (the result of which is given as a youngest common ancestor, i.e., the latest possible date of divergence.) However, hominins discovered more recently are somewhat older than the presumed rate of evolution would suggest.

*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.

## Morphology

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 c.c. Although this is more than the average endocranial volume of chimpanzee brains (360 c.c.) the earliest Australopiths (A. anamensis) appear to have been within the chimpanzee range, whereas some later Australopith fossils 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. In several species there is a considerable degree of sexual dimorphism, males being larger than females. Modern humans do not display the same degree of sexual dimorphism as *Australopithecus* appears to have. 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. New research suggests, however, that australopithecines exhibited a lesser degree of sexual dimorphism than these figures suggest, but the issue is not settled.

According to one scholar, A. Zihlman, *Australopithecus* body proportions closely resemble those of bonobos (*Pan paniscus*), leading evolutionary biologists such as Jeremy Griffith to suggest that bonobos may be phenotypically similar to *Australopithecus*. Furthermore, thermoregulatory models suggest that Australopithecus species were fully hair covered, more like chimpanzees and bonobos, and unlike humans.

## Species variations

Opinions differ as to whether the species *A. aethiopicus*, *A. boisei*, and *A. robustus* should be included within the genus *Australopithecus*, and no current consensus exists as to whether they should be placed in a distinct genus, *Paranthropus*, which is suggested along with Homo to have developed as part of a clade with A. africanus as its basal root. Until the last half-decade, the majority of the scientific community included all the species shown in the box at the top of this article in a single genus. The postulated genus *Paranthropus* was morphologically distinct from *Australopithecus*, and its specialized morphology implies that its behaviour may have been quite different from that of its ancestors, although it has been suggested that the distinctive features of *A. aethiopicus*, *A. boisei*, and *A. robustus* may have evolved independently. In reviewing both the literature and fossil record in 2007 Wood and Constantino recognised two competing hypotheses, one that aethiopicus, boisei and robustus are a distinct clade with boisei and robustus evolving from aethiopicus. The second is that there was some parallel evolution of two distinct lines, with boisei evolving from aethiopicus, but robustus evolving separately from A. Africanus. (They also acknowledge that Skelton & McHenry in 1992 proposed that aethiopicus is separate from a boisei/robustus clade).

## Evolutionary role

The fossil record seems to indicate that *Australopithecus* is the common ancestor of the distinct group of hominids now called *Paranthropus* (the "robust australopiths"), and most likely the genus *Homo*, which includes modern humans. Although the intelligence of these early hominids was likely no more sophisticated than in modern apes, the bipedal stature is the key element that distinguishes the group from previous primates, which were quadrupeds. The morphology of *Australopithecus* upset what scientists previously believed — namely, that strongly increased brain size had preceded bipedalism.

If *A. afarensis* was the definite hominid that left the footprints at Laetoli, that strengthens the notion that *A. afarensis* had a small brain, but was a biped. Fossil evidence such as this makes it clear that bipedalism far predated large brains. However, it remains a matter of controversy as to how bipedalism first emerged (several concepts are still being studied). 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. However, many anthropologists argue that these advantages were not large enough to cause the emergence of bipedalism.

A recent study of primate evolution and morphology noted that all apes, both modern and fossil, show skeletal adaptations to erect posture of the trunk, and that 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 suggested that erect, straight-legged walking originated as an adaptation to tree-dwelling. Studies of modern orangutans in Sumatra have shown that these apes use four legs when walking on large, stable branches, and swing underneath slightly smaller branches, but are bipedal and keep their legs very straight when walking on multiple flexible branches under 4 cm diameter, while also using their arms for balance and additional support. This enables them to get nearer to the edge of the tree canopy to get fruit or cross to another tree.

