This paper is testing the hypothesis that A. robustus in South Africa evolved independently and parallel to the Eastern robust species comprised of P. aethiopicus and P. boisei, which evolved from the main gracile majority and migrated to Lake Turkana. In order to ascertain whether or not this is a feasible hypothesis, I compared not only divergent "robust" Australopithecine fossil records against each other but also against earlier hominins that share physical and phylogenetic similarities. This includes A. afarensis and A. africanus, the former displaying highly plesiomorphic characteristics unique to the Eastern clade and the latter displaying many morphological similarities which may have been derived by A. robustus species in the same geographical locations. Because post-cranial remains for Paranthropus are limited, the examination of dentition and craniofacial morphology will be the primary source of information in determining these differences. The types of wear, the size and morphology of the dental record we do have will show any differences in diet as well as factors like disease or times of malnutrition manifested by enamel hypoplasia on the teeth. Other considerations like the topographical features of the teeth and canine/post-canine size will be included as well. Research will include what paleogeographical and paleoclimatical conditions existed, bone/dentition comparison and craniofacial examination in order to determine if there are sufficient variables to facilitate independent diversion of the species.
Methods and Materials
The primary specimen casts examined by hand were A. afarensis (AL-288), P. aethiopicus (KNM-WT-17000), P. boisei (OH5, KNM-ER-406), A. africanus< (Sts 5) and A. robustus (SK 23/48). Other samples examined and compared via online and book imagery are Peninj mandible, Omo L. 7a-125, Sts 71 and SK 8. From the specimen casts high resolution digital images were taken and imported into Adobe Flash, which were then used to create a visual analysis tool where opacity levels could be adjusted for comparison. Angles imported into the program included the top, front and side (placed according to a Frankfort Mandibular Plane angle for consistency).
I attempted to find evidence in the paleoenvironmental and paleoecological records for East and South Africa respectively for the time period we suspect Paranthropus lived (~3.7-1.2 mya). I also attempted to determine if the geographical distance would facilitate allopatric, parapatric, or sympatric speciation or if these features could have developed through other means such as climate adaptation. This was done through the mapping of the areas involved with online tools that show both topography and satellite views, then piecing them together to gain a better knowledge of the area. These highly detailed images were then imported into 3D software to recreate an accurate view of the land to help in the hypothesis. The rendering process yielded a 'fly over' of the landscape from Hadar (Ethiopia) to Taung (South Africa), which helped in determining if a geographic barrier of lakes existed along the East African Rift. Lastly attempts to find and use evidence from paleobotany and other disciplines to find what type of flora/fauna existed parallel to the "robusts" and how that may or may not have effected their ability to migrate to other areas was carried out using primarily information from online and textbook sources. My primary concerns are as follows:
Did "robust" Australopithecines like P. boisei and A. robustus evolve in Eastern Africa and Southern Africa (respectively) parallel to one another in the Late Pliocene (~2.4mya), or did they evolve from a common shared gracile ancestor and radiate to the locations their specimens would eventually be found? Are there sufficient differences between dentition, cranial morphology and diet such as to indicate that they developed these traits independently of each other? Would there have been any way they could develop these traits around the same time because of climate and subsequent diet changes? What are/were the main differences in regard to paleogeographical and paleoclimatical variables (if any) between Lake Turkana and South Africa? What are/were the features of the geography of LT and SA as well as in between them? Could the East African rift have been a viable route for Eastern "robusts" to travel south? Were there any barriers, diet or geography limitations to facilitate speciation?
One of the major points of contention in this debate is the dating of the "robust" specimens and the accuracy we can place them in the chronological timeline. In the East Turkana Basin, most specimens according to Grine (1988) have been "reasonably well constrained" (i.e. their dates are not as disputed) but in the South there remains some discussion about the accuracy of the dating given, in this case for A. robustus/crassidens [Grine et al., 1988]. The current chronology developed for the four main locations of "robust" hominins in the East is based mainly on conventional K/Ar and (40)Ar/(39)Ar dating on anorthoclase feldspar separated from pumice clasts contained within volcanic ash layers [Grine et al, 1988]. In the south, especially at Swartkrans, a number of dating methods have been used, including paleomagnetic sequencing, faunal correlations with radiometrically dated sediments in eastern Africa and thermoluminescence (TL) signal values of quartz sand grains [Grine, 1988; p232]. More recently, advanced dating techniques such as gamma spectrometric measurements and elemental analyses of a range of sediment samples, U-series isotopic and uranium uptake history measurements have been used, but because of the large discrepancy of the dates there remains some discussion of actual ages [Curnoe, Grün, Taylor, and Thackeray, 2002].
