The Evolutionary History of the Australopiths
© Springer Science+Business Media, LLC 2010
Published: 27 July 2010
The australopiths are a group of early hominins (humans and their close extinct relatives) that lived in Africa between approximately 4.1 and 1.4 million years ago. Formerly known as the australopithecines, they are not a “natural” group, in that they do not represent all of the descendants of a single common ancestor (i.e., they are not a “clade”). Rather, they are grouped together informally because nearly all share a similar adaptive grade (i.e., they have similar adaptations). In particular, they are bipedal apes that, to a greater or lesser extent, exhibit enlarged molar and premolar teeth (postcanine megadontia) and other associated modifications to their feeding apparatuses. Dietary adaptations clearly played an important role in shaping their evolutionary history. They also are distinguished by their lack of derived features typically associated with the genus Homo, such as a large brain, a broad complement of adaptations for manual dexterity, and advanced tool use. However, Homo is almost certainly descended from an australopith ancestor, so at least one or some australopiths belong directly to the human lineage. Regardless, australopiths had a rich evolutionary history deserving of study independent of questions about our direct ancestry. They were diverse, geographically widespread, and anatomically derived, they lived through periods of pronounced climate change, and their story dominates the narrative of human evolution for millions of years.
KeywordsAustralopithecus Paranthropus Hominin Human evolution
History of Discovery
The specimen’s morphology was broadly ape-like, and it was clearly a juvenile because it still possessed some of its milk (deciduous) teeth. However, its canine was slightly smaller and its face was slightly less projecting than one might have expected of a juvenile ape at a similar stage of development. Most importantly, its brain was small but appeared to be reorganized so that its spinal cord exited the cranial cavity through a hole (the foramen magnum) that was positioned farther forward on the skull than is typical in apes. This was a critical observation because an anteriorly positioned foramen magnum suggests that the vertebral column was vertically oriented and positioned directly beneath the skull, as in humans. In other words, it appeared to Dart as if the specimen had been bipedal. In contrast, the foramen magnum in nonhuman apes is positioned posteriorly on the skull and faces backward, corresponding to an inclined vertebral column. Dart (1925) concluded that the specimen, now widely known as the Taung child, was an extinct human ancestor, and he assigned it the name Australopithecus africanus, meaning “southern ape of Africa.”
In the second half of the twentieth century, discoveries continued in southern Africa, but australopiths were also found in eastern Africa. The first notable such discovery was made in Olduvai Gorge in Tanzania (Leakey 1959). The Gorge is part of the East African Rift Valley system, which is significant because the Rift preserves volcanic sediments that can be dated using radiometric methods. Australopiths from Olduvai are now known to be as old as approximately 1.8 million years ago (e.g., Leakey et al. 1961; Walter et al. 1991), which makes them much older than had originally been thought (e.g., Washburn 1960). Indeed, the dating of the Olduvai australopiths contributed to the idea that entire epochs of Earth’s history were older than previously surmised. Subsequent discoveries (e.g., Leakey and Walker 1976; Johanson and White 1979; Howell 1978; Walker et al. 1986) at other Rift Valley site complexes (e.g., Koobi Fora, West Turkana, Omo Shungura, Hadar) vastly expanded the number of australopith specimens and species, as well as the time range from which they were known to have existed. More recently, australopith fossils have also been discovered in both south and north central Africa (Brunet et al. 1996; Kullmer et al. 1999). Australopiths are currently unknown outside of Africa.
