The Australopithecines are a diverse group of hominins, comprising various species. Australopithecus is the given group or genus name. It stems from the Latin word Australo, meaning “southern,” and the Greek word pithecus, meaning “ape.” Within this section, we will outline these differing species’ geological and temporal distributions across Africa, unique derived and/or shared traits, and importance in the fossil record.
Figure 9.12: Robust Australopithecines such as Paranthropus boisei had large molars and chewing muscles. Credit: Paranthropus boisei skull by Durova is under a CC BY-SA 3.0 License.
Between 3 mya and 1 mya, there seems to be differences in dietary strategy between different species of hominins designated as Australopithecines. A pattern of larger posterior dentition (even relative to the incisors and canines in the front of the mouth), thick enamel, and cranial evidence for extremely large chewing muscles is far more pronounced in a group known as the robust australopithecines. This pattern is extreme relative to their earlier contemporaries or predecessors, the gracile australopithecines, and is certainly larger than those seen in early Homo, which emerged during this time. This pattern of incredibly large hind dentition (and very small anterior dentition) has led people to refer to robust australopithecines as megadont hominins (Figure 9.12).
Because of these differences, this section has been divided into “gracile” and “robust” Australopithecines, highlighting the morphological differences between the two groups (which many researchers have designated as separate genera: Australopithecus and Paranthropus, respectively) and then focusing on the individual species. It is worth noting, however, that not all researchers accept these clades as biologically or genetically distinct, with some researchers insisting that the relative gracile and robust features found in these species are due to parallel evolutionary events toward similar dietary niches.
Despite this genus’ ancestral traits and small cranial capacity, all members show evidence of bipedal locomotion. It is generally accepted that Australopithecus species display varying degrees of arborealism along with bipedality.
Gracile Australopithecines
This section describes individual species from across Africa. These species are called “gracile australopithecines” because of their smaller and less robust features compared to the divergent “robust” group. Numerous Australopithecine species have been named, but some are only based on a handful of fossil finds, whose designations are controversial.
East African Australopithecines
East African Australopithecines are found throughout the EARS, and they include the earliest species associated with this genus. Numerous fossil-yielding sites, such as Olduvai, Turkana, and Laetoli, have excellent, datable stratigraphy, owing to the layers of volcanic tufts that have accumulated over millions of years. These tufts may be dated using absolute dating techniques, such as Potassium-Argon dating (described in Chapter 7). This means that it is possible to know a relatively refined date for any fossil if the context (i.e., exact location) of that find is known. Similarly, comparisons between the faunal assemblages of these stratigraphic layers have allowed researchers to chronologically identify environmental changes.
The earliest known Australopithecine is dated to 4.2 mya to 3.8 mya. Australopithecus anamensis (after “Anam,” meaning “lake” from the Turkana region in Kenya; Leakey et al. 1995; Patterson and Howells 1967) is currently found from sites in the Turkana region (Kenya) and Middle Awash (Ethiopia; Figure 9.13). Recently, a 2019 find from Ethiopia, named MRD, after Miro Dora where it was found, was discovered by an Ethiopian herder named Ali Bereino. It is one of the most complete cranial finds of this species (Ward et al. 1999). A small brain size (370 cc), relatively large canines, projecting cheekbones, and earholes show more ancestral features as compared to those of more recent Australopithecines. The most important element discovered with this species is a fragment of a tibia (shinbone), which demonstrates features associated with weight transfer during bipedal walking. Similarly, the earliest found hominin femur belongs to this species. Ancestral traits in the upper limb (such as the humerus) indicate some retained arboreal locomotion.
Some researchers suggest that Au. anamensis is an intermediate form of the chronospecies that becomes Au. afarensis, evolving from Ar. ramidus. However, this is debated, with other researchers suggesting morphological similarities and affinities with more recent species instead. Almost 100 specimens, representing over 20 individuals, have been found to date (Leakey et al. 1995; McHenry 2009; Ward et al. 1999).
Au. afarensis is one of the oldest and most well-known australopithecine species and consists of a large number of fossil remains. Au. afarensis (which means “from the Afar region”) is dated to between 2.9 mya and 3.9 mya and is found in sites all along the EARS system, in Tanzania, Kenya, and Ethiopia (Figure 9.14). The most famous individual from this species is a partial female skeleton discovered in Hadar (Ethiopia), later nicknamed “Lucy,” after the Beatles’ song “Lucy in the Sky with Diamonds,” which was played in celebration of the find (Johanson et al. 1978; Kimbel and Delezene 2009). This skeleton was found in 1974 by Donald Johanson and dates to approximately 3.2 mya. In addition, in 2002 a juvenile of the species was found by Zeresenay Alemseged and given the name “Selam” (meaning “peace,” DIK 1-1), though it is popularly known as “Lucy’s Child” or as the “Dikika Child” (Alemseged et al. 2006). Similarly, the “Laetoli Footprints” (discussed in Chapter 7; Hay and Leakey 1982; Leakey and Hay 1979) have drawn much attention.
