Chapter 11: The Earliest Hominids
A Very Brief HistoryThe etymology of the word Australopithecus is, "southern ape," which harkens back to the name given to the original specimen. In 1924, Raymond Dart examined the strange, immature skull of a small-brained creature extracted from the cave breccia of a South African mine. This was the first discovery of an Australopithecine, and was named Taung, after the site where it was found; its species designation Dart gave it was apt-- Australopithecus africanus, or "southern ape from Africa". The acknowledgement of Taung as a member of the human lineage remained hotly contested for many years after the initial finding, primarily because of preconceived notions of which characteristics embodied the human precursor. Also, many researchers of Dart's time were convinced that the human line started in Asia and not Africa, despite the paucity of evidence from both regions. Additionally, with the previous discoveries of a fossil "hominid" called Eoanthopus dawsonii, the phylogenetic position of Taung (as well as many other fossils) remained questionable. It was not until 1953 that E. dawsonii, more commonly known as the Piltdown Man, was exposed as a scientific fraud (to read more about the Piltdown hoax, click here).
The Piltdown hoax did much to reveal preconceptions and underlying motives within the study of human origins. It too perfectly resolved a primary issue - placing the evolution of the brain ahead of locomotory developments - and remains one of the most interesting case studies of the scientific establishment. However, after the discovery of the hoax, once again our origins had to be refigured.
Ardipithecus ramidusThe earliest fossil hominid - while not classified as an Australopithecine - reaches back in time to 4.4 Mya. This specimen, discovered in 1992 by Tim White (University of California at Berkeley) off the Middle Awash River in Ethiopia, has been variably reported as an ancestor to all hominids, or the ancestor to hominids and modern chimpanzees.
Australopithecus anamensisThe genus Australopithecus spans a portion of geologic time from approximately 4.2 Mya to about 1.0 Mya. The earliest form, a species called Australopithecus anamensis, was only recently announced by Meave Leakey from the Kenya National Museum (1994). Dated from between 4.2 and 3.5 Mya, these fossils exhibit characteristics of the postcranium which suggest that A. anamensis was bipedal, although dental and mandibular morphology indicates a more apelike physical disposition. These fossils have been found in the western Lake Turkana region in northern Kenya.
"Lucy" - belongs to the species, A. afarensis. Found at sites in East Africa (e.g., Laetoli, Hadar), A. afarensis - which spans a time range between 3.8-2.8 Mya - has provided a great deal of fossil information concerning the morphology of early hominids. Most of the characteristics are continuous, and their condition within A. afarensis lies somewhere between other hominids (relatively lower), and chimpanzees (relatively higher):
Later AustralopithecinesThe remaining groups of Australopithecines can be divided along two lines: geographic distribution, and general morphological features. By geography, these hominids can be subdivided into South African and East African groups; along morphological lines, they can be grouped as either gracile (lightly built), or robust (heavily built).
South African Forms
A. africanusA. africanus includes both Sts 5 (named "Mrs. Ples"), and the previously mentioned Taung child. A. africanus is very similar to A. afarensis in postcranial anatomy, with elements of the pelvis, ribs, and vertebrae exhibiting similar characteristics. The cranium of A. africanus is somewhat different, with a higher degree of cranial flexion and no crests (sagittal or compound T-N), as observed in A. afarensis. These specimens have been found predominantly at the South African sites of Sterkfontein and Makapansgat, in addition to Taung.
A. robustusA. robustus is also within the genus Australopithecus; however, these fossils differ markedly from the more gracile forms. Their crania exhibit remarkably well-fortified structures and buttresses to counteract increased forces in mastication (chewing). Some of these characteristics include a large sagittal crest, as well as flaring zygomatic arches (cheekbones), both of which provide additional leverage and anchor points for the muscles of mastication. Additionally, the teeth of A. robustus are very large, particularly in comparison to the anterior (front) dentition.
East African Forms
A. aethiopicusA. aethiopicus, discovered by Alan Walker (Penn State University), stirred quite a controversy over the phylogenetic arrangement of the hominid lineage. Until this discovery, most phylogenies had the Australopithecines evolving down one exclusive lineage, while Homo evolved separately down another (although both shared a common ancestor, presumably A. afarensis). A major portion of this puzzle included a tight relatioship between the robust Australopithecines, an arrangement which required rethinking because of the A. aethiopicus specimen, KNM-ER 17000 (aptly named the Black Skull). Because it bore a number of similarities with A. afarensis which suggest its placement as an intermediate form between A. afarensis and A. boisei, the previous arrangement with the South African A. robustus became questionable. Were the South African A. robustus fossils still part of a common lineage including A. boisei, or could South and East African Australopithecines have evolved independently of each other?
A. boiseiA. boisei, the East African robust form, is thought to have descended from A. aethiopicus. This species exhibits a hyper-robust morphology, with exaggerated features similar in kind to those of A. aethiopicus, but far greater in degree. The massive dentition is coupled with a sagittal crest; increased dishing of the face, or orthognathism, characterizes these forms. This reduction in postnasal facial projection probably came as a result of increased demands to combat masticatory forces. Specimens include OH 5 and KNM-ER 406, among others. Whereas earlier interpretations of their dental specialization offered dietary shifts (e.g., consuming a tougher, lower quality diet), recent analyses of dental microwear suggest that A. boisei incorporated a diverse range of foods in its diet.
The principle feature which binds all hominids together into a unique group is their choice of locomotion. All hominids exhibit features of their anatomy which indicate habitual bipedalism. There are a great many features which provide direct or subtle evidence for this behavior; a some of the main features of the skeletal system include:
Other features include asymmetry of the distal femoral condyles and proximal tibial condyles; relative positioning of the ischium and the iliac crest; angle of the femoral neck. Additionally, reconfiguration of muscular attachments, especially the hip abductors, is required to maintain proper balance while moving in a bipedal gait. These characteristics, when observed in fossilized skeletal elements, argue for an organism which utilized a highly specialized mode of locomotion shared with all human beings today.
Theories on the Origin of Bipedality
A new member of the family - A. garhiFound at and around a site called Bouri, in Ethiopia, this species was only recently named. The general morphology of this hominid is similar to the other Australopithecines, though A. garhi has a sagittal crest and their canines are generally larger. And surprisingly, there is evidence that this species of Australopithecine used stone tools, a trait generally associated only with the genus Homo. The discovery of this species prompts researchers to ask new questions about the lineage leading to Homo sapiens sapiens, since it is dated at just about the time to be the ancestor of the first Homo species.
Click here for a comparison of numerous hominid (and great ape) crania. Or visit an online resource for fossil hominids - a very handy list with nicknames, museum specimen numbers, descriptions, and links.