The australopiths were not the first hominins. They were preceded by earlier taxa which are generally not assigned either a formal or informal group name, but which are referred to here as pre-australopiths. One might also refer to them as basal hominins. There are four such species assigned to three genera. Sahelanthropus tchadensis is the earliest known putative hominin and is derived from sediments that are approximately seven million years old from Chad (Brunet et al. 2002). Orrorin tugenensis is dated to six million years ago from Kenya and is the earliest known hominin to preserve compelling postcranial evidence that it walked bipedally (Senut et al. 2001; Galik et al. 2004; Richmond and Jungers 2008). Ardipithecus kadabba is a poorly-known species appearing at 5.7 million years ago from Ethiopia (Haile-Selassie et al. 2004) that may be ancestral to the better known Ardipithecus ramidus (White et al. 1994). This latter species, known from Ethiopia and, possibly, Kenya at 4.4 million years ago, is the best known of the four and is represented by more than 100 specimens, including a partial skeleton (e.g., White et al. 2009). Interestingly, that skeleton preserves an unexpected combination of primitive traits. It has almost none of the derived traits typically associated with bipedalism, and it is said to lack many of the traits seen in the extant apes associated with suspension and vertical climbing (modes of locomotion that rely heavily on the upper limbs for propulsion and support; Lovejoy et al. 2009a, b, c). Accordingly, although all of these species share a small number of derived cranial traits with later hominins (including a reduced canine and, in some species, an anteriorly positioned foramen magnum; White et al. 1994, 2009; Senut et al. 2001; Brunet et al. 2002; Haile-Selassie et al. 2004; Strait and Grine 2004), there is a reasonable possibility that some of them (particularly Ardipithecus) may not be hominins. If so, then the derived traits they share with later hominins must have evolved in parallel (i.e., independently). Evaluating the hominin status of the pre-australopiths will be a major priority of paleoanthropology in the coming years.
Australopith Taxonomy, Distribution, and Chronology
Australopiths were once known as australopithecines, and the latter is the name that is probably most familiar to students, educators, or casual readers. However, this name is now used less and less frequently because the term australopithecine implies the existence of a formal taxonomic group, the Australopithecinae, that is no longer recognized. In contrast, the term australopith does not relate to any formal taxonomic group and thus is a more convenient shorthand label for these species.
Temporal range (million years)
Other key facts
The first known hominin species to exhibit enlarged molar and premolar teeth (postcanine megadontia)
Allia Bay, Kenya
Evidence from the knee joint indicates bipedal locomotion
3.9 (or 3.7)–3.0
Called Praeanthropus afarensis by some workers
One of the best known fossil hominin species. The species to which the partial skeleton nicknamed “Lucy” belongs
Associated with fossilized footprints from Laetoli indicating bipedalism
Koro Toro, Chad
Poorly-known species represented by only fragmentary specimens. Attributed by some workers to A. afarensis. One of only two hominin species known from north central Africa
West Turkana, Kenya
Poorly-known species best represented by a damaged cranium that preserves small molar teeth and facial morphology resembling that of some specimens of the genus Homo
Taung, South Africa Sterkfontein, South Africa Makapansgat, South Africa
The first australopith species to be discovered and one of the best known of all such species
West Turkana, Kenya
The earliest known robust australopith
Omo Shungura, Ethiopia
Possesses some but not all of the derived craniofacial traits characteristic of the other robust species
Possesses huge molar and premolar teeth but lacks the derived craniofacial morphology characteristic of the robust australopiths. Known from only a single specimen, a partial cranium
Olduvai Gorge, Tanzania
The first australopith discovered in eastern Africa
Koobi Fora, Kenya Konso, EthiopiaOmo Shungura, Ethiopia
A robust australopith originally attributed to the genus Zinjanthropus but now commonly attributed to the genus Paranthropus
Well known from jaws, crania, and teeth, but poorly known from postcrania
Malapa, South Africa
Newly discovered species preserving an intriguing mix of australopith-like and Homo-like traits
Drimolen, South Africa Kromdraai, South Africa Swartkrans, South Africa
The only robust australopith known from southern Africa.
Well known from jaws, teeth, and crania but, although postcranial remains are known from the same sites, these are not firmly attributed to the species
A. afarensis is broadly contemporaneous with Kenyanthropus platyops, a species known primarily from a single, badly damaged cranium derived from sediments on the western side of Lake Turkana in Kenya (Leakey et al. 2001). This specimen possesses small molar teeth, and in this respect it differs from most other australopiths (indeed, some workers would not classify it as such). It also exhibits craniofacial features that appear to resemble those in certain much later specimens of the genus Homo, although its brain is small. However, the poor condition of the specimen has led some to speculate that it is distorted and in fact represents simply a cranium of A. afarensis (White 2003). Alternatively, some workers see this species as the phyletic ancestor of certain members of the genus Homo (Leakey et al. 2001), but there are other ways of interpreting the morphological evidence (Strait and Grine 2004; see below).