Geographic and Paleobotanical considerations for divergent evolution
I propose that in addition to being separate due to geographic distance and natural obstacles (discussed below), we can also reclassify which groups hold phylogenetic lineages based on morphological similarities as well. As such we will attempt to determine if the Southern species and Paranthropus (for our purposes this will refer to P. aethiopicus and P. boisei) appear to be a polyphyletic group, with the more pleisomorphic branch in the East and the synapomorphic group in the South, or if they are monophyletic. The primary basis for a polyphyletic hypothesis rests first in the current specimen locations and their proximity to each other within a neatly bifurcated geographic separation between East and South Africa. For primary reference we can use our Topographical Map [Tool 1.2] as a visual aide in establishing not only the location of specific specimens but also to help formulate a potential route they took and the areas they would eventually habitate.
Starting with A. afarensis in northernmost Ethiopia around 3.2mya, we move south along the East African Rift valley 1,181 kilometers until we reach Lake Turkana and the Omo River delta that empties into it. Here we find not only A. afarensis, but P. aethiopicus (2.5mya) and P. boisei (2.3mya) as well along its banks both on east and west sides. Traveling south along the rift from Lake Turkana, we discover A. afarensis (3.3mya) and P. boisei (1.8mya) in Olduvai Gorge almost 740 kilometers away. From Lake Eyasi southward however, currently no other "robust" fossils are reported. It will take another 2,952 kilometers, through Zambia, Malawi, Zimbabwe and most of the northernmost part of South Africa to reach the next deposit of fossils with A. africanus and A. robustus.
We can see that there is no overt physical or geographical obstacle that would have impeded a southern migration for our three Eastern species, but there several other factors that may have limited expansion [Tool 1.2]. A drastic change in the environment and subsequent reallocation/shuffling in their highly specialized food sources could have, in a sense, trapped the Eastern "robusts" within an area that was fit to support them. Just south of Olduvai there are large bodies of water that curve gently along the African Rift and envelope the Gorge, consisting of Lakes Kivu, Taganyika, Rukwa and Nyasa which form a sort of natural water border along the rift. We can surmise that from these lakes, the tributaries and outflows that once fed into the valley would dry up as the water levels of the lakes retracted, as we can see from the still dry washes from the satellite view using Tool 1.2.
As major widespread North Polar glaciation (cooling event) ~2.4-2.5 in the Late Pliocene cooled and dried the planet [Grine, 1988; p408], the once lush areas began to dry out and grasslands took their place, shrinking the forested areas in and around our areas of interest, specifically Olduvai and Lake Turkana. Both terrestrial floral records in Europe and South America [Grine, 1988], as well as deep sea oxygen records by Shackleton et al. (1984) both indicate global "cooling shifts" at this time. The rather sudden truncation of easily obtainable food sources within a mosaic environment would have kept the highly specialized eaters from continuing any farther south due to increased food acquisition pressure and forced them to devote more energy than before to fall-back foods. While specialized, it would not have prevented them from feeding on a diet typical to other primate folivigores or omnivores, this could include tubers, soft (non-toxic) roots, local fruit, [Ungar and Lambert, 2008], succulent plants, insects and fibrous bark elements (see below on Micro-Wear). In addition, shore-side reeds or marshland Gramineae species and montane forest bamboo could have been consumed, as many specimens of both P. boisei and A. robustus have been found in these areas [Verhaegen, 1992; 18]. Primarily these softer, fibrous or pulpy materials can be seen being consumed primarily by P. boisei, without the same pitting or wear patterns that A. robustus displays [Ungar et al. 2008]. While their dentition was highly specialized, they most likely consisted on a wide variety of food sources, including fall-back and seasonal sources which differed between East and South locations.
While many of the lakes in the area fluctuate sharply in water depth throughout the year through flooding and drought, the climate change brought on by the ~2.5 mya cooling event [Grine, 1988; p411] would have further lessened the amount of inflow from river inlets. In cases like Lake Turkana, the effect of such repeated flooding would have been the overly alkaline soil on the western shore is largely sterile save for these spots of growth attributed to the freshwater springs. Natron and Eyasi, other nearby large bodies of water are also alkaline, thus not areas that would sustain much vegetation along their banks due to lack of drainage and regular flooding/drought patterns (1). As the water level decreases and becomes more amictic (non-mixing), the alkaline levels would have risen further even in slightly alkaline lakes over the next million years (2).
