Australopithecus Group

A Australopithecus Group Scott A. Williams Center for the Study of Human Origins, Department of Anthropology, New York University, New York, NY, USA Synonyms Australopiths; Kenyanthropus; Paranthropus; Praeanthropus Definition A genus (or group of multiple genera) of fossil hominins ranging in geological age from the Pliocene through the early Pleistocene (~4.2–1.2 Ma = Mega-annum, or millions of years) of Africa, the name Australopithecus comes from Greek for “ape” (pithekos) and Latin “of the south” (australis), for the geographical location of the first material discovered, in South Africa. genus Australopithecus (but several species are commonly included in a distinct genus, Paranthropus, and one species is named in a distinct genus, Kenyanthropus). Additionally, some researchers suggest that the earliest members of the genus Homo should actually be transferred to Australopithecus, leaving just Homo erectus and its descendants in Homo. The genus Australopithecus is almost certainly paraphyletic, meaning that it does not form a natural group; instead, one or more lineages are more closely related to members of other genera (e.g., Paranthropus or Homo). Australopiths were ape-like hominins that retained relatively small brains and probably some adaptations to the arboreal environment (i.e., long arms for tree climbing), yet were clearly upright bipeds (adapted to walking on two legs), some of which were primitive tool users. Determining which australopith lineage gave rise to later hominins (i.e., Homo) is a major area of focus and controversy in paleoanthropology (Fig. 1). History Introduction The Australopithecus group (“australopiths”) consists of up to twelve species of PlioPleistocene hominins (members of the human lineage following the phylogenetic divergence from chimpanzees and bonobos) mostly included in the The genus Australopithecus was not the first hominin genus other than Homo to be named, but it is the oldest one that is still used (Pithecanthropus, for example, has been subsumed within Homo as H. erectus). Named in 1925 by Raymond Dart, Australopithecus africanus was immediately challenged by Dart’s # Springer International Publishing AG 2016 T.K. Shackelford, V.A. Weekes-Shackelford (eds.), Encyclopedia of Evolutionary Psychological Science, DOI 10.1007/978-3-319-16999-6_3423-1 2 Australopithecus Group Australopithecus Group, Fig. 1 Australopiths in time and space. Species are arranged according to their documented presence in the fossil record (x-axis; Ma = Mega annum: millions of years before present) and approximate geographic locality (y-axis; arranged from south to north). Species known from more than one locality or date, or when a range of dates are published, are shown with vertical and horizontal bars, respectively. Single points indicate that a species is known from one site and date; a dashed line indicates that the date is controversial (as is the case with A. prometheus) or links material not currently assigned to a species to affiliated material (as is the case with the Ledi-Geraru mandible and the A.L. 666-1 palate, the earliest Homo material). Homo material <2 Ma is not shown but is represented in both East and South Africa and elsewhere outside Africa. Color codes indicate different genera: green Ardipithecus, black Australopithecus, blue Kenyanthropus, purple Paranthropus, red Homo senior colleagues in England, largely due to the influence of the later-discredited Piltdown material and the early developmental age of the A. africanus type specimen, a juvenile skull from Taungs. Throughout the 1930s and 1940s, Robert Broom amassed a number of adult specimens that he initially attributed to a new species, A. transvaalensis (then later a new genus, Plesianthropus – “near man”), the most famous of which is the Sts 5 cranium, nicknamed “Mrs. Ples.” Broom and his assistant John Robinson, who carried on work at Sterktontein after Broom’s death in 1951, also discovered Sts 14, the first partial skeleton of an early hominin that preserved numerous associated postcranial (below the skull) bones and therefore provided information on the way in which A. africanus walked. Robinson (1972) and later Owen Lovejoy and others (e.g., Lovejoy et al. 1973) studied this material to understand the anatomy and biomechanics of bipedalism in an early hominin. The new, adult material attributed to A. africanus mostly put to rest the Australopithecus Group idea that Dart’s “African ape of the south” (or “southern ape of Africa”) was anything but a bipedal, early member of the human lineage. Additionally, Broom discovered what he considered two species of a new genus that he named Paranthropus, meaning “alongside man” (P. robustus and P. crassidens; although a minority of researchers today still recognize material from Kromdraii and Swartkrans, respectively, as separate species, the latter material is generally subsumed within the former species). These craniodentally (concerning the skull, including teeth) “robust” hominins were distinct from the more “gracile” A. africanus, which later led to the introduction and common use of “gracile and robust australopithecines” to refer to the two groups, the former