The ancestors of gorillas and chimpanzees are suggested to have become more specialised in climbing vertical tree trunks, using a bent hip and bent knee posture that matches the knuckle-walking posture they use for ground travel. This was due to climate changes around 11 to 12 million years ago that affected forests in East and Central Africa, so periods occurred when openings prevented travel through the tree canopy, and at these times, ancestral hominids could have adapted the erect walking behaviour for ground travel. Humans are closely related to these apes, and share features including wrist bones apparently strengthened for knuckle-walking.

However, the view that human ancestors were knuckle-walkers is now questioned since the anatomy and biomechanics of knuckle-walking in chimpanzees and gorillas are different, suggesting that this ability evolved independently after the last common ancestor with the human lineage. Further comparative analysis with other primates suggests that these wrist-bone adaptations support a palm-based tree walking.

Radical changes in morphology took place before gracile australopiths evolved; the pelvis structure and feet are very similar to modern humans. The teeth have small canines, but australopiths generally evolved a larger postcanine dentition with thicker enamel.

Most species of *Australopithecus* were not any more adept at tool use than modern nonhuman primates, yet modern African apes, chimpanzees, and most recently gorillas, have been known to use simple tools (i.e. cracking open nuts with stones and using long sticks to dig for termites in mounds), and chimpanzees have been observed using spears (not thrown) for hunting.

For a long time, no known stone tools were associated with *A. afarensis*, and paleoanthropologists commonly thought that stone artifacts only dated back to about 2.5 million years ago. However, a 2010 study suggests the hominin species ate meat by carving animal carcasses with stone implements. This finding pushes back the earliest known use of stone tools among hominins to about 3.4 million years ago.

In a pair of companion papers, the discoverers of *A. garhi* note that it is possible A. garhi used stone tools, as remains of this species and butchered animal remains were found close to each other.

## Dentition

Australopithecines have thirty two teeth, like modern humans, but with an intermediate formation; between the great apes and humans. Their molars were parallel, like those of great apes, and they had a slight pre-canine diastema. But, 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 human's do, with low crowns and four low, rounded cusps used for crushing. They have cutting edges on the crests.

Robust australopithecines (like *A. boisei* and *A. robustus*) had larger cheek, or buccal, teeth than the smaller – or gracile – species (like *A. afarensis* and *A. africanus*). It is possible that they had more tough, fibrous plant material in their diets while the smaller species of *Australopithecus* had more meat. But it is also possibly due to their generally larger build requiring more food. Their larger molars do support a slightly different diet, including some hard food.

Australopithecines also had thick enamel, like those in genus *Homo*, while other great apes have markedly thinner enamel. One explanation for the thicker enamel is that these hominins were living more on the ground than in the trees and were foraging for tubers, nuts, and cereal grains. They would also have been eating a lot of gritty dirt with the food, which would wear at enamel, so thicker enamel would be advantageous. Or, it could simply indicate a change in diet. Robust australopithecines wore their molar surfaces down flat, unlike the more gracile species, who kept their crests, which certainly seems to suggest a different diet. The gracile *Australopithecus* had larger incisors, which indicates tearing and more meat in the diet, likely scavenged. The wear patterns on the tooth surfaces support a largely herbivorous diet.

When we examine the buccal microwear patterns on the teeth of *A. afarensis* and *A. anamensis*, we see that *A. afarensis* did not consume a lot of grasses or seeds, but rather ate fruits and leaves, but *A. anamensis* did eat grasses and seeds in addition to fruits and leaves.

## Diet

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In a 1979 preliminary microwear study of *Australopithecus* fossil teeth, anthropologist Alan Walker theorized that robust australopiths were largely frugivorous. *Australopithecus* species mainly ate fruit, vegetables, small lizards, and tubers. 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.

These observations have been interpreted as evidence that *P. robustus* may have fed on hard and brittle foods, such as some nuts and seeds. More recently, new analyses based on three-dimensional renderings of wear facets have confirmed earlier work, but have also suggested that *P. robustus* ate hard foods primarily as a fallback resource, while *A. africanus* ate more mechanically tough foods. A recent study looking at enamel fractures suggests *A. africanus* actually ate more hard foods than *P. robustus*, with double the frequency of antemortem chips.

