COMMENTARY

Paranthropus through the looking glassBernard A. Wooda,1

and David B. Pattersona,b

Most research and public interest in human originsfocuses on taxa that are likely to be our ancestors.There must have been genetic continuity betweenmodern humans and the common ancestor we sharewith chimpanzees and bonobos, and we want to knowwhat each link in this chain looked like and how it be-haved. However, the clear evidence for taxic diversityin the human (aka hominin) clade means that we alsohave close relatives who are not our ancestors (1). Twopapers in PNAS focus on the behavior and paleoenvi-ronmental context of Paranthropus boisei, a distinctiveand long-extinct nonancestral relative that lived along-side our early Homo ancestors in eastern Africa betweenjust less than 3 Ma and just over 1 Ma. Both papers usestable isotopes to track diet during a largely unknown,but likely crucial, period in our evolutionary history.

The first fossil evidence of P. boisei, two upper milkteeth, a very large molar, and a tiny canine, was dis-covered in 1955 at Olduvai Gorge, in Tanzania (2). Themystery of the owner of the unusual teeth was solvedin 1959 when Mary Leakey recognized fragments of afossil hominin cranium eroding from a hillside. TheOlduvai Hominid (OH) 5 cranium had a small (ca.500 cm3) brain—not much bigger than that of a gorillaand about a third the size of that of a modern human—a flat and broad face, large attachment areas forchewing muscles, small incisors and canines, and ex-ceptionally large premolar and molar tooth crowns.Louis Leakey proposed a new taxon, Zinjanthropusboisei (3) for OH 5, but within a few years the newgenus was dropped in favor of Australopithecus, orParanthropus; the latter is our preference. More evi-dence of P. boisei came in 1964 with the discovery atPeninj, just north of Olduvai Gorge, of a large lowerjaw with the same unusual relative tooth size relation-ships seen in OH 5. In addition to the evidence fromOlduvai, cranial and mandibular, but mostly dental,remains assigned to P. boisei have been recoveredfrom sites in Kenya and Ethiopia. Konso in Ethiopiais the furthest north the taxon is recorded, and an

upper jaw fragment from Malema in Malawi is thesouthernmost evidence. However, most of what weknow about P. boisei comes from fossils from KoobiFora on the eastern shore of Lake Turkana (4) and fromsites in the Nachukui Formation on the western side ofthe lake (Fig. 1A).

The cranial and dental morphology of P. boisei is sodistinctive its remains are relatively easy to identify (5).Unique features include its flat, wide, and deep face,flexed cranial base, large and thick lower jaw, andsmall incisors and canines combined with massivechewing teeth. The surface area available for process-ing food is extended both forward—by having premo-lar teeth that look like molars—and backward—by theunusually large third molar tooth crowns, all of whichare capped by exceptionally thick and fast-formingdental enamel (6). Other early hominins, such as Para-nthropus robustus from southern Africa, have similar-looking crania, large postcanine teeth, and thick enamel,but the crania of P. boisei are distinctive, its postcanineteeth are exceptionally large, and the enamel coveringthem is exceptionally thick. The foramen magnum—

where the brain connects with the spinal cord on theunderside of the cranium—is situated almost as far for-ward as it is in modern humans, suggesting that P. boiseiwas capable of walking erect on its hind legs.

A Dietary PuzzleWhy would a hominin need flared cheek bones, alarge mandible, bony sagittal crests, and massivechewing teeth? Several lines of evidence suggest thatP. boisei acquired its distinctive morphology within anecosystem that was trending toward cooler, drier, andmore open conditions, which in turn led to an increasein C4 vegetation on the landscape (Fig. 1B and ref. 7).Stable isotope data extracted from the enamel ofpost-2-Ma P. boisei fossils from Koobi Fora are con-sistent with a diet dominated by C4 foods (Fig. 1 C andD and ref. 8), whereas contemporary evidence of thegenus Homo from Koobi Fora (i.e., Homo habilis and

aCenter for the Advanced Study of Human Paleobiology, The GeorgeWashington University, Washington, DC 20052; and bDepartment of Biology,University of North Georgia, Dahlonega, GA 30597Author contributions: B.A.W. and D.B.P. wrote the paper.The authors declare no competing interest.Published under the PNAS license.See companion articles, “Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia,” 10.1073/pnas.2006982117 and“Isotopic evidence for the timing of the dietary shift toward C4 foods in eastern African Paranthropus,” 10.1073/pnas.2006221117.1To whom correspondence may be addressed. Email: bernardawood@gmail.com.First published September 2, 2020.