later subsuming within it a new species called Australopithecus afarensis (see below). Today, the terms are outdated, both because neither A. africanus nor A. afarensis was particularly gracile in craniodental morphology and because “australopithecine” invokes a taxonomic statement (subfamily designation) that is considered by many to be too broad and therefore inaccurate (i.e., “hominin” refers to a tribe, Hominini, whereas Homininae, a subfamily, includes all African great apes – hominins and members of the chimpanzee/bonobos and gorilla lineages). Therefore, the term “gracile” has been largely abandoned and “robust australopith” is preferred in colloquial reference to members of the genus Paranthropus. In the 1950s, Louis and Mary Leakey discovered Olduvai Hominid 5 (OH5), a cranium belonging to what they called Zinjanthropus boisei (“Boise’s eastern African man”), a hyper-robust species clearly adapted to heavy/tough/repetitive chewing, and hence nicknamed “Nutcracker Man.” In 1962, Sherwood Washburn organized a Wenner-Gren Foundation-funded meeting of prominent paleoanthropologists, primate anatomists, molecular anthropologists, primate behavioral ecologists, and the three primary leaders of the modern synthesis of evolutionary biology, George Simpson, Theodosius Dobzhansky, and Ernst Mayr, to discuss the current state of taxonomy in the human fossil record. While all of the 3 entries in what would become an edited volume (Washburn 1963) are now considered classic contributions, Mayr’s paper on the taxonomy of fossil hominids had a particularly strong influence on what Washburn called the “new physical anthropology.” Recognizing the surfeit of genera in the hominin fossil record, Mayr recommended that just two be recognized, Australopithecus (including Plesianthropus, Paranthropus, Zinjanthropus, and other genus names proposed for early hominins) and Homo (including archaic e.g., Neandertals - and modern humans and a plethora of other genus and other species names now referred to as Homo erectus). Many paleoanthropologists followed suit, including Robinson, who sunk A. africanus into Homo but retained Paranthropus, thus adopting a distinct, two-genus system to that proposed by Mayr (Robinson 1972). In the 1970s, discoveries in East Africa, specifically from Laetoli in Tanzania and Hadar in Ethiopia, led to the naming of a new species of Australopithecus, A. afarensis (although for taxonomic reasons some researchers prefer the genus Praeanthropus – “pre-man”). The material it was based on included the type specimen, a mandible from Laetoli (Laetoli Hominid 4; LH 4) discovered by Mary Leakey and her team, and a partial skeleton from Afar Locality 288 (A.L. 288-1) colloquially known as “Lucy,” along with other craniodental and postcranial material from nearby localities (e.g., A.L. 129), discovered by Donald Johanson and Maurice Taieb. Since the naming of A. afarensis, much more material has been discovered, including the A.L. 333 “First Family,” a collection of craniodental and postcranial fossils attributable to a group of individuals killed around the same time, A.L. 444-2, a large partial skull (Kimbel and Delezene 2009), and DIK-1-1, the skull and partial skeleton of a young individual similar in developmental age to the Taung child, from Dikika, Ethiopia (Alemseged et al. 2006). A. afarensis is both geologically older (3.9–3 Ma) and morphology more primitive in some ways than A. africanus (3.0–2.1 Ma); thus, it was interpreted by Johanson and Tim White to be ancestral to an A. africanus-Paranthropus lineage 4 on one hand, and the genus Homo on the other (Johanson and White 1979). The discovery of the KNM-WT 17000 (“the Black Skull”) cranium from West Lake Turkana in Kenya can be interpreted as a challenge to Johanson and White’s phylogenetic hypothesis (Strait and Grine 1997). Initially named Australopithecus boisei (Walker et al. 1986), it was later subsumed into the species Paranthropus aethiopicus based on a unique combination of features that were in some ways primitive (A. afarensis-like: e.g., very small braincase and a prognathic, or jutting face, as opposed to the flat, “dished” face of P. robustus and P. boisei) and in others derived (P. boisei-like: “robust” features of the skull, including a wide face and a sagittal crest, a protrusion of bone along the midline of the skull). If A. afarensis was ancestral to P. aethiopicus, which was in turn ancestral to P. boisei, then A. africanus was freed from its phylogenetic position between A. afarensis and Paranthropus. An analysis of nasoalvaeolar (nasal and upper jaw) morphology also supports the monophyly (ancestral-descendant relationship) of all Paranthropus species to the exclusion of A. africanus (Villmoare and Kimbel 2011). Multiple new species have been discovered and named throughout the 1990s and 2000s, falling geologically and phylogenetically on both sides of A. afarensis. White and colleagues discovered new craniodental and postcranial material at Aramis in Ethiopia and named a new species, Australopithecus ramidus, which was later transferred to a new genus, Ardipithecus, based on its distinct and more primitive traits. Ardipithecus ramidus is now known from a partial