The canines and molars of Au. afarensis are reduced relative to great apes but are larger than those found in modern humans (indicative of a generalist diet); in addition, Au. afarensis has a prognathic face (the face below the eyes juts anteriorly) and robust facial features that indicate relatively strong chewing musculature (compared with Homo) but which are less extreme than in Paranthropus. Despite a reduction in canine size in this species, large overall size variation indicates high levels of sexual dimorphism.
Skeletal evidence indicates that this species was bipedal, as its pelvis and lower limb demonstrate a humanlike femoral neck, valgus knee, and bowl-shaped hip (Figure 9.15). More evidence of bipedalism is found in the footprints of this species. Au. afarensis is associated with the Laetoli Footprints, a 24-meter trackway of hominin fossil footprints preserved in volcanic ash discovered by Mary Leakey in Tanzania and dated to 3.5 mya to 3 mya. This set of prints is thought to have been produced by three bipedal individuals as there are no knuckle imprints, no opposable big toes, and a clear arch is present. The infants of this species are thought to have been more arboreal than the adults, as discovered through analyses of the foot bones of the Dikika Child dated to 3.32 mya (Alemseged et al. 2006).
Although not found in direct association with stone tools, potential evidence for cut marks on bones, found at Dikika, and dated to 3.39 mya indicates a possible temporal/ geographic overlap between meat eating, tool use, and this species. However, this evidence is fiercely debated. Others have associated the cut marks with the earliest tool finds from Lomekwi, Kenya, temporally (3.3 mya) and in close geographic proximity to this species.
South African Australopithecines
Since the discovery of the Taung Child, there have been numerous Australopithecine discoveries from the region known as “The Cradle of Humankind,” which was recently given UNESCO World Heritage Site status as “The Fossil Hominid Sites of South Africa.” The limestone caves found in the Cradle allow for the excellent preservation of fossils. Past animals navigating the landscape and falling into cave openings, or caves used as dens by carnivores, led to the accumulation of deposits over millions of years. Many of the hominin fossils, encased in breccia (hard, calcareous sedimentary rock), are recently exposed from limestone quarries mined in the previous century. This means that extracting fossils requires excellent and detailed exposed work, often by a team of skilled technicians.
While these sites have historically been difficult to date, with mixed assemblages accumulated over large time periods, advances in techniques such as uranium-series dating have allowed for greater accuracy. Historically, the excellent faunal record from East Africa has been used to compare sites based on relative dating, whereby environmental and faunal changes and extinction events allow us to know which hominin finds are relatively younger or older than others.
The discovery of the Taung Child in 1924 (discussed in the Special Topic box “The Taung Child” below) shifted the focus of palaeoanthropological research from Europe to Africa, although acceptance of this shift was slow (Broom 1947; Dart 1925). The species to which it is assigned, Australopithecus africanus (name meaning “Southern Ape of Africa”), is currently dated to between 3.3 mya and 2.1 mya (Pickering and Kramers 2010), with discoveries from Sterkfontein, Taung, Makapansgat, and Gladysvale in South Africa (Figure 9.16). A relatively large brain (400 cc to 500 cc), small canines without an associated diastema, and more rounded cranium and smaller teeth than Au. afarensis indicate some derived traits. Similarly, the postcranial remains (in particular, the pelvis) indicate bipedalism. However, the sloping face and curved phalanges (indicative of retained arboreal locomotor abilities) show some ancestral features. Although not in direct association with stone tools, a 2015 study noted that the trabecular bone morphology of the hand was consistent with forceful tool manufacture and use, suggesting potential early tool abilities.
Another famous Au. africanus skull (the skull of “Mrs. Ples”) was previously attributed to Plesianthropus transvaalensis, meaning “near human from the Transvaal,” the old name for Gauteng Province, South Africa (Broom 1947, 1950). The name was shortened by contemporary journalists to “Ples” (Figure 9.17). Due to the prevailing mores of the time, the assumed female found herself married, at least in name, and has become widely known as “Mrs. Ples.” It was later reassigned to Au. africanus and is now argued by some to be a young male rather than an adult female cranium (Thackeray 2000, Thackeray et al. 2002).
