Australopithecus

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Australopithecus "gracile australopithecine" Temporal range: Pliocene–Pleistocene, 3.9–1.7Ma PreЄ Є O S D C P T J K Pg N ↓ Descendant taxon Homo survives to present
Australopithecus afarensis
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Primates
Family:	Hominidae
Tribe:	Hominini
Subtribe:	†Australopithecina
Genus:	†Australopithecus R.A. Dart, 1925
Type species
†Australopithecus africanus Dart, 1925
Species
†A. africanus †A. garhi †A. sediba Also called Paranthropus †P. aethiopicus †P. robustus †P. boisei Also called Praeanthropus †A. afarensis †A. anamensis †A. bahrelghazali

Australopithecus (pronounced AW-struh/strey-loh-PITH-i-kuhs, BE: [ˌɒstrələ(ʊ)ˈpɪθɪkəs] AE [ˌɑ:strəloʊˈpɪθɪkəs]; from Latin australis "southern", Greek πίθηκος pithekos "ape") is an extinct genus of hominids. From the evidence gathered by palaeontologists and archaeologists, it appears that the Australopithecus genus evolved in eastern Africa around four million years ago before spreading throughout the continent and eventually becoming extinct two million years ago. During this time period a number of australopith species emerged, including Australopithecus afarensis, A. africanus, A. anamensis, A. bahrelghazali, A. garhi and A. sediba.

Academics still debate whether certain African hominid species of this time, such as A. robustus and A. boisei, constitute members of the same genus. If so, they would be considered robust australopiths while the others would be gracile australopiths. However, if these species do constitute their own genus, they may be given their own name, Paranthropus.

Archaeologists and palaeontologists widely hold that the australopiths played a significant part in human evolution, being the first of the hominins to show presence of a gene that causes increased length and ability of neurons in the brain, the duplicated SRGAP2 gene.^[1] One of the australopith species eventually evolved into the Homo genus in Africa around two million years ago, which contained within it species like Homo habilis, H. ergaster, and eventually the modern human species, H. sapiens sapiens.^[2]

Evolution

Map of the fossil sites of the early Australopithecines in Africa.

A reconstruction of Australopithecus afarensis.

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.^[3] The footprints have generally been classified as australopith because that is the only form of prehuman known to have existed in that region at that time.

Skulls of
1. Gorilla 2. Australopithecus 3. Homo erectus 4. Neanderthal (La Chapelle aux Saints) 5. Steinheim Skull 6. Modern Homo sapiens

Australopithecus anamensis, Australopithecus afarensis and Australopithecus africanus are among the most famous of the extinct hominins. A. africanus were once regarded as 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 A. afarensis or an even earlier form, possibly Kenyanthropus platyops), or both developed from a yet possibly unknown common ancestor independently.

According to the Chimpanzee Genome Project, both 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, if we assume 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 molecular clock would theorize.^[4]

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.^[4]

Morphology

Original skull of Mrs. Ples, a female Australopithecus africanus.

Taung Child by Cicero Moraes, Arc-Team, Antrocom NPO, Museum of the University of Padua

The brains of most species of Australopithecus were roughly 35% of the size of that of a modern human brain. 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 variations of Australopithecus there is a considerable degree of sexual dimorphism, in this case males being larger than females.^[5]

According to A. Zihlman, Australopithecus body proportions closely resemble those of bonobos (Pan paniscus),^[6] leading evolutionary biologists like Jeremy Griffith to suggest that bonobos may be phenotypically similar to Australopithecus.^[7]

Modern hominids do not appear to display sexual dimorphism to the same degree — particularly, modern humans display a low degree of sexual dimorphism, with males being only 15% larger than females, on average. In Australopithecus, however, males can be up to 50% larger than females. New research suggests that sexual dimorphism may be less pronounced than this, but there is still debate on the subject.^[5]

Species variations

Opinions differ as to whether the species aethiopicus, boisei, and robustus should be included within the genus Australopithecus, and there is no current consensus as to whether they should be placed in a distinct genus, Paranthropus, which is suggested to have developed from the ancestral Australopithecus line.^{[citation needed]} Up 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 aethiopicus, "boisei, and robustus may have evolved independently.

