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Australopithecus
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The genus Australopithecus is a group of extinct hominids, the gracile australopithecines, closely related to humans.
EvolutionGracile australopithecines shared several traits with modern apes and humans, and were widespread throughout Eastern and Northern Africa by a time between 3.0 and 3.9 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania. These hominid footprints are remarkably similar to modern humans and have been dated as 3.7 million years old. Until recently, the footprints have generally been classified as australopithecine 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 hominids.
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The genus Australopithecus is a group of extinct hominids, the gracile australopithecines, closely related to humans.
EvolutionGracile australopithecines shared several traits with modern apes and humans, and were widespread throughout Eastern and Northern Africa by a time between 3.0 and 3.9 million years ago. The earliest evidence of fundamentally bipedal hominids can be observed at the site of Laetoli in Tanzania. These hominid footprints are remarkably similar to modern humans and have been dated as 3.7 million years old. Until recently, the footprints have generally been classified as australopithecine 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 hominids. 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 slower, as opposed to quicker, from the date suggested by a gene clock (the result of which is given as an "youngest common ancestor", i.e., the latest possible date of diversion.) However, hominids 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 humans and chimpanzees diverged once, then interbred around one million years after diverging.
MorphologyThe 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 no more than 1.2 and 1.4 m (approx. 4 to 4.5 feet) tall. In several variations of australopithecine there is a considerable degree of sexual dimorphism, meaning that males are 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 australopithecines, however, males can be up to 50% larger than females. New research suggests that sexual dimorphism may be far less pronounced than this, but there is still much debate on the subject.
Species variationsAlthough 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 ancestor.
Evolutionary roleThe fossil record seems to indicate that Australopithecus is the common ancestor of the distinct group of hominids, now called Paranthropus (the "robust australopithecines"), 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 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. Climate changes around 11 to 12 million years ago 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. It is suggested that the ancestors of gorillas and chimpanzees became more specialised in climbing vertical tree trunks or lianas, using a bent hip and bent knee posture which matches the knuckle-walking posture they use for ground travel. Humans are closely related to these apes, and share features including wrist bones apparently strengthened for knuckle walking.
Radical changes in morphology took place before gracile australopithecines evolved; the pelvis structure and feet are very similar to modern humans. The teeth have small canines, but australopithecines generally evolved a larger post-canine dentition with thicker enamel. Australopithecines faced one particular challenge while living on the savanna.
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.
DietIn a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that robust australopithecines were largely frugivorous. However, newer methods of studying fossils have suggested the possibility that Australopithecus was omnivorous. In 1992, trace element studies of the strontium/calcium ratios in robust australopithecine fossils suggested the possibility of animal consumption, as did a 1994 using stable carbon isotopic analysis.
Notable Specimens
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