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Homo rudolfensis) has a more C3 signal (9). Most C4 biomass in thetropics are grasses, and the chewing-dominated dentition of P. boiseiwould have enabled it to process grass, or sedges, which given thestrength of the C4 signal may have been their staple diet (10, 11).However, the samemorphology would have also allowed P. boisei toprocess nuts and hard-shelled seeds. These could have been eitheradditions to their normal diet or they could have been fallback foodsthat would have kept them going until they could return to eatingtheir preferred food (12). The distinctive craniodental morphology ofP. boisei appears as a package that undergoes little change over thecourse of a million years (13). Whatever niche P. boisei occupied, thatniche and P. boisei’s adaptive response to it were remarkably dura-ble. The exception to this morphological conservatism concernsfossils older than 2.3 Ma (14) from two locations—the ShunguraFormation in the Lower Omo Valley of Ethiopia and west of LakeTurkana. The best-known of these fossils are a 2.6-My-old lower jawfound in Ethiopia and a ca. 2.5-My-old cranium recovered from thewest of Lake Turkana. Many, but not all, researchers put them in aseparate species, Paranthropus aethiopicus. Unfortunately, no toothcrowns were preserved in either the lower jaw or the cranium, but thespace occupied by the roots of the postcanine teeth suggests thatthe crowns of the premolars andmolarsmust have been similar in sizeto those of P. boisei. Compared with P. boisei, P. aethiopicus has amore projecting face, a more ape-like (i.e., less flexed) cranial base,larger incisors and canines, and simpler premolar crowns and roots.The earliest probable evidence for P. aethiopicus, a piece of upper

jaw and a shin bone from Laetoli in Tanzania (Fig. 1A), has beendated to ca. 2.66 Ma.

In PNAS, Wynn et al. (15) exploit evidence from the Lower OmoValley (LOV) to explore the dietary ecology of the P. aethiopicus–P. boisei lineage, and Negash et al. (16) provide a comparative con-text for these changes by tracking the dietary ecology of the broaderherbivore community in the LOV between 3.5 and 2.0 Ma. The LOV,which is one of the few locations in eastern Africa where there is amore-or-less-continuous sedimentary record of this period of humanevolutionary history, has been subjected to the type of careful sys-tematic geological (17) and paleontological (18, 19) analysis thatprovides the necessary context for investigating what preceded theprolonged period of P. boisei stasis that is recorded in younger sedi-ments at sites around Lake Turkana. Both studies use a method calledchange point detection to search for shifts in stable isotope values.Stable isotope analysis is destructive, but over the years sampling andanalytical techniques have been refined in ways that reduce thedamage to the fossils, while at the same time increasing precision(20). Thankfully, these developments, and the potential implicationsof any results, persuaded the curators responsible for these col-lections that these minimally destructive analyses were justified.

Tracking Diets through TimeWynn et al. (15) document an isotopic transition in P. aethiopicusthat apparently precedes the morphological transition betweenP. aethiopicus and P. boisei (Fig. 1 C and D). Stable isotopic data

Lower Omo Valley

Koobi ForaNachukui

Olduvai Beds I & II

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Fig. 1. Eastern African context of Paranthropus in the LOV. (A) Spatial distribution of Paranthropus-bearing localities in eastern Africa. (B)Vegetation change in eastern Africa over the past 5 My as indicated by fraction woody cover estimates. (C) New LOV hominin δ13C values withinthe context of existing eastern African hominin data. (D) New LOV hominin δ13C values in the context of existing Turkana Basin hominin values. (E)Summary of change points in new LOV faunal δ13C values.

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collected from enamel record the types of foods an organism wasconsuming when that particular tooth was forming. Therefore,these data are reflective of behavior, in this case food choice.Wynn et al.’s (15) findings are consistent with the expectation thatbehavioral change signaled by stable isotope data will precede amorphological response that requires population-level shiftsacross many generations.

However, changes in hominin diet and morphology in the LOVdid not occur within an ecological vacuum. Negash et al. (16) usestable isotope data from the herbivores that lived alongside Pa-ranthropus and our ancestors to better understand the ecologicalcontext of hominin dietary change. These data indicate a het-erogeneous response to the elevated prevalence of C4 vegetationin the LOV, with evidence of several dietary shifts between 2.8 and2.2 Ma (Fig. 1E). Some of the families they investigated (e.g.,antelopes and pigs) underwent a series of changes in stable iso-tope signal, whereas monkeys (including the large-bodied ba-boon species Theropithecus) only changed their dietary signalonce, at approximately the same time as the hominins. These datapoint to a complex relationship between environmental changeand dietary adaptation in the mammal communities that are

sampled in the LOV, something that is also seen at other sites inAfrica (21), but most sites lack the age control and sample sizesthat enabled Negash et al.’s (16) careful analysis.

