The Australopithecines, though not included in the genus Homo, are considered the first true humanlike beings, reason being they share many skeletal traits that are clearly hominid.
Australopithecines date back to approximately between 4.5 and 5 Ma (million years ago). Their skeletal structure indicates an obvious upright posture, with long legs. The skull of an australopithecus indicates a proportionally large brain. The pelvis is a significant skeletal feature. It is shape adapted to support bipedalism, and strikingly resembles that of a human, modern or otherwise. Australopithecines are estimated to have weighed around 130 pounds, females often being less than 77 pounds, and males often weighing minimally 155 pounds.
An australopithecus skull was much larger, proportionally, than that of modern apes of the same size. The lower facial area of the skull projects outward slightly, and the lower canines elevate themselves somewhat beyond the level of the other teeth. The tooth position on the jaws is somewhat elongated, and almost parallel on the sides.
The youngest known australopithecine, Australopithecus africanus, may have stood approximately 4-5 feet tall, and would have weighted between 65-150 pounds (and like all australopithecines, the females would more than likely be the lighter). The brain size is averaged at about 450 ml. It had a round yet projecting face, with very humanlike teeth, especially notably less prominent canines. The overall tooth size is about equal to that of small modern gorillas.
The Australopithecus afarensis is what is most commonly thought of when this genus is referred to. More specifically, the individual skeleton dubbed "Lucy." Lucy was the skeleton of a small female standing in at 3.5 feet, and weighing only an estimated 65 pounds. The evidence of her small weight was that her leg size was unusually short for a modern human stature, but quite adept for her suggested size. From less complete remains it has been determined that larger members of her species, all male, would have weighed roughly 130-175 pounds. Lucy's skeleton was apt for bipedal mobility, and her brain size was larger than that of any modern day primates of her stature.
Other species of australopithcines are known, not just the two stated specifically. However, the differences between some of these species are very minute. More often than not all australopiths fit into the same generic skeletal characteristics, with mere size, proportion, or slight shape variations between them.
Lucy and the Tuang Child
"Lucy" is what most people make a connection to when they reffer to Australopithecus. The partially complete skeleton of a hominid dubbed "Lucy" was the first recognized australopith, discovered by Donald Johanson in 1973-74. The find brought the knowledge of human evolution to new heights, with Lucy being classified as Australopithecus afarensis, the first in a later line of discoveries to be recognized in this genus. But, not the first discovered.
Raymond Dart was technically the first to discover what was the skull of a juvenile Australopithecus africanus, this find would not be credited until much more information on the genus had been recorded post Lucy. This skull was dubbed the "Tuang Child."
The Tuang Child an example of A. africanus, and Lucy that of A. afarensis, there can be seen significant physical differences which may explain why the Dart's discovery wasn't credited sooner. Chronoligically, Dart's discovery came much earlier than Johanson's. However, the species africanus is, in actuality, the youngest known australopith.
A. Afarensis, the species Lucy belongs to, is a predecessor to A. africanus. The skull had an undeniably sharp protrusion to it, as well as many more apelike than humanlike skull features, as prior stated.
A. africanus, though, appeared much more semblant of early Homo. The face is clearly less projected than that of afarensis or any other australopithecine. The head appears to have a much more round rather than jagged look, with much less of an extrusion in the brow region. Because of very humanlike appearances, it is commonly beleived africanus is the direct link to H. habilis, the earliest known direct anscestor to modern day Homo sapiens. No conclusion can be derived as to whether or not A. Africanus is or is not, in fact, the latest australopith, and precursor to our very genetic line.