In 2008, nine-year-old Matthew Berger, son of paleoanthropologist Lee Berger, noted a clavicle bone in some leftover mining breccia in the Malapa Fossil Site (South Africa). After rigorous studies, the species, Australopithecus sediba (meaning “fountain” or “wellspring” in the South African language of Sesotho), was named in 2010 (Figure 9.18; Berger et al. 2010). The first type specimen belongs to a juvenile male, Karabo (MH1), but the species is known from at least six partial skeletons, from infants through adults. These specimens are currently dated to 1.97 mya (Dirks et al. 2010). The discoverers have argued that Au. sediba shows mosaic features between Au. africanus and the genus, Homo, which potentially indicates a transitional species, although this is heavily debated. These features include a small brain size (Australopithecus-like; 420 cc to 450 cc) but gracile mandible and small teeth (Homo-like). Similarly, the postcranial skeletons are also said to have mosaic features: scientists have interpreted this mixture of traits (such as a robust ankle but evidence for an arch in the foot) as a transitional phase between a body previously adapted to arborealism (particularly in evidence from the bones of the wrist) to one that adapted to bipedal ground walking. Some researchers have argued that Au. sediba shows a modern hand morphology (shorter fingers and a longer thumb), indicating that adaptations to tool manufacture and use may be present in this species.
Another famous Australopithecine find from South Africa is that of the nearly complete skeleton now known as “Little Foot” (Clarke 1998, 2013). Little Foot (StW 573) is potentially the earliest dated South African hominin fossil, dating to 3.7 mya, based on radiostopic techniques, although some argue that it is younger than 3 mya (Pickering and Kramers 2010). The name is jokingly in contrast to the cryptid species “bigfoot” and is named because the initial discovery of four ankle bones indicated bipedality. Little Foot was discovered by Ron Clarke in 1994, when he came across the ankle bones while sorting through monkey fossils in the University of Witwatersrand collections (Clarke and Tobias 1995). He asked Stephen Motsumi and Nkwane Molefe to identify the known records of the fossils, which allowed them to find the rest of the specimen within just days of searching the Sterkfontein Caves’ Silberberg Grotto.
The discoverers of Little Foot insist that other fossil finds, previously identified as Au. Africanus, be placed in this new species based on shared ancestral traits with older East African Australopithecines (Clarke and Kuman 2019). These include features such as a relatively large brain size (408 cc), robust zygomatic arch, and a flatter midface. Furthermore, the discoverers have argued that the heavy anterior dental wear patterns, relatively large anterior dentition, and smaller hind dentition of this specimen more closely resemble that of Au. anamensis or Au. afarensis. It has thus been placed in the species Australopithecus prometheus. This species name refers to a previously defunct taxon named by Raymond Dart. The species designation was, through analyzing Little Foot, revived by Ron Clarke, who insists that many other fossil hominin specimens have prematurely been placed into Au. africanus. Others say that it is more likely that Au. africanus is a more variable species and not representative of two distinct species.
Paranthropus “Robust” Australopithecines
In the robust australopithecines, the specialized nature of the teeth and masticatory system, such as flaring zygomatic arches (cheekbones), accommodate very large temporalis (chewing) muscles.These features also include a large, broad, dish-shaped face and and a large mandible with extremely large posterior dentition (referred to as megadonts) and hyper-thick enamel (Kimbel 2015; Lee-Thorp 2011; Wood 2010). Research has revolved around the shared adaptations of these “robust” australopithecines, linking their morphologies to a diet of hard and/or tough foods (Brain 1967; Rak 1988). Some argued that the diet of the robust australopithecines was so specific that any change in environment would have accelerated their extinction. The generalist nature of the teeth of the gracile australopithecines, and of early Homo, would have made them more capable of adapting to environmental change. However, some have suggested that the features of the robust australopithecines might have developed as an effective response to what are known as fallback foods in hard times rather than indicating a lack of adaptability.
There are currently three widely accepted robust australopithecus or, Paranthropus, species: P. aethiopicus, which has more ancestral traits, and P. boisei and P. robustus, which are more derived in their features (Strait et al. 1997; Wood and Schroer 2017). These three species have been grouped together by a majority of scholars as a single genus as they share more derived features (are more closely related to each other; or, in other words, are monophyletic) than the other australopithecines (Grine 1988; Hlazo 2015; Strait et al. 1997; Wood 2010 ). While researchers have mostly agreed to use the umbrella term Paranthropus, there are those who disagree (Constantino and Wood 2004, 2007; Wood 2010).
