| Gracile australopith Temporal range: Pliocene - Pleistocene 3.9–1.7 Ma Descendant taxon Homo survives to present |
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| Australopithecus afarensis | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Infraclass: | Eutheria |
| Order: | Primates |
| Family: | Hominidae |
| Subfamily: | Homininae |
| Tribe: | Hominini |
| Subtribe: | Hominina |
| Genus: | Australopithecus R.A. Dart, 1925 |
| Species | |
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†A. africanus |
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Australopithecus (Latin australis "southern", Greek πίθηκος pithekos "ape") is a genus of hominids that is now extinct. From the evidence gathered by palaeontologists and archaeologists, it appears that the Australopithecus genus evolved in eastern Africa around 4 million years ago before spreading throughout the continent and eventually becoming extinct 2 million years ago. During this time period various forms of australopiths existed, including Australopithecus anamensis, A. afarensis, A. sediba, and A. africanus. There is still some debate amongst academics 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 to be robust australopiths whilst the others would be considered gracile australopiths. However, if these species do indeed constitute their own genus, then they may be given their own name, the Paranthropus.
It is widely held by archaeologists and palaeontologists that the australopiths played a significant part in human evolution, and it was one of the australopith species that eventually evolved into the Homo genus in Africa around 2 million years ago, which contained within it species like Homo habilis, H. ergaster and eventually the modern human species, H. sapiens sapiens.[1]
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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.[2] Until recently, the footprints have generally been classified as australopith because that had been the only form of pre-human known to have existed in that region at that time; however, some scholars have considered reassigning them to a yet unidentified very early species of the genus Homo.
Australopithecus anamensis, Australopithecus afarensis and Australopithecus africanus are among the most famous of the extinct hominins. A. africanus used to be 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 5 to 6 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 diversion.) However, hominins discovered more recently are somewhat older than the molecular clock would theorize. Sahelanthropus tchadensis, commonly called "Toumai" is about 7 million years old and Orrorin tugenensis lived at least 6 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 an evolutionary 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.[3]
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 between 1.2 to 1.4 m (3 ft 10 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. 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.[4]
Although opinions differ as to whether the species aethiopicus, boisei and robustus should be included within the genus Australopithecus, the current consensus in the scientific community is that they should be placed in a distinct genus, Paranthropus, which is believed to have developed from the ancestral Australopithecus line. 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. However, Paranthropus was morphologically distinct from Australopithecus, and its specialized morphology also implies that its behavior was quite different from that of its ancestors.
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 modern apes, the bipedal stature is the key evidence which distinguishes the group from previous primates who are quadrupeds. The morphology of Australopithecus upsets what scientists previously believed, namely, that large brains preceded bipedalism. If A. afarensis was the definite hominid which left the footprints at Laetoli, it strengthens the notion that A. afarensis had a small brain but was a biped. Fossil evidence such as this has made it clear that bipedalism far predated large brains. However, it remains a matter of controversy how bipedalism first evolved millions of years ago (several concepts are still being studied). The advantages of bipedalism allowed hands to be free for grasping objects (e.g. carrying 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 evolution of bipedalism.
A recent study of primate evolution and morphology noted that all apes, both modern and fossil, show skeletal adaptations to upright posture of the trunk, and that fossils such as Orrorin tugenensis indicate bipedalism around 6 million years ago, around the time of the split between humans and chimpanzees indicated by genetic studies. This suggested that upright, straight-legged walking originally evolved 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.[5]
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 which 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 that there were periods when openings prevented travel through the tree canopy, and at these times ancestral hominids could have adapted the upright walking behaviour for ground travel. Humans are closely related to these apes, and share features including wrist bones apparently strengthened for knuckle-walking.[6] 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.[7] Further comparative analysis with other primates suggests these wrist bone adaptations support a palm based tree walking.[7]
Radical changes in morphology took place before gracile australopiths evolved; the pelvis structure and feet are very similar to modern humans.[8] The teeth have small canines, but australopiths generally evolved a larger post-canine dentition with thicker enamel.[9]
Most species of Australopithecus were not any more adept at tool use than modern non-human 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.
In a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that robust australopiths were largely frugivorous.[10] 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.[11] These observations have been interpreted as evidence that Paranthropus robustus may have fed on hard and brittle foods like some nuts and seeds.[11] 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.[12]
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.[13]
The first australopithecine to be discovered and documented was a fossil of a three year old Australopithecus africanus which was 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. 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 which 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.[14]
The first australopithecine to be 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.[14]
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".[15]
According to molecular evolution principles Human evolutionary genetics (Jobling, Hurles and Tyler-Smith, 2004), the most recent common ancestor of humans and chimpanzees had as much pre-human traits as chimp-like traits. In other words, progressing along the chimpanzee line means losing some human-like traits and gaining chimpanzee traits, whereas progressing along the Human line means losing some chimp-like traits and gaining human traits. A fossil close to the last common ancestor is thus difficult to place in a precise chimp or human lineage because it has not accumulated enough differences since the two lines have diverged. For instance, the most recent Paranthropus fossils are 1.2 million years old, fairly recent compared to the time since the divergence of humans and gorillas (7-9 mya), therefore many of the paranthropus traits are very much like gorillas (see Homininae). In contrast, it is more difficult to place a 3 or 4 million years old australopithecines fossil in the chimp or human lineages. A. africanus seem to be close to the human lineage, whereas A. afarensis are closer to the chimp lineage, with the exception of Selam (3.3Mya) who has been placed in A. afarensis, but has more human traits than Lucy (3.2Mya) for instance, and would better be placed in A. africanus. The mainstream view among paleontologists can be found in this page and in the main "human evolution" page, but similar conclusions were reached by at least two other biologists, independently: the author of the “Paranthropus aethiopicus” page of the Online Biology Dictionary and Richard Dawkins in his book The Ancestor's Tale "According to this theory, chimps and bonobos are descended from Australopithecus Gracile type species while gorillas are descended from Australopithecus Robustus (Parnthropus) type species. These apes were once bipedal but then lost this ability when they were forced back into the semi-forest, presumably by those Australopithecines who eventually became us"; In short, ancestors of chimpanzees are Australopithecus afarensis and ancestors of gorillas are Paranthropus (see tree on Homininae).
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