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iGracile australopithecines
Fossil range: Pliocene
Austrolopithecus africanus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Primates
Family: Hominidae
Subfamily: Homininae
Tribe: Hominini
Subtribe: Hominina
Genus: Australopithecus
R.A. Dart, 1925
Species

A. afarensis ("Lucy")
A. africanus
A. anamensis
A. bahrelghazali
A. garhi
Formerly Australopithecus,
now Paranthropus
P. aethiopicus
P. robustus
P. boisei

The gracile australopithecines (members of the genus Australopithecus) (Latin australis "of the south", Greek pithekos "ape") are a group of extinct hominids that are closely related to humans.

Evolution

Gracile australopithecines shared several traits with modern apes and humans and were widespread throughout Eastern and Southern Africa as early as 4 to as late as 1.2 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 positively 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 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 5-6 million years ago, if we assume a constant rate of evolution. However, more recently discovered hominids 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; the location of the mastoid of both indicate that they were bipedal and had therefore diverged from the common ancestor much further back along the evolutionary trail. 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. [1]

As molecular evidence has accumulated, the constant-rate assumption has proven false—or at least overly general. However, while the molecular clock cannot be blindly assumed to be true, it does hold in many cases, and these can be tested for. For example, molecular clock users are developing workaround solutions using a number of statistical approaches including maximum likelihood techniques and later Bayesian modeling.

Morphology

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 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 15% larger than females, on average. In Australopithecines, 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 variations

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 at right in a single genus. Paranthropus, being more massive and robust, was also morphologically distinct from Australopithecus, and its specialized physiology also implies that its behavior was quite different from that of its ancestor.

Evolutionary role

Laetoliafar.jpg

Re-creation of A. afarensis from Laetoli (American Museum)Template:3d alt

The fossil record seems to indicate that Australopithecus is the common ancestor of the distinct group of hominines, now called Paranthropus (the "robust australopithecines"), and most likely the genus Homo which includes modern humans. Although the intelligence of these early hominines 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 preceeded bipedalism. If A. afarensis was the definite hominine 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.

Radical changes in morphology took place before gracial Australopithecines evolved; the pelvis structure and feet are almost indistinguishable in comparison to modern humans. The teeth are aligned just as modern humans with small canines; however, the evolution of Paranthropus evolved a larger thicker dentition. Australopithecines faced one particular challenge while living on the savanna. They were the slowest-moving primates at the time and many fell prey to carnivorous creatures (lions and the extinct Dinofelis).

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 (ie. cracking open nuts with stones and using long sticks to dig for termites in mounds). However, Australopithecus garhi does appear to have been the most advanced of the line with its presumably older stone tool artifacts than the earliest genus homo member known so far Homo habilis. A. garhi's remains have been found with tools and butchered animal remains, suggesting the incipience of a very primitive tool industry. This led many scientists to suspect that A. garhi may be the ancestor of the Homo genus. However, further evidence may help anthropologists and scientists to determine the true ancestor species.

Diet

In a 1979 preliminary microwear study of Australopithecus fossil teeth, anthropologist Alan Walker theorized that Austrolopithecus may have been fruitarian.[2] However, newer methods of studying fossils have shown that Australopithecus was likely omnivorous. In 1992, isotope studies of the strontium/calcium ratios in Australopithecus fossils showed that the species almost certainly consumed animals. These findings were confirmed in 1994 using stable carbon isotropic analysis.[3]

Notable Specimens

References

  1. Bower, Bruce (May 20, 2006). "Hybrid-Driven Evolution: Genomes show complexity of human-chimp split". Science News 169 (20): 308.
  2. Billings, Tom. Humanity's Evolutionary Prehistoric Diet and Ape Diets--continued, Part D).
  3. Billings, Tom. Comparative Anatomy and Physiology Brought Up to Date--continued, Part 3B).

External links

See also

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