A. afarensis disappears at around three million years ago, but the eastern African fossil record is poor following that time, so the precise date at which the species goes extinct is not known with certainty. Hominins reappear starting at approximately 2.7 million years ago in the Omo Shungura site complex (Suwa et al. 1996), and these seem to represent multiple distinct species. Among them is the first of the taxa known as the robust australopiths, Paranthropus aethiopicus (Arambourg and Coppens 1967). The robust species are all characterized by enlarged cheek teeth, massive chewing muscles, and modifications to their facial skeleton that are thought to either increase the leverage of those muscles or to buttress the face against the loads imposed by high or repetitive bite forces that would have been applied to the teeth (e.g., Rak 1983). P. aethiopicus has these traits but, unlike the other robust species, combines them with large anterior teeth (incisors and canines), a highly projecting face, and a small brain (Walker et al. 1986).
P. aethiopicus is broadly contemporaneous with Australopithecus garhi (Asfaw et al. 1999), a species that blurs the distinction between robust and gracile australopiths. This species resembles the gracile taxa in that it preserves a broadly A. afarensis-like craniofacial skeleton, but it resembles robust taxa in that it possesses absolutely massive molar and premolar teeth. However, the details of the surface anatomy of these teeth appear to differ from those of P. aethiopicus and the later robust australopiths. The species is known from only a single specimen from Ethiopia, and it is dated to approximately 2.5 million years ago. Interestingly, the specimen derives from strata that also preserve stone tools and animal bones with cut marks (de Heinzelin et al. 1999), but there is no way of firmly associating these finds with A. garhi. Some workers imply that A. garhi is a suitable ancestor of Homo (Asfaw et al. 1999), but there is no direct evidence suggesting this to be the case (Strait and Grine 1999, 2004). It is equally or more likely that A. garhi represents a terminal descendant of A. afarensis that evolved massive cheek teeth in parallel with the robust australopiths (Strait et al. 2007).
A. africanus is succeeded in the southern African fossil record by a robust australopith, Paranthropus robustus. This species is said to have appeared at approximately 1.8 million years ago (e.g., Vrba 1995). It is likely to have persisted for at least a few hundred thousand years. Like P. boisei, it has derived craniodental features associated with generating and withstanding high or repetitive bite forces, although the expression of these traits is not as extreme as is seen in the eastern African species (Rak 1983). Interestingly, P. robustus is derived from sediments that also preserve postcranial remains that may indicate a more advanced degree of bipedalism and manual dexterity than is seen in A. afarensis and A. africanus (Susman 1988a, b, 1994). However, these fossils cannot be assigned to P. robustus with certainty because individuals of the genus Homo are also found in those layers (e.g., Trinkaus and Long 1990).
A new southern African australopith, Australopithecus sediba, has just been described from the site of Malapa. It is currently known from two partial skeletons, although future work may reveal more specimens. This species preserves an interesting mix of anatomical traits that include some australopith-like features (including a small brain) and some Homo-like features. It appears to either slightly predate or be roughly contemporaneous with P. robustus.
Note that debate persists regarding the precise cladistic relationships of early hominins, and the cladogram depicted in Fig. 6a represents only one of several phylogenetic hypotheses. However, most workers would probably accept some form of the simplified cladograms shown in Fig. 6b, c.
Naturally, phyletic trees are like cladograms in that they represent hypotheses, and the phylogenetic hypothesis depicted in Fig. 7 is only one of several that might explain the pattern of early human evolution.
The australopiths lived during a period of pronounced climatic change. During the late Miocene and into the Pliocene, African climates became cooler and dryer. Over time, this led to a fragmentation of African forests that almost certainly played a role in hominin origins and the evolution of both bipedalism and the pre-australopiths. Then, between three and two million years ago, the onset of northern hemisphere glaciation led to increased climatic variability as climates shifted strongly back and forth from cool and dry to warm and wet over relatively short time periods (Potts 1998). This pattern continued throughout the Pleistocene. Although most australopiths are associated with so-called mosaic environments in which multiple types of habitat were present (e.g., woodland, savannah, gallery forest, bushland, etc.), the earlier australopiths are generally associated with habitats that were more heavily wooded than those associated with later australopiths (e.g., Reed 1997). In order to cope with changing and unstable habitats, it is generally thought that many of the derived anatomical traits seen in australopiths are adaptations that enabled them to be behaviorally flexible, particularly concerning their diet (e.g., Wood and Strait 2004).