Evolutionary role

Cast of the skeleton of Lucy, an Australopithecus afarensis.

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. Though the intelligence of these early hominids was likely no more sophisticated than modern apes, the bipedal stature is the key evidence that distinguishes the group from previous primates, who were quadrupeds. The morphology of Australopithecus upsets what scientists previously believed—namely, that large brains 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 how bipedalism first emerged (several concepts are still being studied). The advantages of bipedalism left 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.

Skull of the Taung child.

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 showed that these apes use four legs when walking on large stable branches, swing underneath slightly smaller branches, but are bipedal and keep their legs very straight when walking on multiple small 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.^[8]

It is suggested that the ancestors of gorillas and chimpanzees became 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 eleven to twelve million years ago that affected forests in East and Central Africa so that there were periods 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.^[9]

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 this ability evolved independently after the last common ancestor with the human lineage.^[10] Further comparative analysis with other primates suggests these wrist bone adaptations support a palm based tree walking.^[10]

Radical changes in morphology took place before gracile australopiths evolved; the pelvis structure and feet are very similar to modern humans.^[11] The teeth have small canines, but australopiths generally evolved a larger post-canine dentition with thicker enamel.^[12]

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.

However, some have argued that A. garhi used stone tools due to a loose association of this species and butchered animal remains.

Diet

In a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that robust australopiths were largely frugivorous.^[13] Australopithecus mainly ate fruit, vegetables, and tubers. Much research has focused on a comparison between the South African species Australopithecus africanus and Paranthropus robustus. Early analyses of dental microwear in these two species showed that compared to Paranthropus robustus, Australopithecus africanus had fewer microwear features and more scratches as opposed to pits on its molar wear facets.^[14]

These observations have been interpreted as evidence that Paranthropus robustus may have fed on hard and brittle foods like some nuts and seeds.^[14] More recently new analyses based on three-dimensional renderings of wear facets have confirmed earlier work but have also suggested that Paranthropus robustus ate hard foods primarily as a fallback resource while Australopithecus africanus ate more mechanically tough foods.^[15]

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.^[16]

In 2005, fossils of animal bones with butchery marks dating 2.6 million years old were found at the site of Gona, Ethiopia. There is fossil evidence of meat consumption for at least three species of hominins occurring at around 2.6-2.5 Ma: Australopithecus africanus, Australopithecus garhi, and Paranthropus aethiopicus.^[17]

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.^[18]

History of study

The first australopithecine discovered and documented was a fossil of a three year old Australopithecus africanus discovered 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 who published his findings in Nature magazine in February 1925. Dart realised that the fossil contained a number of humanoid features, and so came to the conclusion that this was an early ancestor of humans.^[19]

Ten years later, he and the Scottish paleontologist Robert Broom set about to search for more early hominin specimens, and at several sites they found further A. africanus remains as well as fossils of a species Broom named Paranthropus (which would now be recognised as Paranthropus robustus). Initially, anthropologists were largely hostile to the idea that these discoveries were anything but apes, though this changed during the latter years of the 1940s.^[19]

The first australopithecine discovered in eastern Africa was a skull belonging to an Australopithecus 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.^[19]

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 probably around 3 million years old. It belongs to Australopithecus prometheus^[20][21] Also in the same cave scientists have recently discovered a new australopithecine in South Africa. The fossils of Australopithecus sediba, which lived 1.9 million years ago, were found in Malapa cave in South Africa. It is thought Australopithecus africanus probably gave rise to Australopithecus sediba, which some scientists think possibly evolved into Homo erectus.^[22]

Notable specimens

• 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