Modern day Lake Turkana is surrounded by deciduous bush-land and thicket, with scattered emergent trees and dominant shrubs or trees like Acacia, Commiphora, Salvadora persica, Capparidaceae and Tiliaceae. The herbaceous cover is short and generally discontinuous with dominance of grasses and succulents. Within the basin, vegetation changes gradually with altitude. Between 800 and 1800 m, there is a mosaic landscape of evergreen and semi-evergreen bush lands, evergreen forests and Afroalpine communities [Vincens, Tiercelin, and Buchet, 2006]. Plio-Pleistocene palaeoflora reconstructions show that major changes in vegetation have affected the eastern part of the African continent, linked to regional geography through tectonic activity around 2.5mya. Specifically, three major tectonic/volcanic events occurred, starting in the Late Eocene and ending in the Late Pliocene which "formed barriers and changed the drainage patterns and climate and must have influenced local biota strongly"[Denys et al., 1986]. Formation of high uplands could have created rain-shadow effects for equatorial westerlies during the first of the Late Eocene tectonic activity, as these uplands developed they helped to deflect the passage of the Inter-Tropical Convergence Zone (ITCZ) over East and South Central Africa, pushing it far toward the South during the austral summer and thus inducing a progressive decrease in rainfall and increase in seasonality [Vincens, Tiercelin, and Buchet, 2006]. Further attributing to the viability of a lush vegetation mosaic near waters edge is the fossil record of snails dated to 3.4mya on the banks of Lake Turkana, a species normally found only in tropical rainforests [Cameron, 2004].
A personal hypothesis (I will need another occasion to explore fully) is one of the "robusts" (specifically P. boisei) not only sharing space with graciles, but learning from them and how their unique adaptations kept them from being prey but also doomed them to stagnation and eventual demise. With the adaptation of meat consumption ~2.6mya by A. garhi [Kennedy (nd), 3], one group of gracile Australopithecines gained an advantage that would help them not only to adapt better to changing surroundings, but would lead to huge leaps in evolutionary development. Even before the neural controls for delayed consumption and food transport were to develop in the hominin brain [Kennedy (nd), 11], the scavenging of meat from carcasses played an important role in hominin development. As Paranthropus looked on, content (or stuck) with the diet it was suited best for, graciles began exploring new food acquisition techniques and moved invariably into danger as they looked for carrion or picked over kills of larger animals. This could have made them easy prey for carnivores like jackals, hyena or predatory cats [Grine, 1988] in the area, most likely as they picked at the kill or let their guard down to eat. The low, flat savannah-mosaic grasslands would have made line of sight ideal for a first row seat to indicators like circling carrion birds or even allow them to witness the graciles (or other "robusts" for that matter) getting killed by predators nearby. We can reason that Paranthropus either could not (due to its highly specialized masticatory process and character displacement pressures) or chose not to explore the dietary concept of meat appropriation. Perhaps they saw the graciles or more adventurous (or hungry) "robusts" moving south of the gorge and dying by predation or maybe they just were not willing to potentially forfeit their spots in a highly competitive mosaic environment. [Treves and Palmqvist, 2006] While this does apply somewhat anthropomorphized guesses as to their intelligence and reasoning/causation abilities, it would at least give us one hypothesis as to why they would not have wanted to venture south in addition to the environmental restrictions. Forced to spend more time acquiring a broad diet range in a more arid environment, coupled with the threat of predation (see below) and natural barriers formed by increasingly alkaline water bodies (less viable long term food and water sources) to the south, they may have simply been forced to stay where they were. What they gained in specialization to a harsh changing environment, would have also meant that adapting further to a different food source (eating meat) might have just been too great a change to occur. Just as environmental pressures pressed Australopithecus, other fauna would have been similarly forced to adapt, including predators and the type of prey they chose. In both East and South African sites we have direct and indirect examples of carnivore alteration. In the East we have what could be seen as post-mortem alteration on our primary P. boisei sample, OH 5, where the mastoid and zygomatic regions were chewed by medium sized carnivores [Grine et al, 1988; p463]. This does not necessarily indicate ante-mortem causes due to direct predation of carnivores, but it does reinforce the idea of increased carnivore activity in the Eastern section near Olduvai in relation to environmental pressures. In the South however we have much clearer evidence of carnivore alteration, including cranial puncturing (direct attack) and gnawing (scavenging) of A. robustus samples of SK 54, SK 3978, SK 82 and SK 859 in or around Swartkrans [Brain, 1981].
Micro-abrasive wear and difference of dentition
One compelling difference between the two groups that could potentially be seen as a further defining factor is the type of wear on the hyper-robust dentition for P. boisei versus A. robustus. Micro-abrasive wear examination on the post-canine dentition on both species reveal that P. boisei may have had far less dietary options, lending to our hypothesis that they would have been more confined to more specific areas due to these constraints. In an examination of P. boisei molars, [Ungar, Grine and Teaford, 2008] found that of the seven specimens we have, none showed any evidence on the micro-abrasive level of having eaten hard or brittle foods (compared to A. robustus samples).