Less is known about locomotion in other australopith species, but based on the fossils that are preserved, there is nothing to indicate that locomotion in them would have been fundamentally different from that of A. afarensis and A. africanus. One exception might be that some modern-appearing fossils from the southern African cave site of Swartkrans might indicate that P. robustus had a more human-like mode of locomotion than the gracile australopiths (Susman 1988a), but this possibility cannot be established with certainty because fossils of the genus Homo are also known from this cave. Moreover, postcranial fossils derived from the same site complex as A. garhi appear to exhibit longer hind limbs than A. afarensis, but these fossils have not been attributed to A. garhi or any other australopith (Asfaw et al. 1999). Aspects of the australopith proximal femur (the part of the thigh bone that contributes to the hip joint) appear to have been highly conservative and were present in at least one pre-australopith, Orrorin. These traits pertain to maintaining balance while shifting weight from one limb to another during bipedalism. This suggests that components of australopith locomotor behavior persisted for over four million years (Richmond and Jungers 2008). However, other pre-australopiths, particularly Ardipithecus, had radically different postcranial skeletons that exhibit barely any adaptations for bipedalism (Lovejoy et al. 2009a, b, c). If Ardipithecus is a hominin, then it appears as if the evolution of locomotion was complex during the earliest third of human evolution.
Diet: Unlike postcranial anatomy, aspects of craniodental morphology are known for every australopith species. With the exception of Kenyanthropus (Leakey et al. 2001) and A. sediba (Berger et al. 2010), all australopiths exhibit postcanine megadontia (e.g., McHenry 1984). This megadontia is initially expressed primarily in the molars, but in later species it is expressed to a high degree in the premolars as well. Relative to living chimpanzees and gorillas, all australopith species also exhibit thick enamel (the highly mineralized outer coating of the tooth crowns) on their postcanine teeth (e.g., Martin 1985; Olejniczak et al. 2008). As megadontia increases, so too does the size and robusticity of the mandible. Simultaneously, the chewing muscles become proportionally larger and their positions on the skull shift so as to maximize their leverage. Finally, several of the australopiths exhibit derived traits on their facial skeletons that act to withstand high stresses imposed by feeding (e.g., Rak 1983). Collectively, all of these adaptations point to the probability that australopiths had the ability to process foods that were mechanically resistant (e.g., Jolly 1970; Peters 1987).
Like most apes, australopiths undoubtedly would have preferred to eat soft, sweet, fleshy fruit, but when those resources were not available, they would have had the ability to “fall back” on foods that were less desirable and more difficult to process (e.g., Marshall and Wrangham 2007). In this manner, they were likely to have been ecological generalists well suited to respond to changes in their habitats (Wood and Strait 2004). Indeed, analyses of stable (nonradioactive) isotopes preserved in the tooth enamel of P. robustus indicate that at least this species shifted its diet seasonally (Sponheimer et al. 2006). The precise nature of the fallback foods eaten by australopiths is a matter of debate. On mechanical grounds, some workers suggest that australopith facial traits are adaptations for feeding on large, hard objects like large nuts and seeds (e.g. Peters 1987; Strait et al. 2009). However, studies of dental microwear (the microscopic damage done to teeth by food and grit) suggest that few australopiths routinely fed on hard objects and that some may have fallen back on tough, rather than hard, vegetation (Scott et al. 2005; Grine et al. 2006a, b; Ungar et al. 2008). The mechanical and microwear data are compatible if large hard objects were selectively very important but consumed very rarely or if large hard objects do not tend to leave microwear signals (as has been suggested; Lawn and Lee 2009). Regardless, it seems evident that selective pressures related to diet and feeding profoundly influenced the evolution of australopiths.
