The "Robust" Australopiths

Citation: Constantino, P. J. (2013) The "Robust" Australopiths. Nature Education Knowledge 4(1):1

The "robust" australopiths are a group of hominins with large cheek teeth and strongly built jaws that lived alongside the earliest members of our own genus, Homo, approximately 2.5-1.4 million years ago. Why they ultimately went extinct while we set off to conquer the world is still a mystery.

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Introduction: Who are the "Robust" Australopiths?

The "robust" australopiths are a group of fossil hominins that existed in East and southern Africa between approximately 2.5 and 1.4 million years ago (Ma). They are referred to here as members of the genus Paranthropus, though considerable disagreement about their proper taxonomy persists (see below). They are characterized by several features of the skull that give them a "robust" appearance when compared to other, more gracile hominins. The most notable of these features are large, thickly enameled, postcanine teeth that were supported by deep and broad mandibular corpora with tall and broad rami (Fig. 1). Other notable features include zygomatic (cheek) bones that were extended both laterally and anteriorly, a face that was more orthognathic (i.e., pulled back towards the rest of the skull) than in other australopiths, and the occasional presence of bony crests on the top and back of the skull, presumably for the attachment of large jaw muscles. Taken together, these traits suggest an animal that could both generate and dissipate high bite forces, and they imply that at least some portion of the Paranthropus diet was particularly difficult to break down^1-4.

The “robust” skull morphology of <i>Paranthropus</i>, represented by OH 5.

Figure 1: The “robust” skull morphology of Paranthropus, represented by OH 5.

Though the mandible shown is a hypothetical model created to fit the OH 5 cranium, the accuracy of its morphology was later confirmed with the discovery of the Peninj mandible. The labeled traits represent adaptations to either generating or resisting high masticatory forces.

The Fossil Evidence

Southern Africa

The first fossils attributed to the genus Paranthropus were announced in a 1938 Nature paper by Robert Broom, who was then working for the Transvaal Museum (now the Ditsong National Museum of Natural History) in Pretoria, South Africa⁵. The fossils were discovered by a schoolboy in Kromdraai Cave, just a couple miles upstream from the Sterkfontein site where Broom had recently uncovered additional hominin fossils (which would ultimately be attributed to Australopithecus africanus) (Fig. 2). The Kromdraai remains were assembled to form most of a single, presumably male, cranium, which became the type specimen of Paranthropus robustus Broom, 1938. This specimen's discovery came at an important time, for along with the Sterkfontein fossils it lent much needed support to Raymond Dart's then-controversial claim that his juvenile A. africanus cranium from Taung, South Africa was more closely related to humans than to other living apes⁶. Together, these new fossils provided tangible evidence that humans evolved just like every other species on earth, and that at least part of this evolution occurred in South Africa.

Figure 2: Paranthropus robustus and Australopithecus africanus from southern Africa.

P. robustus is represented by the SK 48 cranium from Swartkrans and is shown in anterior (a), inferior (c), and superior (d) views. A. africanus is represented by Sts 5 from Sterkfontein and is shown in anterior view only (b). SK 48 is distinguished from Sts 5 by many of the traits depicted in Figure 1.

A decade later, a second species was added to the genus Paranthropus. This species included fossils recovered by Broom and his colleague John Robinson from Swartkrans Cave, only about 3 miles from Kromdraai (and less than a mile downstream from Sterkfontein)⁷. Subsequent fieldwork at Swartkrans resulted in the accumulation of a sizeable collection of hominins, most of which (but not all⁸) were assigned to P. crassidens. Most researchers (e.g.,^9-11) have done away with the specific distinction between the Kromdraai and Swartkrans hypodigms and now include specimens from both sites in the species P. robustus (but see^12-15). Additional P. robustus specimens have since been discovered at the nearby sites of Drimolen^11,16, Gondolin¹⁷, and Cooper's Cave^18-20.

