Australopithecus

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Australopithecus

The gracile australopithecines (members of the genus Australopithecus) 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 4.4 to as late as 1.7 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. They have generally been classified as Australopithicine until recently, because that had been the only form of pre-human known to have existed in that region at that time. Australopithecus afarensis. A. afarensis and A. 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 an as 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 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.

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 tall. In several variations of Australopithecine there is a considerable degree of sexual dimorphism in which males are larger than females. The evolutionary adaptive changes in more advanced hominines do not appear to display the dimorphism, thereby leaving modern human sexes at almost the same height with the average male slightly taller than the average female.

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

The 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 hominines was likely no more sophisticated than modern apes (more or less), 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 complex brains came first and was followed by bipedalism. If A. afarensis was the definite hominine which left the footprints at Laetoli, it even strengthens the notion that A. afarensis had a small brain but was a biped. It remains a matter of controversy how bipedalism first evolved millions of years ago (several concepts are still being studied). However, the advantages of bipedalism allowed hands to be free for grasping objects, and allowed the eyes to look over tall grasses for possible food sources or predators.

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. gahri'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.

References

White, Tim D., et al. "Asa Issie, Aramis and the Origin of Australopithecus." Nature 440 (April 13, 2006), 883-89.

External links

Why australopithecines became bipedal