COMMENTARY
Paranthropus through the looking glass COMMENTARY Bernard A. Wooda,1 and David B. Pattersona,b
Most research and public interest in human origins upper jaw fragment from Malema in Malawi is the focuses on taxa that are likely to be our ancestors. southernmost evidence. However, most of what we There must have been genetic continuity between know about P. boisei comes from fossils from Koobi modern humans and the common ancestor we share Fora on the eastern shore of Lake Turkana (4) and from with chimpanzees and bonobos, and we want to know sites in the Nachukui Formation on the western side of what each link in this chain looked like and how it be- the lake (Fig. 1A). haved. However, the clear evidence for taxic diversity The cranial and dental morphology of P.boisei is so in the human (aka hominin) clade means that we also distinctive its remains are relatively easy to identify (5). have close relatives who are not our ancestors (1). Two Unique features include its flat, wide, and deep face, papers in PNAS focus on the behavior and paleoenvi- flexed cranial base, large and thick lower jaw, and ronmental context of Paranthropus boisei, a distinctive small incisors and canines combined with massive and long-extinct nonancestral relative that lived along- chewing teeth. The surface area available for process- side our early Homo ancestors in eastern Africa between ing food is extended both forward—by having premo- just less than 3 Ma and just over 1 Ma. Both papers use lar teeth that look like molars—and backward—by the stable isotopes to track diet during a largely unknown, unusually large third molar tooth crowns, all of which but likely crucial, period in our evolutionary history. are capped by exceptionally thick and fast-forming The first fossil evidence of P. boisei, two upper milk dental enamel (6). Other early hominins, such as Para- teeth, a very large molar, and a tiny canine, was dis- nthropus robustus from southern Africa, have similar- covered in 1955 at Olduvai Gorge, in Tanzania (2). The looking crania, large postcanine teeth, and thick enamel, mystery of the owner of the unusual teeth was solved but the crania of P. b o i s e i are distinctive, its postcanine in 1959 when Mary Leakey recognized fragments of a teeth are exceptionally large, and the enamel covering fossil hominin cranium eroding from a hillside. The them is exceptionally thick. The foramen magnum— Olduvai Hominid (OH) 5 cranium had a small (ca. where the brain connects with the spinal cord on the 500 cm3) brain—not much bigger than that of a gorilla underside of the cranium—is situated almost as far for- and about a third the size of that of a modern human— ward as it is in modern humans, suggesting that P. b o i s e i a flat and broad face, large attachment areas for was capable of walking erect on its hind legs. chewing muscles, small incisors and canines, and ex- ceptionally large premolar and molar tooth crowns. A Dietary Puzzle Louis Leakey proposed a new taxon, Zinjanthropus Why would a hominin need flared cheek bones, a boisei (3) for OH 5, but within a few years the new large mandible, bony sagittal crests, and massive genus was dropped in favor of Australopithecus,or chewing teeth? Several lines of evidence suggest that Paranthropus; the latter is our preference. More evi- P. boisei acquired its distinctive morphology within an dence of P. boisei came in 1964 with the discovery at ecosystem that was trending toward cooler, drier, and Peninj, just north of Olduvai Gorge, of a large lower more open conditions, which in turn led to an increase
jaw with the same unusual relative tooth size relation- in C4 vegetation on the landscape (Fig. 1B and ref. 7). ships seen in OH 5. In addition to the evidence from Stable isotope data extracted from the enamel of Olduvai, cranial and mandibular, but mostly dental, post-2-Ma P. boisei fossils from Koobi Fora are con-
remains assigned to P. boisei have been recovered sistent with a diet dominated by C4 foods (Fig. 1 C and from sites in Kenya and Ethiopia. Konso in Ethiopia D and ref. 8), whereas contemporary evidence of the is the furthest north the taxon is recorded, and an genus Homo from Koobi Fora (i.e., Homo habilis and
aCenter for the Advanced Study of Human Paleobiology, The George Washington University, Washington, DC 20052; and bDepartment of Biology, University of North Georgia, Dahlonega, GA 30597 Author 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: [email protected].
www.pnas.org/cgi/doi/10.1073/pnas.2016445117 PNAS Latest Articles | 1of3 Downloaded by guest on September 28, 2021 A B
Lower Omo Valley Konso Nachukui C Koobi Fora
Chesowanja
Peninj Laetoli Olduvai Beds I & II
250 km N Malema D E
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 within the 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.