"All of these specimens lacked the extremes of The area-scale fractal complexity (Asfc is the slope of the steepest part of the curve fit to the log-log plot of relative area over the range of scales multiplied by -1000) evinced by Lophocebus albigena (Mangabey) and especially Cebus paella (Tufted Capuchin), both known to consume hard, brittle foods. Paranthropus boisei molars also lacked the extremes of epLsar seen in Trachypithecus cristata (Silvered Leaf Monkey) and Alouatta palliate (Howler Monkey), both known to consume tough leaves and stems. The P. boisei individuals examined evidently avoided such metabolically challenging foods, at least in the days before death. This is notably consistent with Walker's early assertion that P. boisei microwear patterns resemble those of living frugivores, and differ from those of living grazers, leaf browsers, and bone feeders. Comparisons with the South African hominins suggest that while Paranthropus boisei may have consumed foods with similar ranges of toughness as those eaten by Australopithecus africanus, the eastern African "robust" hominin did not eat harder and brittler foods than the South African "gracile" form. Further, the patterns for P. boisei and A. robustus are very different. Paranthropus robustus likely ate foods that were on average much harder and less tough than P. boisei. The differences in both central tendencies and ranges of variation suggest different feeding strategies, and by implication, that the two species of Paranthropus probably had markedly different diets or foraging strategies" [Ungar et al. 2008]. (See Table 1.1)
Primates that consume hard, brittle foods tend to have heavily pitted, complex microwear surface textures, whereas those that eat tough leaves or stems have more anisotropic surfaces dominated by long, parallel striations [Ungar, Grine and Teaford, 2008]. If these findings carry over to future P. boisei samples, it would further reinforce the idea that they most likely were consuming primarily tough, bark-like, fibrous material that was gritty and required many mastication cycles to break down completely. This would lead to heavy cranial scaffolding we see like mid-sagittal cresting, heavy mandibular corpora and larger zygomatic region as the muscles needed to chew such material would be considerable. Thus we have a discrepancy between adaptive morphology being a clue to what they could have eaten, versus the microwear data that shows what they most likely ate primarily.
In contrast, microwear study of A. robustus hints more at a diet of hard, brittle objects that were used as a supplement or "fall back" in periodic increments (9). This would indicate an even higher degree of environmental plasticity in regard to food acquisition, a flexibility needed in what appears to be a much drier, harsher environment than the Eastern australiopiths endured. This can be based on modern models for living African apes, occlusal surface topography and paleoenvironmental data of resources available at the time. "Liem's Paradox" states that specialized morphology can allow for a broader diet wherein a species may actively avoid the very foods to which it is adapted when other, more preferred resources are available [Ungar, Grine and Teaford, 2008]. We can see examples of this in modern primates like mountain lowland Gorilla and Orangutan species, both often feeding on bark and the tough fibrous layers beneath the bark when fruit or other food is not available, falling back on available supplements even for long stretches of time. Normally ~16-37% of an Orangutan diet can be bark and during these periods of food stress or loss due to fire this amount can triple and last months (Caldecott, 2005). While the climate and environment alone could not establish a clear discrepancy or independent divergence, it can help us understand how they may (or may not) have radiated during their lifetime.
Table 1.1 [Ungar, Grine and Teaford, 2008]
"To paraphrase a famous cri de coeur, one is tempted to exclaim, Cherish your primitive traits!-they may yet have their day." – P.V Tobias [Grine, 1988]
Morphological and Geographical Comparison of East versus South robust species
The first group consists of A. afarensis, P. aethiopicus and P. boisei, the first two not only sharing many primitive features and an overlap in existence, but are often found in relatively close proximity to one another in the two sites where P. aethiopicus remains are discovered. While P. boisei remains the ‘wild card’ due to rather extreme morphological differences, we can still establish a synapomorphic lineage between A. afarensis, P. aethiopicus and P. boisei because of their similar geographical territory and traits. Because so little sample record remains of P. aethiopicus, it is difficult to make a clear phylogenetic link between it and the well represented A. afarensis/P. boisei findings, but through the following morphological similarities I hope to draw a convincing hypothetical line nonetheless.