As a collective, this genus spans 2.7 mya to 1.0 mya, although the dates of the individual species differ. The earliest of the Paranthropus species, Paranthropus aethiopicus, is dated to between 2.7 mya and 2.3 mya and currently found in Tanzania, Kenya, and Ethiopia in the EARS system (Figure 9.19; Constantino and Wood 2007; Hlazo 2015; Kimbel 2015; Walker et al. 1986; White 1988). It is well known because of one specimen known as the “Black Skull” (KNM–WT 17000), so called because of the mineral manganese that stained it black during fossilization (Kimbel 2015). As with all robust Australopithecines, P. aethiopicus has the shared derived traits of large, flat premolars and molars; large, flaring zygomatic arches for accommodating large chewing muscles (the temporalis muscle); a sagittal crest (ridge on the top of the skull) for increased muscle attachment of the chewing muscles to the skull; and a robust mandible and supraorbital torus (brow ridge). However, only a few teeth have been found. A proximal tibia indicates bipedality and similar body size to Au. afarensis. In recent years, researchers have discovered and assigned a proximal tibia and juvenile cranium (L.338y-6) to the species (Wood and Boyle 2016).
First attributed as Zinjanthropus boisei (with the first discovery going by the nickname “Zinj” or sometimes “Nutcracker Man”), Paranthropus boisei was discovered in 1959 by Mary Leakey (see Figure 9.20 and 9.21; Hay 1990; Leakey 1959). This “robust” australopith species is distributed across countries in East Africa at sites such as Kenya (Koobi Fora, West Turkana, and Chesowanja), Malawi (Malema-Chiwondo), Tanzania (Olduvai Gorge and Peninj), and Ethiopia (Omo River Basin and Konso). The hypodigm, sample of fossils whose features define the group, has been found by researchers to date to roughly 2.4 mya to 1.4 mya. Due to the nature of its exaggerated, larger, and more robust features, P. boisei has been termed hyper-robust—that is, even more heavily built than other robust species, with very large, flat posterior dentition (Kimbel 2015). Tools dated to 2.5 mya in Ethiopia have been argued to possibly belong to this species. Despite the cranial features of P. boisei indicating a tough diet of tubers, nuts, and seeds, isotopes indicate a diet high in C4 foods (e.g., grasses, such as sedges). Another famous specimen from this species is the Peninj mandible from Tanzania, found in 1964 by Kimoya Kimeu.
Paranthropus robustus was the first taxon to be discovered within the genus in Kromdraai B by a schoolboy named Gert Terblanche; subsequent fossil discoveries were made by researcher Robert Broom in 1938 (Figure 9.22; Broom 1938a, 1938b, 1950), with the holotype specimen TM 1517 (Broom 1938a, 1938b, 1950; Hlazo 2018). Paranthropus robustus dates approximately from 2.0 mya to 1 mya and is the only taxon from the genus to be discovered in South Africa. Several of these fossils are fragmentary in nature, distorted, and not well preserved because they have been recovered from quarry breccia using explosives. P. robustus features are neither as “hyper-robust” as P. boisei nor as ancestral as P. aethiopicus; instead, they have been described as being less derived, more general features that are shared with both East African species (e.g., the sagittal crest and zygomatic flaring; Rak 1983; Walker and Leakey 1988). Enamel hypoplasia is also common in this species, possibly because of instability in the development of large, thick-enameled dentition.
Comparisons between Gracile and Robust Australopiths
Comparisons between gracile and robust australopithecines may indicate different phylogenetic groupings or parallel evolution in several species. In general, the robust australopithecines have large temporalis (chewing) muscles, as indicated by flaring zygomatic arches, sagittal crests, and robust mandibles (jawbones). Their hind dentition is large (megadont), with low cusps and thick enamel. Within the gracile australopithecines, researchers have debated the relatedness of the species, or even whether these species should be lumped together to represent more variable or polytypic species. Often researchers will attempt to draw chronospecific trajectories, with one taxon said to evolve into another over time.
The well-known fossil of a juvenile Australopithecine, the “Taung Child,” was the first early hominin evidence ever discovered and was the first to demonstrate our common human heritage in Africa (Figure 9.23; Dart 1925). The tiny facial skeleton and natural endocast were discovered in 1924 by a local quarryman in the North West Province in South Africa and were painstakingly removed from the surrounding cement-like breccia by Raymond Dart using his wife’s knitting needles. When first shared with the scientific community in 1925, it was discounted as being nothing more than a young monkey of some kind. Prevailing biases of the time made it too difficult to contemplate that this small-brained hominin could have anything to do with our own history. The fact that it was discovered in Africa simply served to strengthen this bias.