skeleton and a plethora of other specimens (White et al. 2009). Another new species, Australopithecus anamensis, is additionally thought to be more primitive and potentially ancestral to A. afarensis. Specimens later attributed to A. anamensis were initially discovered in the 1960s at Kanapoi in Kenya. New material discovered in the 1980s and 1990s, including both craniodental and postcranial fossils, led Meave Leakey and her team to name the species in 1995 (Leakey et al. 1995). White and his team working at Aramis and Asa Issie in Ethiopia Australopithecus Group discovered new material that they attributed to A. anamensis, including jaws and a femur (White et al. 2006). Ar. ramidus and A. anamensis date to 4.4 and 4.2–3.9 Ma, respectively, and form what White and colleagues describe as an ancestor-descendant relationship (White et al. 2006). In South Africa, a nearly complete skeleton from Sterkfontein nicknamed “Little Foot” (StW 573) was recently re-dated to ~3.7 Ma (Granger et al. 2015); if correct, it represents the oldest material in South Africa and is contemporaneous with A. afarensis. Its discoverer, Ron Clarke, has resurrected Dart’s species name for Australopithecus material from the South African site of Makapansgat, A. prometheus. Other new species, either contemporary with or post-dating A. afarensis, include A. barelghazali (Brunet et al. 1995) in Central Africa (Chad, 3.6 Ma), A. garhi (Asfaw et al. 1999) and A. deyiremeda (Haile-Selassie et al. 2015) in Ethiopia (2.5 and 3.4 Ma, respectively), and A. sediba (Berger et al. 2010) in South Africa. The latter species is the best known, represented by two partial skeletons from Malapa (Malapa Hominin 1, or MH1, and MH2, 2.0 Ma), along with additional material from multiple other individuals. Both A. garhi and A. sediba have been argued to be the sister taxon (most closely related and sharing an exclusive relationship) to the genus Homo but neither is universally supported. A third new taxon, Kenyanthropus platyops (“flat-faced man from Kenya”), was named by Meave Leakey and colleagues in 2001 and also proposed as a candidate for that phylogenetic position (Leakey et al. 2001). Represented by a nearly complete but damaged cranium from West Turkana (KNM-WT 40000), the cranium is argued to share derived traits with members of the genus Homo, including a flat face with a non-jutting jaw. One major problem stems from the relative ambiguity of the genus designation and how to decide what defines a particular genus. Bernard Wood and Mark Collard (1999) suggested that the earliest members of the genus Homo (H. habilis and H. rudolfensis) should be transferred to the genus Australopithecus based on an adaptive zone Australopithecus Group model, where factors like body size, diet, and locomotion separate species into Australopithecus and Homo. This requires both craniodental and postcranial fossils to assess, however, and adequate material is not available for most species. A. sediba, which is adequately preserved and argued to share derived traits of the skull, teeth, and postcranium with members of the genus Homo, retains relatively small body size and adaptations to climbing, so would be classified in Australopithecus under this system, whereas it might be subsumed in the genus Homo or placed in a new genus under a strict, cladistic (i.e., character and parsimony-based) analysis (Berger et al. 2010; Dembo et al. 2015). 5 centers on the interpretation of these features as primitive, nonfunctional traits in a committed terrestrial biped vs. retained adaptations to climbing. Suffice to say, it seems likely that A. afarensis was a committed terrestrial biped that could forage and take shelter in the trees (Ruff et al. 2016). Unexpectedly, A. africanus seems to be better adapted to climbing than A. afarensis (Berger 2002). This suggests that either A. africanus has become secondarily more arboreal or that it retains the primitive condition and A. afarensis is more derived toward terrestrial bipedalism, perhaps in convergence with later members of the genus Homo; recent work on early hominin calcaneal robusticity has been interpreted in support of the latter hypothesis (Prang 2015). Locomotion One thing that is clear is that members of the Australopithecus group demonstrate unequivocal evidence for bipedal locomotion, unlike earlier potential members of the hominin lineage (e.g., Sahelanthropus tchadensis, Orrorin tugenensis, Ardipithecus kadabba). A. anamensis, A. afarensis, and A. africanus show derived features of the foot, knee, ankle, pelvis, spine, and cranial base consistent with adaptation to habitual bipedalism (Robinson 1972; Lovejoy 2005). These features include a non-grasping foot with an adducted hallux (in-line big toe) and a large calcaneus (heel bone), a flat ankle joint, a large plateau at the knee joint, valgus (inward) angle of the femora, short and curved blades of the pelvis, sigmoidal curvature of the vertebral column with both thoracic kyphosis (posterior curvature) and lumbar lordosis (anterior curvature), and an anteriorly positioned, horizontally angled foramen magnum (“big hole” at the base of the skull) through which the spinal cord passes. Additionally, the