Of the observable cranial features exhibited, there are morphological elements we can rule out based on ontogenetic processes that would be strictly environmentally or diet based in their formation, thus less a clue to phylogenetic clade and more of similar outside stimuli or environmental pressure and sexual dimorphism depending on the degree. These traits are mostly the pronounced sagittal crest, extreme flared zygomatic regions and compound nuchal crest associated with the mastication process, all of which makes these species’ unique and provides an interesting clue to its adaptation during this period of climate/diet change. In order for our theory to work, we will have to suppose that the before mentioned paleogeographical, predatorial and paleoenvironmental pressures would have been sufficient to not only keep southern species like A. africanus from moving northward, but also to have kept our eastern species from moving southward and provided enough pressures to facilitate the drastic morphological adaptations we see in P. boisei.
A. afarensis - P. aethiopicus - P. boisei
"Predominately pleisomorphic anatomy in cranium and dentition, with a high degree of sexual dimorphism and a mixture of primitive and familiar traits"
"The craniofacial morphology is extreme in nearly every sense, with substantial mid-facial prognathism, more forwardly positioned zygomatic process roots and a forwardly sloping zygomatic facial surface"
"P. aethiopicus, displayed a high level of prognathism and derived pleisomorphic characteristics, P. boisei is a stark contrast in many regards."
- High degree of prognathism
- Laterally thickened supraorbital bars
- Broad post-orbital breadth
- Zygomatic region is broad and massive
- Shallow mandibular fossa
- Weakly developed articular eminence
- Developed ontogenetic masticatory elements
- Dentition seems like the species was in an evolutionarily transitional state
- Less advanced molarization on P3 teeth
- Lack of a second distinct cusp
- Large canines and post-canine dentition
- Traces of shearing wear on the elongated distal crests of the lower canines.
- Strong mid-facial prognathism
- Posteriorly accentuated sagittal crest
- Extensive compound temporonuchal crest
- Flat mandibular fossa
- Large incisors and canines
- Forwardly sloping zygomatic facial surface
- Less subnasal prognathism
- Variably orthognathic face
- Larger braincase
- Mid-cranial sagittal cresting
- Absence of pneumatized bone in temporal squama
- Smaller overall post cranial size
- Sexual dimorphism is reduced
- Smaller overall body size
- Little to no canines
- "without any trace of a canine fossa"
- Large, molarized pre-molars
- Excessive development in the labiolingual diameter of the post-canines
- Well defined areas of hypoplasic enamel
A. africanus - A. robustus
"Moderate upper mid-facial prognathism is less severe than A. afarensis, but still pleisomorphic compared to A. robustus."
"A small post-glenoid process and plate-like tympanic element converge with modern human morphology in comparison with all our previous species discussed"
- Higher, shorter brain case
- Narrower cranial base
- Posteriorly located foramen magnum
- Moderate upper mid-facial prognathism
- Larger post-canine teeth
- Centrally crowded molar cusp
- P3 is uniformly bicuspid
- Canine wear is exclusively apical
- Less primitive occlusial wear than seen in A. afarensis.
- Mandibular fossa is deep with a strong articular eminence, compared to a relatively weak eminence in other Australopithecines.
- More orthognathic facial aspect
- Variably deep mandibular fossa
- Highly modified canine fossa
- Less prominent anterior pillar than A. africanus
- Anterior placement of the zygomatic bones
- Increase in brain size over A. africanus.
- More anterior foramen magnum position
- Absence of pneumatized bone in the temporal squama (also shared by P. boisei).
- Hyper-robust, post-canine teeth
- Molarized premolars
- Lower deciduous first molars
- Very thick enamel caps worn to flat occlusial planes.
- Marked decrease in canine size
From ~3.7 to 3.0mya A. afarensis had a range from modern day Hadar Ethiopia to Olduvai Gorge, with a known specimen in Koro Toro (Chad) to the east. Nearly 90% of the hypodym for A. afarensis comes from the Hadar Formation, with 367 fossils being recovered from this single location and providing us with a majority of what we know about the species [Kimbel et al., 2007]. From our Topographical Map [Tool 1.2] we can see a clear majority dispersion moving south along the East African Rift valley into the Lake Turkana delta as well as some dispersed along its shores, before finally coming to the Laotoli site (~45km from Olduvai itself). This southern migration along the curve of the East African Rift valley may have been a result of growing environmental pressures brought on by the beginnings of the Late Pliocene glaciation cooling period, which may have led them along this path as freshwater and food sources diminished.
Craniofacial morphology displays a high degree of prognathism, with laterally thickened supraorbital bars, and broad post-orbital breadth relative to other facial breadth dimensions. The zygomatic region is broad and massive, with a shallow mandibular fossa bounded anteriorly by a weak to moderately developed articular eminence. Aside from strong prognathic facial features, the other features of note are the dentition and mandible morphology. Like P. aethiopicus, they display large canines and post-canine dentition, with some revealing traces of shearing wear on the elongated distal crests of the lower canines [Kimbel, 2007].