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, fossils of animal bones with butchery marks dating 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.

A study in 2018 found non-carious cervical lesions, caused by acid erosion, on the teeth of *A. africanus* suggesting the individual ate a lot of acidic fruits.

## History of study

Main articles: Taung Child
The type specimen for genus *Australopithecus* 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 ancestor of humans. Later, Scottish paleontologist Robert Broom and Dart set about to search for more early hominin specimens, and at several sites they found more *A. africanus* remains, as well as fossils of a species Broom named *Paranthropus* (which would now be recognised as *P. robustus*). Initially, anthropologists were largely hostile to the idea that these discoveries were anything but apes, though this changed during the late 1940s.. By 1950, Mayr was treating *Australopithecus* as a species of Homo, *Homo transvaalensis*, on the grounds that all bipedal apes should be treated as part of Homo. However, the contra view taken by Robinson in 1954, excluding Australopiths from Homo, became the prevalent view in the 1950s.

The first australopithecine discovered in eastern Africa was a skull belonging to an *A. boisei* that was excavated in 1959 in the Olduvai Gorge in Tanzania by Mary Leakey. Since then, the Leakey family have continued to excavate the gorge, uncovering further evidence for australopithecines, as well as for *Homo habilis* and *Homo erectus*. The scientific community took 20 years to widely accept *Australopithecus* as a member of the family tree.

Then, 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.

*A. afarensis*, *A. anamensis*, and *A. bahrelghazali* were split off into the genus *Praeanthropus*, but this genus been largely dismissed.

## Phylogeny

A taxonomy of the *Australopithecus* within the great apes is assessed as follows, with *Paranthropus* and *Homo* emerging among the *Australopithecus*. The genus *Australopithecus* with conventional definitions is assessed to be highly paraphyletic, i.e. it is not a natural group, and the genera *Kenyanthropus*, *Paranthropus* and *Homo* are included. The exact phylogeny within *Australopithecus* is still highly controversial. Approximate radiation dates of daughter clades is shown in Millions of years ago (Mya). Sahelanthropus, *Orrorin,* and Ardipithecus, possibly sisters to *Australopithecus*, are not shown here.

## Inconsistent taxonomy

Even though *Australopithecus* is classified as a "genus", several other genera appear to have emerged within it: *Homo, Kenyanthropus* and *Paranthropus*. This genus is thus regarded as an entrenched paraphyletic wastebasket taxon. Resolving this into monophyletic groupings would require extensive renaming of species in the binomial nomenclature. Possibilities suggested have been to rename *Homo sapiens* to *Australopithecus sapiens* (or even *Pan sapiens*), or to rename all the *Australopithecus* species.

## Notable specimens

* KT-12/H1, an *A. bahrelghazali* mandibular fragment, discovered 1995 in Sahara, Chad
* AL 129-1, an *A. afarensis* knee joint, discovered 1973 in Hadar, Ethiopia
* Karabo, a juvenile male *A. sediba*, discovered in South Africa
* Laetoli footprints, preserved hominin footprints in Tanzania
* Lucy, a 40%-complete skeleton of a female *A. afarensis*, discovered 1974 in Hadar, Ethiopia
* Selam, remains of a three-year-old *A. afarensis* female, discovered in Dikika, Ethiopia
* STS 5 (Mrs. Ples), the most complete skull of an *A. africanus* ever found in South Africa
* STS 14, remains of an *A. africanus*, discovered 1947 in Sterkfontein, South Africa
* STS 71, skull of an *A. africanus*, discovered 1947 in Sterkfontein, South Africa
* Taung Child, skull of a young *A. africanus*, discovered 1924 in Taung, South Africa

## Gallery

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.Cast of the skeleton of Lucy, an *A. afarensis*Skull of the Taung child