Australopithecines and Homo habilis: Australopithecines

Scientific classification

Kingdom - Animalia
Phylum - Chordata
Class - Mammalia
Infra class - Eutheria
Order - Primates
Family - Hominidae
Subfamily - Hominidae
Tribe - Hominini
Sub tribe - Australopithecina
Genus - Australopithecus

The hominid family originated between 10 and 5 million years ago, with a single species of bipedal ape. As usually happens with newly established mammalian lineages: The 1^st specie gave rise by stages to a range of descendants, producing a relatively luxuriant evolutionary bush. Individual branches were pruned away from time to time as species became extinct and new ones were added. Eventually, the hominid bush was reduced to just Homo sapiens. No less than 3 perhaps 6 and maybe more hominid species coexisted 2 million years ago in Africa. Those species can be divided into 2 groups:

With relatively large brains and small cheek teeth.
Large-brained species were member of the genus Homo.

So far, only one species has been formally named in this group Homo habilis.

If several species of Homo existed at this time, only one of them could have been ancestral to modern humans, while the rest became extinct. Labeling of the 2^nd group is more contentious. We will call them australopithecines; they all became extinct.

Evolution

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

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. The footprints have generally been classified as australopith because that is the only form of pre-human known to have existed in that region at that time.

Cranium

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

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 divergence.) 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

Morphology

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

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.

Anatomy and biology of the australopithecines:

Bipedal apes with modified dentition.
Lived in more open environmental setting, not the open plains of bushland and wooden Savannah.
Hominid structure of teeth and jaws appear to have required more grinding that an ape’s diet.
Male australopithecines were larger in body size. 20-40% taller, 30-40% heavier than females.
Australopithecines were social animals.
Foraging strategies of hominids were not dramatically different: australopithecines have been carnivores.
Australopithecines were principally vegetarian.
Australopithecines of 2 million years ago occurred in 2 forms:

Gracile (means slender)
Robust.

So far, only one gracile australopithecines has been identified whereas, as many as 4 robust species have been named. In South Africa, the gracile species is australopithecus africanus and the robust australopithecus robustus. The robust australopithecine in East Africa is australopithecus boisei. Naming gracile species in East Africa is more contentious; some apply the name australopithecus africanus to some specimens. The term gracile and robust implies substantial anatomical differences between the 2 forms. One small and delicately built. The other bigger and more massive. Scholars realize the difference between the 2 forms is mainly in the dental and facial adaptations to chewing. The robust forms have bigger grinding teeth, more robust jaws and more bulky chewing muscles and muscle attachments.
The remainder of the skeleton:The gracile and robust australopithecines were roughly comparable, with the robust having a slightly greater stature. Brain size, based on small specimens, gives the robust an edge over the gracile. Both are close to 500 cm³. Most scholars agreed hominids did not leave Africa before about 1 million years ago, when Homo Erectus expand to Eurasia.

Australopithecines anatomy

Teeth, jaw and cranial anatomy are one functional complex.
Difference between the 2 forms of australopithecine is that the robust species have taken this adaptation to an extreme, having enormous, flat molars and relatively small blade-like incisors and canines.
In all hominid, the tooth row is tucked under the face more than the apes, giving a less projecting facial profile and increasing chewing efficiency.
Australopithecines tucking under are particularly marked.
The robusticity of the lower jaw (mandible) that is characteristic of hominids compared with apes is particularly apparent in the robust species, reflecting more powerful chewing action.
Extra muscle in the robust has 2 anatomical consequences:

One of the muscle that power the lower jaw-the temporal muscle-is anchored to a raised bony crest that run along the top of the cranium, front to back.
This so-called sagittal crest also in gorillas is absent in gracile australopithecines.
The great site of the temporal in robust and a second chewing muscle, the masseter, causes the cheekbones (the zygomatic arch) to be exaggerated and flared forward.