East Africa

Not long after the discovery of Paranthropus robustus at Swartkrans, evidence of a similar megadont hominin was discovered by Louis Leakey in East Africa. The first evidence consisted of two teeth found in 1955 at Olduvai Gorge in Tanzania²¹. However, the taxonomy of these teeth was uncertain until the discovery in 1959 of OH 5, a well-preserved adolescent cranium also from Olduvai Gorge²²^. Leakey thought the OH 5 cranium was distinct from both Australopithecus and Paranthropus, and consequently he attributed it to a new genus and species, Zinjanthropus boisei. Ultimately, however, Leakey was convinced by his fellow paleoanthropologists that while his fossils likely represented a new species, they were not distinct enough from the specimens at Kromdraai and Swartkrans to warrant the formulation of a separate genus. A few years later, Louis and his wife Mary announced the discovery of the Peninj mandible from the western shore of Lake Natron in Tanzania²³. Together with the OH 5 cranium, the nearly complete mandible showed this East African species to be an even more "robust" hominin than its counterparts in southern Africa. Subsequently, the hypodigm of what we now refer to as Paranthropus boisei was augmented by discoveries from the Omo region²⁴ and Konso in Ethiopia²⁵, from Koobi Fora^26-28, West Turkana²⁹, and the Baringo basin of Kenya^30,31, and from considerably further south in Malema, Malawi³². Some of the geologically oldest of these remains, particularly from the Omo region, were ultimately attributed to Paranthropus aethiopicus, the last Paranthropus taxon to be officially recognized. Initially announced as a separate genus, Paraustralopithecus Arambourg and Coppens, 1968, these fossils have since been subsumed in the genus Paranthropus. However, most scientists retain the specific distinction for the fossils of this area that predate 2.3 Ma based on observed changes in dental and mandibular morphology that appear to have occurred around this time^33-37(but see^38,39). Most researchers (e.g.,^40,41) also include the cranium KNM-WT 17000 and the mandible KNM-WT 16005 from West Turkana in the P. aethiopicus hypodigm. KNM-WT 17000, more popularly known as the "Black Skull" due to the high concentrations of manganese in the soil that turned the bones a blue-black color upon fossilization, has several traits that link it with Paranthropus such as anteriorly positioned cheek bones and presumably large postcanine teeth (based mostly on tooth root size - the cranium is edentulous except for an associated left P⁴⁴¹). However, it also has traits that link it with the earlier species Australopithecus afarensis such as increased prognathism and a more posteriorly positioned sagittal crest that merges with the nuchal crest (see Ward and Hammond's Scitable article for a summary of the non-robust australopiths). It thus appears that even if P. boisei and P. aethiopicus are retained as separate species, they likely represent chronospecies of the same lineage (i.e., a single ancestor/descendant anagenetic line). How this lineage is related to P. robustus from southern Africa is a topic of debate (see below). For a list of known Paranthropus sites and associated contextual information, see Table 1.