We do not know what the temporal and geographic ranges ofan extinct hominin taxon like P. boisei were. Its presence at a fossilsite is evidence that it was living at that time and in that place, butwe should not assume that existing site samples circumscribe thetemporal or geographic range of a taxon. Each site is like a win-dow that gives us access to what is going on in just one roomwithin a large house, but it is likely that important events in theevolutionary history of a taxon took place in presently windowlessrooms. The task of paleoanthropologists is to either find addi-tional sites (i.e., new windows) or improve the view through theexisting windows. In Lewis Carroll’s 1871 novel Through theLooking Glass, when Alice decides to climb through a mirror sheenters a fantastical world where everything is topsy-turvy. Theworld of P. boisei is not topsy-turvy, but it is sufficiently unlikecontemporary analogs that we have to interpret it cautiously andon its own terms (22, 23). These two contributions substantiallyimprove the view through an important existing window into thepaleobiology of P. boisei.

1 B. Wood, E. K. Boyle, Hominin taxic diversity: Fact or fantasy? Am. J. Phys. Anthropol. 159 (suppl. 61), S37–S78 (2016).2 L. S. B. Leakey, Recent discoveries at Olduvai Gorge, Tanganyika. Nature 181, 1099–1103 (1958).3 L. S. B. Leakey, A new fossil skull from Olduvai. Nature 184, 491–493 (1959).4 B. A. Wood, Koobi Fora Research Project: Hominid Cranial Remains (Oxford University Press, New York, 1991), vol. 4.5 B. Wood, P. Constantino, Paranthropus boisei: Fifty years of evidence and analysis. Am. J. Phys. Anthropol. 134 (suppl. 45), 106–132 (2007).6 A. D. Beynon, B. A. Wood, Variations in enamel thickness and structure in East African hominids. Am. J. Phys. Anthropol. 70, 177–193 (1986).7 N. E. Levin, Environment and climate of early human evolution. Annu. Rev. Earth Planet. Sci. 43, 405–429 (2015).8 T. E. Cerling et al., Stable isotope-based diet reconstructions of Turkana Basin hominins. Proc. Natl. Acad. Sci. U.S.A. 110, 10501–10506 (2013).9 D. B. Patterson et al., Comparative isotopic evidence from East Turkana supports a dietary shift within the genus Homo. Nat. Ecol. Evol. 3, 1048–1056 (2019).

10 M. Sponheimer et al., Using carbon isotopes to track dietary change in modern, historical, and ancient primates. Am. J. Phys. Anthropol. 140, 661–670 (2009).11 O. C. C. Paine et al., Grass leaves as potential hominin dietary resources. J. Hum. Evol. 117, 44–52 (2018).12 J. E. Lambert, C. A. Chapman, R. W. Wrangham, N. L. Conklin-Brittain, Hardness of cercopithecine foods: Implications for the critical function of enamel thickness

in exploiting fallback foods. Am. J. Phys. Anthropol. 125, 363–368 (2004).13 B. Wood, C. Wood, L. Konigsberg, Paranthropus boisei: An example of evolutionary stasis? Am. J. Phys. Anthropol. 95, 117–136 (1994).14 G. Suwa, T. D. White, F. C. Howell, Mandibular postcanine dentition from the Shungura Formation, Ethiopia: Crown morphology, taxonomic allocations, and

Plio-Pleistocene hominid evolution. Am. J. Phys. Anthropol. 101, 247–282 (1996).15 J. G. Wynn et al., Isotopic evidence for the timing of the dietary shift toward C4 foods in eastern African Paranthropus. Proc. Natl. Acad. Sci. U.S.A. 117, 21978–

21984 (2020).16 E. W. Negash et al., Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia. Proc. Natl. Acad. Sci. U.S.A. 117, 21921–21927 (2020).17 F. H. Brown, I. McDougall, Geochronology of the Turkana depression of northern Kenya and southern Ethiopia. Evol. Anthropol. 20, 217–227 (2011).18 R. Bobe, A. K. Behrensmeyer, The expansion of grassland ecosystems in Africa in relation to mammalian evolution and the origin of the genus Homo.

Palaeogeogr. Palaeoclimatol. Palaeoecol. 207, 399–420 (2004).19 Z. Alemseged, R. Bobe, D. Geraads, “Comparability of fossil data and its significance for the interpretation of hominin environments” in Hominin Environments in

the East African Pliocene: An Assessment of the Faunal Evidence, R. Bobe, Z. Alemseged, A. K. Behrensmeyer, Eds. (Springer, Dordrecht, 2007), pp. 159–181.20 M. Sponheimer et al., Isotopic evidence for dietary variability in the early hominin Paranthropus robustus. Science 314, 980–982 (2006).21 D. B. Patterson et al., Landscape scale heterogeneity in the East Turkana ecosystem during the OkoteMember (1.56–1.38 Ma). J. Hum. Evol. 112, 148–161 (2017).22 A. K. Behrensmeyer, Four million years of African herbivory. Proc. Natl. Acad. Sci. U.S.A. 112, 11428–11429 (2015).23 J. T. Faith, J. Rowan, A. Du, Early hominins evolved within non-analog ecosystems. Proc. Natl. Acad. Sci. U.S.A. 116, 21478–21483 (2019).

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