Australopithecus africanus essay

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April 10, 2010

Australopithecus africanus

The first member of its genus to be discovered, Australopithecus africanus is the oldest species of hominin to be found in southern Africa. Cave sites where it is found have been dated approximately to 3-2.0 ma based mostly on biochronological methods (dating methods utilizing the relative chronologies of non-hominin animal fossils).  Its morphology is similar to Australopithecus afarensis, but it has important differences in the skull and teeth.  The fact that Au. africanus shares some morphological features with Au. afarensis, others with members of the Paranthropus genus, and others with early Homo species makes it a difficult species to place in the hominin lineage.  Thus, understanding Au. africanus is central to understanding early hominin phylogeny.

The first specimen of Au. africanus to be found, in 1924, was a juvenile skull from the site of Taung in South Africa.  The biologist Raymond Dart believed that this specimen was a member of the hominin clade based on the forward positioning of the foramen magnum (the hole in the base of the skull where the spinal cord connects with the brain), which is seen in humans and other bipedal hominins.  Many scientists at the time did not believe Dart’s assertion and thought the skull was of a non-hominin ape, particularly because they thought that hominins would have larger brains than that possessed by Au. africanus.  This belief was held in part because of a fossil skull and jaw found in England, called Piltdown Man, which had a large brain like a human but with a jaw and teeth that were more primitive (more like those of an ape).  This fossil played to scientists’ beliefs that the big change between humans and apes would be seen first in the size of the brain; it also showed human evolution to have occurred (at least partially) in Europe, which agreed with many scientists’ Eurocentric views better than would an African evolution for humans.

In time, the Piltdown fossil was shown to be a fake, simply a human skull and an orangutan jaw with filed down teeth, but for the first part of the 20th century Piltdown Man was considered to be the best case for a member of the lineage between apes and humans.  It was not until the 1950s that Au. africanus was recognized by the scientific community as such, a true hominin. By this time, a number of cranial and other postcranial (skeletal material not from the skull) specimens attributed to Au. africanus had been found in limestone caves at Sterkfontein, Taung, and Makapansgat, all in South Africa.  These sites are limestone caves that were eaten away by rainwater and filled with animal remains and sediments from the surface.

Because of this context, the sites at which Au. africanus has been found do not have easily defined layers and dating of the sites is difficult, especially since South Africa lacks volcanic layers that would allow for radioactive isotopic dating (dating of the volcanic material using the timing of decaying isotopes within the material).  Thus, these sites are primarily dated using biochronological methods.  The fauna that are used to date these sites have also led scientists to reconstruct the habitats at which Au. africanus lived as woodland and open woodland savanna.

The morphology of Au. africanus is similar to A. afarensis in many ways.  For instance, it is small-bodied compared to later hominins and possesses the pelvic structures and adaptations to the legs and feet that characterize habitual bipeds, such as a broad, short pelvis and a valgus knee (a knee that is angled underneath the body).  It also has curved phalanges (finger bones) like Au. afarensis; this fact, coupled with finding remains in areas reconstructed as wooded environments, has led to the possibility that Au. africanus spent at least some time in trees.  Au. africanus also lacks many features associated with consumption of hard foods; for example, Au. africanus lacks sagittal crests (crests along the midline of the skull where chewing muscles attach) and flared zygomatics (cheek bones), which are found in most specimens assigned to Paranthropus boisei and robustus (see essays for these species).

However, Au. africanus has a slightly larger estimated brain size than Au. afarensis, and has larger post-canine teeth (molars and premolars) and smaller anterior teeth (incisors and canines) than does Au. afarensis, traits it shares with members of Paranthropus.  These traits are derived relative to A. afarensis; that is, they are different than the condition found in A. afarensis and have evolved in the lineage leading to Au. africanus.  Au. africanus also has a slightly less prognathic (projecting) face, although this trait is variable in the species.  This species also has a more flexed basicranium (a skull base that is more angulated in the center).  In addition, Au. africanus has a trait called anterior or nasal pillars, which are a buttressing of bone on either side of the nasal opening of the skull on the maxilla (the bone comprising the majority of the face).  Although the exact function of this trait is not well understood, some scientists have suggested that this is an adaptation to the forces of chewing hard foods.  This trait is also commonly seen in P. robustus, but is not present in other hominins.

Au. africanus also has derived traits that are not found in any of the other australopiths or Paranthropus members.  These include a taller frontal bone (the bone that makes up the forehead) and an occipital bone (the bone at the back of the skull) with a longer and flatter orientation on the bottom and a higher point of directional change towards the top of the bone, which results in a higher, more rounded shape for the back of the skull.  These derived traits are also found in Homo species.

Overall, these traits suggest that Au. africanus may have evolved from Au. afarensis or a similar, as of yet unknown, hominin.  The changes in the dentition and buttressing in the face may indicate that Au. africanus was eating a harder or tougher diet than was Au. afarensis.  However, studies of microwear (the scratches and pits left on the teeth by chewing of food) suggest that Au. africanus was not eating a diet similar to that of members of the paranthropines.

The relationship of Au. africanus to other hominins is not well understood. Most scientists agree that Au. africanus evolved from Au. afarensis or a similar hominin, but the relationship between it and later hominins is unclear.  This cloudiness results in part because A. africanus is old enough to be an ancestor to many different hominins, and in part because it shares some traits with different groups of these hominins but not others.  Because Au. africanus retained some primitive characters (those characters that are found in a common ancestor and are unchanged in its descendants; in contrast to derived characteristics), such as the lack of sagittal crests and flared zygomatics, that are shared with early members of the Homo lineage, Au. africanus was originally considered to be a direct ancestor to the Homo lineage.  In addition, Au. africanus shares derived characters of the skull (listed above) with Homo and no other hominins.  However, Au. africanus shares some derived traits with all members of the Paranthropus genus, but not with Homo, which indicate it might be an ancestor to the Paranthropus genus.  It also shares the presence of anterior pillars with only P. robustus, which may indicate that it is a direct ancestor to only this species.  This last hypothesis would indicate that the Paranthropus genus was not monophyletic (did not contain an ancestor species and all of its descendent species), since Au. africanus is not included in this genus but is most closely related to a member of this genus.  Thus, where Au. africanus truly fits in is important to understanding many of the relationships within the hominin lineage.

Another important topic of discussion surrounding Au. africanus is the level of variation seen between individuals of this species.  The variation in many characteristics such as molar size and facial structure is larger than that seen in any living apes.  This point has led some scientists to believe that the species Au. africanus actually consists of two separate species.  These differences still stand even when considering Au. africanus to be a very sexually dimorphic species (a species with two body forms and/or sizes, one male and one female, a trait that occurs in many primates, including humans).  If Au. africanus was actually two species, some of our hypotheses about the relationship of Au. afrricanus to other hominins could be refined.