Question

What factors explain the evolution of the robust faces and jaws of Australopithecus, especially in the early Pleistocene? Focus on the combination of selective pressures and nonevolutionary factors that influence skeletal morphology.

Answer 1

Australopithecus species played a significant role in human evolution, the genus Homo sapiens is derived from them. They were the the first hominids to have certain genes called ,the duplicated SRGAP2, which increased the length and ability of the neurons in their brain. One of the Australopithecus species gradually became the Homo genus in Africa around two million years ago (e.g. Homo habilis), and from it gradually evolved the modern humans, H. sapiens sapiens.

"Robust" australopithecous are a group of fossil hominins, which were found in East and southern Africa between 2.5 to1.4 million years ago . They were thought to be from the members of the genus Paranthropus, They are characterized by several features of the skull which give them a "robust" appearance when compared to other gracile hominins. The most specific features are large, thickly enameled, postcanine teeth supported by deep and broad mandibular corpora ,with tall and broad rami . Other notable features are zygomatic (cheek) bones which were extended laterally and anteriorly, and a face which was more orthognathic ( that is pulled back towards than the rest of the skull) when compared to other Australopithecus, and ocassional presence of bony crests on the top and back of the skull, probably for the attachment of large jaw muscles. All these traits together, suggests that it was an animal which could both generate and dissipate high bite forces, and this implies that some portion of their diet was difficult to break down.

Robust australopithecines (like A. boisei and A. robustus) had larger cheek, or buccal, teeth than the smaller – or gracile – species (like A. afarensis and A. africanus) this indiacates, that they might be eating tough, fibrous plant materials in their diet, at the same time the smaller species of Australopithecus had more meat. But the larger body size might be another cause to have more food. Their larger molars support a slightly different diet, having some hard food.

Australopithecines also had thick enamel, as in Homo sapiens, the reason behind this might be, these hominins were living more on the ground than on the trees and were foraging on tubers, nuts, and cereal grains. They might be eating a lot of gritty dirt with the food, which may be responsible for wear of enamel, so in that case the thicker enamels are advantageous for them. Or, there might be a change in diet.

Robust australopithecines were having their molar surfaces down flat, differing from the gracile species, who kept their crests. This also suggest that they were having a different diet. The gracile Australopithecus had larger incisors, which indicates tearing of food was there, means they were eating more meat in their diet but the wear patterns on their tooth surfaces ,supports having large herbivorous diet.

When the buccal microwear patterns on the teeth of A. afarensis and A. anamensis,were observed it is seen that, A. afarensis did not consume a lot of grasses or seeds, but rather ate fruits and leaves, but A. anamensis used to eat grasses and seeds besides fruits and leaves.

In the skull, the biting and chewing puts selective pressure or load ,so these might be the factors which might have influenced evolution of robust faces and jaws in Australopithecus in early pleistocene period.