Laetoli footprints preserve trace fossil evidence of bipedalism in the probable hominin that made them, A. afarensis (Masao et al. 2016). Much discussion has been devoted to the retention of primitive features in the postcranial skeleton of A. afarensis and its implications for climbing vs. terrestrial bipedalism in this species (see Ward 2002 for a review). Disagreement Brain Size, Body Size, and Sexual Dimorphism The late Miocene putative hominin, Sahelanthropus tchadensis, and early Pliocene Ar. ramidus both have chimpanzee-sized brains, with cranial capacities ranging from 360 to 370 cc (cm3) and 300 to 350 cc, respectively (Zollikofer et al. 2005; White et al. 2009) (chimpanzees average ~366 cc; Grabowski et al. 2016). Members of the Australopithecus group are characterized by an average cranial capacity of ~450 cc, ranging from ~400 to 500 cc (Falk et al. 2000). For comparison, early members of the genus Homo have slightly larger brains (624 cc in H. habilis, but as low as 509 cc; ~750 cc for early H. erectus, but as low as 546 cc) (Grabowski 2016); however, H. floresiensis and the recently discovered species H. naledi overlap with the australopiths (426 cc and 465–560 cc, respectively) (Berger et al. 2015; Grabowski et al. 2016). Early hominins also vary in body size, again with overlap between australopiths and some members of the genus Homo (e.g., Australopithecus average ~ 32 kg; H. habilis average ~ 33 kg) (Grabowski et al. 2016). Relative brain size is probably more relevant than raw brain size because brain size and body size are evolutionarily correlated and therefore have coevolved (Grabowski 2016). When body size is taken into Australopithecus Group 6 account and an encephalization quotient (EQ) is calculated by comparing observed brain size relative to predicted brain size given regression estimates on body weight, the difference between chimpanzees (EQ = 2.4) and australopiths (e.g., A. afarensis, EQ = 3.2; A. africanus = 3.8; A. sediba = 3.9) becomes more pronounced, and members of the genus Homo (except H. floresiensis, EQ = 3.8) become more distinct from Australopithecus (e.g., H. habilis, EQ = 5.0; early H. erectus = 4.6; later H. erectus = 6.0; modern H. sapiens = 7.6 (Grabowski et al. 2016). Sexual dimorphism, differences in morphology (size and shape) between sexes of a single species, can only be estimated for reasonably well-documented extinct species. A. afarensis meets this criterion, but researchers do not agree on the degree of sexual dimorphism in this taxon (see Reno and Lovejoy 2015 for a recent review and perspective on new material), although it is almost certainly greater than that observed in modern humans. Given this non-consensus and lack of a clear relationship between sexual dimorphism (in body size or canine size) and sociosexual system amongst primates, little can be said about australopith social groups at this time. Tool Use Darwin proposed that bipedalism evolved in the context of freeing of the hands for tool and weapon use; however, until recently, the oldest tools in the archaeological record (the Oldowan) were 2.6 Ma, greatly postdating the emergence of the hominin lineage and thought to be associated with either early Homo or Paranthropus. Newly discovered, more primitive stone tools from West Turkana, Kenya (Lomekwi 3), dated to 3.3 Ma (Harmand et al. 2015) suggest that earlier species of Australopithecus, or perhaps K. platyops, which was discovered nearby, were tool users. It is notable that bone tools may have been used by P. robustus for activities like digging (e.g., termite mounds; Backwell and d’Errico 2001); similarly, other non-stone materials such as wood and bamboo may have been used by early hominins but would be unlikely to preserve in the fossil record. Conclusion Australopiths are a group of Plio-Pleistocene hominins with a large temporal and geographic distribution, ranging over 3 million years (>4–1 Ma) throughout East and South Africa and into Central Africa. In addition to Australopithecus, other genera are often included in the group or even subsumed within Australopithecus, including Paranthropus, Kenyanthropus, and arguably Ardipithecus. The group is clearly paraphyletic, that is, it is not a natural group because at least some lineages are more closely related to the genus Homo (e.g., K. platyops or A. sediba). Future fossil and archaeological discoveries will undoubtedly both clarify and complicate our understanding of relationships amongst the Australopithecus group of hominins. Cross-References ▶ Ardipithecus Group ▶ Ardipithecus kadabba ▶ Ardipithecus ramidus ▶ Australopithecus afarensis ▶ Australopithecus africanus ▶ Australopithecus anamensis ▶ Australopithecus garhi ▶ Bipedal locomotion ▶ Hominid Evolution; Homo erectus ▶ Homo floresiensis ▶ Homo Group ▶ Homo habilis ▶ Homo heidelbergensis ▶ Homo neanderthalensis ▶ Homo rudolfensis ▶ Orrorin tugenensis ▶ Paranthropous Group ▶ Paranthropus aethiopicus ▶ Paranthropus boisei ▶ Paranthropus robustus ▶ Sahelanthropus tchadensis ▶ Sexual Size Dimorphism ▶ Sexual Size Dimorphism in Chimpanzees Australopithecus Group References Alemseged, Z., Spoor, K., Bobe, W. 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