Region	Site	Geologic Formation	Estimated Age of Hominins (Myr)	Dating Method	Key Specimens	Species Present
East Africa	West Turkana, Kenya	Nachukui	2.5-2.35	radiometric; marker beds	KNM-WT 17000 (cranium), KNM-WT 16005 (mandible)	P. aethiopicus
	West Turkana, Kenya	Nachukui	2.3-1.6	radiometric; marker beds	Various specimens	P. boisei
	Koobi Fora, Kenya	Koobi Fora	2.2-1.88	radiometric; tephrostratigraphy; fission-track, marker beds	KNM-ER 1500 (partial skeleton) and others	P. boisei
			1.88-1.65		KNM-ER 406, 407, 732 (all crania) and others	P. boisei
			1.65-1.39		KNM-ER 729, 3230 (both mandibles) and others	P. boisei
	Omo, Ethiopia	Shungura	2.6-2.3	radiometric; marker beds	Omo 18-18 (edentulous mandible; holotype of P. aethiopicus), and others, mostly isolated teeth	P. aethiopicus
	Omo, Ethiopia	Shungura	2.3-1.2	radiometric; marker beds	Various specimens, mostly teeth	P. boisei
	Chesowanja, Kenya	Chemoigut	2.0-1.5	biostratigraphy; radiometric of capping layer	CH1 (partial cranium), other fragments	P. boisei
	Konso, Ethiopia	Konso	1.4	radiometric; tephrostratigraphy; marker beds	KGA 10-525 (skull), and others	P. boisei
	Malema, Malawi	Chiwondo	1.5	biostratigraphy	RC 911 (maxilla)	P. boisei
	Peninj, Tanzania	Humbu	1.7-1.3	radiometric; magnetostratigraphy	Peninj mandible	P. boisei
	Olduvai Gorge, Tanzania	Olduvai	1.9-1.7	radiometric; biostratigraphy	OH 5 (cranium; holotype of P. boisei)	P. boisei
	Olduvai Gorge, Tanzania	Olduvai	1.7-1.2	radiometric; biostratigraphy	Various specimens	P. boisei
Southern Africa	Swartkrans, South Africa	Monte Christo	1.8-1.5	biostratigraphy	>300 Paranthropus specimens total, mostly isolated dental remains, including SK6 (holotype of P. crassidens)	P. robustus (P. crassidens)
			1.5-1.0			P. robustus (P. crassidens)
			1.5-1.0			P. robustus (P. crassidens)
	Kromdraai, South Africa	Monte Christo	2.0-1.5	biostratigraphy; reversed polarity	Close to 30 Paranthropus specimens, including TM1517 (skull; holotype of P. robustus)	P. robustus
	Sterkfontein, South Africa (M5B)	Monte Christo	1.4-1.1	magnetostratigraphy	Two specimens: Stw 566 & Stw 569	P. robustus
	Drimolen, South Africa	Monte Christo	2.0-1.5	Overall faunal assemblage composition; no absoute dates	>80 hominins, including DNH 7 (nearly complete female skull) and DNH 8 (male mandible)	P. robustus
	Gondolin, South Africa	Eccles	1.9-1.5	biostratigraphy (tentative)	GDA-2: a very large mandibular M2	P. sp.
	Cooper's Cave, South Africa	Monte Christo	1.5-1.4	radiometric; biostratigraphy	COB 101 (partial skull) and others, mostly teeth but also postcrania	P. robustus
*Table 1: The Paranthropus* fossil evidence.**

Paranthropus or Australopithecus?

Ever since Broom's 1949 announcement of a new type of hominin from Swartkrans, paleoanthropologists have been debating the taxonomy and phylogeny of the "robust" australopiths ^{9,10,22,42-47} (Fig. 3). Placing the three commonly recognized species (P. robustus, P. boisei, and P. aethiopicus) in their own genus requires that they are (1) adaptively different from Australopithecus, and (2) monophyletic (i.e., more closely related to each other than to any other species). The current debate largely centers around the latter criterion. Many researchers (e.g.,⁴⁸) believe that the shared skull morphology of the "robust" australopiths is homoplasious (i.e., independently evolved in two or more of the taxa) and thus place them in the genus Australopithecus by default. Homoplasy does appear to have been prevalent in the evolution of African fauna throughout the course of human evolution^49,50, even in the hominins themselves ^35,51-54, but several pieces of evidence are nevertheless consistent with Paranthropus monophyly ^25,37,55,56. Most telling is the fact that in a thorough cladistic analysis of hominin relationships, Paranthropus monophyly was supported in every instance, even when masticatory characters were excluded⁵⁶. Therefore, retention of Paranthropus as a distinct genus is warranted until convincing evidence demonstrates otherwise.

Figure 3: Proposed phylogenies for the “robust” hominins.

The first (a) suggests that the “robust” hominins from East and southern Africa form a monophyletic clade and hence can be grouped into the genus Paranthropus. The second (b) posits that “robust” hominins originated independently in East and southern Africa. The third (c), originally proposed by Skelton and McHenry⁴⁸, links P. boisei with P. robustus to the exclusion of P. aethiopicus. Proponents of the latter two models place the “robust” hominins in the genus Australopithecus. Most paleoanthropologists subscribe to either phylogeny ‘a’ or phylogeny ‘b’. Advocates of ‘a’ and ‘c’ must explain how the less derived (i.e., less “robust”) P. robustus evolved from the more derived and geologically older P. boisei.