An additional point regarding A. afarensis is that they display certain mandibular morphology that is independently derived more from G. gorilla than from the Pan paniscus/troglodytes clade that other australopithecines display. This mainly includes the mandibular ramus, which according to Yoel Rak is species specific, falls into two configurations: one of gorillas and the other of humans, two chimpanzee species and orangutans. If this is the case, he points out, it makes A. afarensis a less likely candidate as a modern human ancestor [Rak, 2006].
P. aethiopicus lived ~2.7–2.3mya in areas where A. afarensis was also found, but due to the lack of available fossils we base our differential strictly on KNM-WT-17000 "Black Skull" found in Nachakui Formation, West Turkana (Kenya). The craniofacial morphology is extreme in nearly every sense, with substantial mid-facial prognathism, more forwardly positioned zygomatic process roots and a forwardly sloping zygomatic facial surface which results in a central hollow for the nasal cavity (concave).
P. aethiopicus retains a number of plesiomorphic characters; strong mid-facial prognathism, posteriorly accentuated sagittal crest and extensive compound temporonuchal crest, flat mandibular fossa with low, indistinct articular eminence. Additionally, like A. afarensis, they also display large incisors and canines [Kimbel, 2007]. Even with this small amount of evidence, it is still hypothesized by myself and further reinforced by Leakey and Walker below that P. aethiopicus is the evolutionary transition between A. afarensis and P. boisei based on cranial morphology and a clear robust masticatory signal that would indicate transitory evolution. Leakey  interpreted KNM-WT-17000 as an early, primitive P. boisei specimen, which for our hypothesis we will adopt as well. One way to see KNM-WT-17000 is that there is a ‘typical’ P. boisei face present, but with a more prognathic palate [Leakey and Walker, 1988; Grine, 1988]. This consensus among those who discovered the specimen is that its character combination is incompatible with decent from A. africanus, which was the supposed previous phylogenetic precursor [Vrba, 1988; Grine, 1988]. Since we have no other specimens for differential comparison, we can also surmise that this particular P. aethiopicus specimen would be male and thus we can infer that many of the features we see are one extreme side of how the species would have compared to A. afarensis, taking into consideration sexual dimorphism and changing environmental pressures.
P. boisei first appears ~2.5–1.4mya, which has some 100,000 year overlap with P. aethiopicus in roughly the same areas. Its dispersement would indicate that as the climate continued to change, it became highly specialized for not only the grasslands, but marshes and wooded areas just outside the shores of Lake Turkana. If this hypothesis is correct, it would mean that P. boisei was incredibly plastic in adapting to the changing world around it, becoming more suited in a broad range of food acquisition.
While our hypothesized transitionary precursor, P. aethiopicus, displayed a high level of prognathism and pleisomorphic characteristics, P. boisei is a stark contrast in many regards. P. boisei displays less subnasal prognathism with a more variably orthognathic face, larger braincase (~500-530cc), mid-cranial sagittal cresting, and absence of pneumatized bone in the temporal squama [Kimbel, 2007]. P. boisei shows a progression toward a more modern direction relative to P. aethiopicus as well, as sexual dimorphism is reduced and a smaller overall body size. Less sexual dimorphism can also indicate less male competition for females, perhaps from an increased focus on the finding and keeping their specific food sources.
Dentition wise P. boisei have tiny canines, more reduced relative to the size of the post-canine teeth and "without any trace of a canine fossa" [Kimbel, 2007]. The post-canine teeth are part of what makes this species so special, with strikingly large, molarized pre-molars and tremendous cheek-teeth megadontia. They also show excessive development in the labiolingual diameter of the postcanines, which is something that sets them apart from the southern "robust" species. The dentition on most of the individuals found displays well defined areas of hypoplasic enamel, normally corresponding to what would be considered the early and latter periods of childhood [Tobias, 1967; p143]. This would indicate that the youth of Olduvai were subjected to systemic, frequent and most likely severe dietary upsets or periods of malnutrition, from which the enamel formation was impaired. However this degree of hypoplasia could also be attributed to several other factors as well, including malaria or other blood borne diseases carried by mosquitoes in the marshes that P. boisei would search for food. Further, dysentery from poor diet in times of lean food sources or potentially other infections that could have lead to poor development and stunted osteoblast activity. In addition, the masticatory stresses were undeniably intense, with every post-canine tooth showing dentine exposure and far greater levels of maxillary attrition than comparable gracile australopithecines [Tobias, 1967]. This kind of wear and level of attrition could indicate gritty or rough fibrous material diet, but perhaps not necessarily the same as A. robustus to the south (See above section on this specifically).