Sexual dimorphism: Sexual dimorphism refers to the phenomenon in which males and females of a species differ with respect to size and/or shape. The most common manifestation of dimorphism among anthropoid primates is when males have a larger body mass and larger, more projecting canine teeth than females (e.g., Plavcan 2001). Dimorphism in these traits is, in turn, correlated with social behavior insofar as highly dimorphic primates tend to be polygynous such that social groups are centered around several breeding females, one breeding male, and their offspring. This is a simplification, and polygynous groups can take multiple forms, but the single male–multifemale organization is a component of most such groups. Dimorphism in body mass and canine size is thought to have evolved due to sexual selection because large body and canine size are advantageous for males as they compete with each other to become the breeding male in a group and because females may preferentially select those traits in their breeding partners. Several, although not all, australopiths appear to be highly dimorphic with respect to both cranial and postcranial dimensions (e.g., Plavcan 2001, 2003; Gordon et al. 2008), and these data may suggest that several of these species were highly dimorphic in body mass as well, although this latter point is disputed (e.g., Reno et al. 2003). Interestingly, hominins appear to be unique among anthropoids in that canine and body size dimorphism appear to have been decoupled (Plavcan 2001). Thus, although several australopiths may have been quite dimorphic in body mass, they exhibit reduced dimorphism in canine size. One possible explanation for this unique pattern is that hominins were polygynous, but that canine size was no longer a target of sexual selection. If true, the reason why canine size was no longer being selected remains unclear. However, those workers that challenge the notion that australopiths, particularly A. afarensis, were dimorphic in body mass (e.g., Reno et al. 2003) suggest that these species may have been monogamous (i.e., their social groups were centered around a single male and a single female; e.g., Lovejoy 1981). However, this hypothesis is very difficult to test.
Australopiths disappear after 1.4 million years ago. The last surviving species are P. boisei in eastern Africa and P. robustus in southern Africa. Unfortunately, the African fossil record is poor after this point, so we cannot rule out the possibility that australopiths persisted for some time before eventually going extinct. However, at some point, they eventually succumbed. The reason for their extinction is unclear. During the time in which both they and Homo existed, fossils of australopiths were much more numerous at most sites (e.g., Wood and Strait 2004), yet it is only Homo that survived. Moreover, australopiths were probably ecological generalists that were capable of living in different types of habitats and consuming different types of food, and this flexibility ought to have made them resistant to extinction (Wood and Strait 2004), although all species, regardless of their adaptations, eventually go extinct at some time. One might point to any number of factors to explain their demise (predation, competition with other hominins, competition with nonhominin mammals; Klein 1988; Wood and Strait 2004), but it is proposed here that the reason concerns dietary ecology. It is evident from their craniodental morphology that australopiths were under intense selection pressure to modify their feeding apparatus, and strong selection carries with it a strong risk of extinction. Two hypotheses are posed here to explain their disappearance. First, the critical resources that australopiths fell back on during periods of resource scarcity may have themselves disappeared, perhaps as a result of climate change. Without these key fallback foods, australopiths might have been unable to survive. Alternatively, the fallback foods might have evolved to become so mechanically resistant that not even robust australopiths could access them. This, too, might have led to australopith extinction. Unfortunately, these hypotheses are difficult to test and, at the present time, must be considered mere conjecture.
The australopiths are gone, but in much the same way that birds are dinosaurs, we are australopiths in that we are almost certainly descended from one of them. It is possible that the australopiths on our direct lineage are not yet known to science, but it is highly unlikely that australopiths represent an entirely distinct clade whose evolutionary history is completely independent from ours. Interestingly, members of our own genus (Homo) appear to have succeeded by abandoning the adaptations that made australopiths successful (e.g., large jaws, massive cheek teeth, huge chewing muscles). In their place, early members of the genus Homo may have become behaviorally flexible by evolving large brains, more dextrous hands, and advanced tool use. This strategy evidently proved to be more successful in navigating the vicissitudes of the Pleistocene.
I thank Will Harcourt-Smith and Niles Eldredge for the invitation to submit this article. Eric Delson and Bernard Wood kindly provided images for some of the figures. This paper was improved considerably by comments by the editors, Eric Delson, and an anonymous reviewer.
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