Biology and Behavior

Despite the fact that Paranthropus is one of the most well represented hominin groups in the fossil record, there is still much we do not know about the basic biology of this genus. Part of this stems from the fact that we can still not confidently recognize Paranthropus postcrania in the fossil record. It is likely that we already have a reasonable sample of these remains, possibly including such specimens as KNM-ER 1500⁵⁷, OH 36⁵⁸, and SK 82 and 97⁵⁹, but we are still unable to unambiguously distinguish them from those of coeval hominins. Solving this problem will allow researchers to provide more reliable estimates of key biological variables such as body size, locomotor habits, and manual dexterity.

The various pieces of evidence we do have about Paranthropus lifeways provide a somewhat contradictory picture of how these hominins grew and behaved on the African landscape. Studies of dental growth and development^60,61, inner ear morphology⁶² and brain shape⁶³all seem to indicate that Paranthropus was more ape-like than initially recognized. Certainly fossils such as KNM-ER 406 and KNM-ER 732 from Koobi Fora seem to indicate that there was a fairly high level of sexual dimorphism, at least in P. boisei⁶⁴. On the other hand, several lines of circumstantial evidence point to a more "advanced" hominin, including studies suggesting that Paranthropus used, if not made, tools. Stone and bone tools have been found at Swartkrans^65-67 and bone tools at Drimolen^16,68, two sites where the vast majority of the hominin remains are attributed to P. robustus. Stone tools have also been found in the Oldowan Infill (aka M5B⁶⁹) of Sterkfontein's Member 5, where approximately half of the specimens are believed to be those of P. robustus⁷⁰. Paranthropus tool use is also supported by a series of studies that tentatively assigned Swartkrans hand fossils with a modern human-like precision grip to P. robustus^65,71-74. Given that our closest living primate relatives use, and occasionally make, tools^75-79, it should not be surprising that Paranthropus did so as well. However, stone tool knapping has long been seen as a major behavioral difference between Paranthropus and the larger brained Homo⁸⁰. Therefore, acceptance of the idea that Paranthropus was using and possibly preparing such tools will require evidence that is more compelling than the loose associations presented to date.

It appears that Paranthropus was one of the first hominin taxa to routinely venture into open grassland areas ^18,25,81-83 (but see⁸⁴) possibly to acquire novel open habitat resources such as termites ⁶⁷ or plant underground storage organs such as tubers, bulbs, and grass corms ^85-88. Biochemical analyses of the diet of P. robustus imply that these hominins were omnivorous ^89-93 and possibly even changed their diets seasonally ⁹⁴. However, recent studies of P. boisei's dental microwear and stable isotope composition paint a different picture — one of a limited, C4-based diet^95,96. Going forward, it will be important to determine whether this behavioral variation is ecologically driven, as we see in modern gorilla diets⁹⁷, or whether it reveals fundamental differences in the adaptive strategies of two independently evolving lineages.

Paranthropus disappears from the fossil record sometime between 1.4 and 0.9 Ma, after a geologic lifespan of just over a million years ^18,25,69. The cause(s) of their extinction is a mystery. Early notions that they had become too specialized to cope with changing environmental conditions have been strongly challenged ^94,95. Competition with Homo is plausible, but indisputable evidence for either direct or indirect interaction between the two genera has yet to be discovered. The fact that such basic information is still lacking for Paranthropus makes them an exciting hominin group to study, both in their own right and because they provide an important comparative context for the evolution of our own genus, Homo, at a time when we were evolving many of the traits that we now associate with being human (see the early Homo Scitable article by Anton and Laird).

Acknowledgements

Thanks are due to Bernard Wood, who was a coauthor on the papers from which much of this review has been drawn.

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Scientific Underpinnings

Did we destroy the Neanderthals? Did primates raft from Africa to South America? What influences the evolution of humans and our primate relatives today? What is it like to search for hominin fossils or to study wild apes? What tools can we use to study rare, endangered primates and help to conserve them in a rapidly changing world? These are some of the diverse questions answered in this topic room.

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