The second group consists of A. africanus and A. robustus, both living in exclusively the area south of modern day Botswana/Zimbabwe based on current findings. Two specimens (KNM-ER-1805 and KNM-ER-1813) that were first designated as A. africanus [Leakey and Walker, 1970] in Koobi Fora have since been determined, due to discrete morphological differences in cranial base, dentition and mandible listed by Wood et al. (1983), to be excluded from consideration as A. africanus [Wood, 1985]. The evidence for considering KNM-ER-1813 specifically is "strongly in favor of being specifically, if not generically, distinct from A. africanus." [Wood, 3; 1985], thus excluding it from being included in the East specimens. In addition, a substantial geographical distance exists between both species and any of the East African "robust” species to the north, which are examined in the sections above.
A. africanus evolved in southern Africa ~3.0-2.5mya in modern day South Africa. Its dispersion includes sites at Sterkfontein and Makapansgat, all of which differ from the East African species in that there are no large (modern) water bodies nearby, a fact that could suggest a very different diet and survival strategy. Within the interval ~2.4 to 2.5mya we see a major biotic turnover in South Africa due to the major global cooling event and subsequent climate change, leaving a clear imprint on the Southern hominins including A. africanus [Grine, 1988]. Generally we find a large variation in A. africanus, one explanation being that the hominin sample is time transgressive right in the middle of this major climate event. With a relatively large mosaic environment to adjust to, different populations could have moved in and out of the Sterkfontein valley over a long time period, leading to variations according to environmental pressures. [Kimbel, 2007; 16] Craniofacial morphology compared to A. afarensis shows a higher, shorter brain case, a narrower cranial base and more posteriorly located foramen magnum. Moderate upper midfacial prognathism is less severe than A. afarensis, but still plesiomorphic compared to A. robustus [Kimbel, 2007; 17]. A. africanus has large post-canine teeth (but smaller than A. robustus) with centrally crowded molar cusp apices in some individuals. The P3 is uniformly bicuspid, canine wear is exclusively apical, and the anterior–posterior adult occlusal wear gradient is weaker, showing a less primitive morphology than seen in A. afarensis. The mandibular fossa is deep with a strong articular eminence, compared to a relatively weak eminence in other australopithecines.
We can estimate that A. robustus evolved ~2.0-1.5mya, in Kromdraai, Swartkrans, Drimolen in South Africa not far from where many A. africanus specimens were found. There remains, however, debate about accurate dating (and classification below) especially at Swartkrans where paleomagnetic sequencing of deposits have been unsuccessful [Grine, 1988; p232]. As with all South African sites, the best estimates for these deposit ages have been based on faunal correlations with radiometrically dated sediments in eastern Africa. As such, the estimates for Swartkrans are; Member 1 (1.8 +/- 0.2mya), Member 2 (1.7-1.5mya) with Member 3 at least several hundred thousand years from Member 1 [Vogel 1985; Grine, 1988].
Further, there is debate on the classification of species by location. With the morphological equivalence and metrical similarities among the Swartkrans Member 1 ("Hanging Remnant" and "Lower Bank"), Member 2 and Member 3 samples indicating that all represent a single species and generally agreed on as P. robustus, there remains the argument by Howell (1978) and Grine (1982, 1985) that these should be given the status of P. crassidens. They also contend that in addition to naming Swartkrans samples P. crassidens, the Kromdraai samples should retain the name P. robustus originally given to them by Broom in 1938 [Grine, 1988; p233]. In addition to being separated by name, it has also been argued [Grine, 1985] that P. crassidens likely evolved from P. robustus or a P. robustus-like ancestor. It has also been suggested that P .robustus was a morphological and phylogenetic intermediate between A. africanus and P. crassidens as well [Grine, 1982; 1988; p241].
Craniofacial morphology overall displays a more orthognathic facial aspect. Specimens in Kromdraai display a variably deep mandibular fossa, small postglenoid process and plate-like tympanic element converge with modern human morphology in comparison with all our previous species discussed. A highly modified canine fossa, less prominent anterior pillar than A. africanus, an anterior placement of the zygomatic bones and basal aspect of the temporal bone all show humanlike characteristics as well [Kimbel, 2007]. Further, there are elements such as a more anterior foramen magnum position, shortened distance between tooth row and mandibular fossa and absence of pneumatized bone in the temporal squama (also shared by P. boisei). Dentition-wise in Swartkrans specimens we see the same hyper-robust, disproportionately large post-canine teeth including buccolingually expanded and molarized premolars, lower deciduous first molars and very thick enamel caps worn to flat occlusal planes. Mandibles are found to be transversely thick with vertically high rami, more robust than A. africanus [Kimbel, 2007]. As large as the dentition is, they are still not quite as large as P. boisei, but share a marked decrease in canine size and increase in enamel thickness compared to A. africanus. P. robustus premolars tend to be large compared to the Swartkrans samples, but their deciduous premolars tend to be smaller than P. crassidens. In addition, discrete traits display intra-sample variability in the Kromdraai samples, with teeth resembling more closely those of A. africanus than those from Swartkrans [Grine, 1988]. A. robustus, unlike the Eastern species, shows more modern morphology with less robusticity than P. boisei and dental abrasion that indicates a far harsher and varied diet.
"The recognition of the biological validity of A. aethiopicus rendered the A. africanus–A. robustus lineage unlikely due to the extensive character reversal it would entail given the former species’ derived zygomaticomaxillary and postcanine dental morphology. While a polyphyletic origin for the "robust" morphology via separate eastern (A. aethiopicus–A. boisei) and southern (A. africanus–A. robustus) lineages is plausible, this scheme does not gain support from phylogenetic analyses of craniofacial characters, which strongly back a monophyletic "robust" clade to the exclusion of A. africanus. Thus, the potential for a single australopith species lineage in southern Africa is weak (and not withstanding the poor chronological resolution within these hominin bearing deposits)." Rak  "The configuration of the infra-orbital facial skeleton of KNM-WT 17000 is the described by Rak (1983) as representing A. boisei. There are no anterior pillars, and the edges of the pyriform aperture are sharp and slightly everted superiorly with a nasomaxillary basin on either side. The nasals are extremely expanded superiorly, reaching above glabella and very narrow inferiorly as in A. boisei. Whatever this says about the relationships between A. boisei and A. robustus, it does appear to indicate that the evolutionary progression of functionally related features is not as Rak has presented them." Walker and Leakey  The question of a monophyletic lineage between Eastern and Southern "robust" species of australopithecines has been debated for some time, but due to an overall lack of specimens and available data we cannot definitively identify any clear conclusion without a doubt. However using the data gathered above, taking into consideration paleoenvironmental and paleoecological factors as well as the distinct morphologies of the individual specimens themselves, I believe the hypothesis for an independent divergent evolution is highly plausible.
Using the Tools created for this purpose, we can clearly see the differences both physically and geographically between the two groups and how they differ or coincide. The glaciation and cooling event of the Late Pliocene and the subsequent climate changes brought rapid, drastic changes to the environment that would force australopithecines to adapt or perish. The 2.5 million year event specifically that heralded a "turnover-pulse" in organisms at the time to change to these altered physical environments could indicate a synchronous evolutionary response in the taxa. A response to climate change could have been immigration/radiation to other areas as food sources were altered, a response that would have occurred much faster than speciation alone [Grine et al, 1988].
This climate change starting before 4.0mya and gradually accelerating further into the Late Pliocene, changed the environment in and around Olduvai to such a degree that decreased food sources and increased predation from equally pressed predators would have essentially created a barrier that kept Paranthropus from radiating past that point. The geographical element of this barrier consisting of increasingly alkaline water bodies to the far south along the East African Rift and an environmental pressure barrier directly to the south in the form of threat of attack by meat eating scavengers and risk of losing highly specific food sources in a savannah-mosaic land area.
It is the inconclusive dating of the southern species of A. robustus/crassidens that presents the largest obstacle to suggest a P. boisei – A. robustus lineage since, if correct, the radiometric and faunal correlated dating used at Swartkrans and Kromdraai would place both A. robustus and A. crassidens later than the K/Ar and (40)Ar/(39)Ar dating used to place P. boisei to the north. If P. boisei evolves in its unique environment around 2.5mya and is confined to only as far south as Olduvai (due to our hypothesized geographical barrier), then it would suggest that A. robustus/crassidens displaying the same "robust" features would be a parallel response to the same climate changes that were occurring in the northeast.
Tool 1.1 = Craniofacial Comparative Cross-section
Tool 1.2 = African Topographic/Satellite Terrain and Chronological Specimen Sites
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&pg=PA191&lpg=PA191&dq=lake+turkana+flora%2Bpliocene&source=web&ots=hQKlGZsJHQ&sig=y3d7j9o3d1ds2olifMYtVkItV8s&hl=en&sa=X&oi=book_result&resnum=2&ct=result#PPA192,M1 - pg192