Homo rudolfensis)hasamoreC3 signal (9). Most C4 biomass in the jaw and a shin bone from Laetoli in Tanzania (Fig. 1A), has been tropics are grasses, and the chewing-dominated dentition of P. b o i s e i dated to ca.2.66Ma. would have enabled it to process grass, or sedges, which given the In PNAS, Wynn et al. (15) exploit evidence from the Lower Omo – strength of the C4 signal may have been their staple diet (10, 11). Valley (LOV) to explore the dietary ecology of the P. a e t h i o p i c u s However, the same morphology would have also allowed P. b o i s e i to P. b o i s e i lineage, and Negash et al. (16) provide a comparative con- process nuts and hard-shelled seeds. These could have been either text for these changes by tracking the dietary ecology of the broader additionstotheirnormaldietorthey could have been fallback foods herbivore community in the LOV between 3.5 and 2.0 Ma. The LOV, that would have kept them going until they could return to eating which is one of the few locations in eastern Africa where there is a their preferred food (12). The distinctive craniodental morphology of more-or-less-continuous sedimentary record of this period of human P. b o i s e i appears as a package that undergoes little change over the evolutionary history, has been subjected to the type of careful sys- course of a million years (13). Whatever niche P. b o i s e i occupied, that tematic geological (17) and paleontological (18, 19) analysis that niche and P. b o i s e i ’s adaptive response to it were remarkably dura- provides the necessary context for investigating what preceded the ble. The exception to this morphological conservatism concerns prolonged period of P. b o i s e i stasis that is recorded in younger sedi- fossils older than 2.3 Ma (14) from two locations—the Shungura ments at sites around Lake Turkana. Both studies use a method called Formation in the Lower Omo Valley of Ethiopia and west of Lake change point detection to search for shifts in stable isotope values. Turkana. The best-known of these fossils are a 2.6-My-old lower jaw Stable isotope analysis is destructive, but over the years sampling and found in Ethiopia and a ca. 2.5-My-old cranium recovered from the analytical techniques have been refined in ways that reduce the west of Lake Turkana. Many, but not all, researchers put them in a damage to the fossils, while at the same time increasing precision separate species, Paranthropus aethiopicus. Unfortunately, no tooth (20). Thankfully, these developments, and the potential implications crowns were preserved in either the lower jaw or the cranium, but the of any results, persuaded the curators responsible for these col- space occupied by the roots of the postcanine teeth suggests that lections that these minimally destructive analyses were justified. the crowns of the premolars and molars must have been similar in size to those of P. b o i s e i .ComparedwithP. b o i s e i , P. a e t h i o p i c u s has a Tracking Diets through Time more projecting face, a more ape-like (i.e., less flexed) cranial base, Wynn et al. (15) document an isotopic transition in P. aethiopicus larger incisors and canines, and simpler premolar crowns and roots. that apparently precedes the morphological transition between The earliest probable evidence for P. a e t h i o p i c u s , a piece of upper P. aethiopicus and P. boisei (Fig. 1 C and D). Stable isotopic data
2of3 | www.pnas.org/cgi/doi/10.1073/pnas.2016445117 Wood and Patterson Downloaded by guest on September 28, 2021 collected from enamel record the types of foods an organism was sampled in the LOV, something that is also seen at other sites in consuming when that particular tooth was forming. Therefore, Africa (21), but most sites lack the age control and sample sizes these data are reflective of behavior, in this case food choice. that enabled Negash et al.’s (16) careful analysis. Wynn et al.’s (15) findings are consistent with the expectation that We do not know what the temporal and geographic ranges of behavioral change signaled by stable isotope data will precede a an extinct hominin taxon like P. boisei were. Its presence at a fossil morphological response that requires population-level shifts site is evidence that it was living at that time and in that place, but across many generations. we should not assume that existing site samples circumscribe the However, changes in hominin diet and morphology in the LOV temporal or geographic range of a taxon. Each site is like a win- did not occur within an ecological vacuum. Negash et al. (16) use dow that gives us access to what is going on in just one room stable isotope data from the herbivores that lived alongside Pa- within a large house, but it is likely that important events in the ranthropus and our ancestors to better understand the ecological evolutionary history of a taxon took place in presently windowless context of hominin dietary change. These data indicate a het- rooms. The task of paleoanthropologists is to either find addi-
erogeneous response to the elevated prevalence of C4 vegetation tional sites (i.e., new windows) or improve the view through the in the LOV, with evidence of several dietary shifts between 2.8 and existing windows. In Lewis Carroll’s 1871 novel Through the 2.2 Ma (Fig. 1E). Some of the families they investigated (e.g., Looking Glass, when Alice decides to climb through a mirror she antelopes and pigs) underwent a series of changes in stable iso- enters a fantastical world where everything is topsy-turvy. The tope signal, whereas monkeys (including the large-bodied ba- world of P. boisei is not topsy-turvy, but it is sufficiently unlike boon species Theropithecus) only changed their dietary signal contemporary analogs that we have to interpret it cautiously and once, at approximately the same time as the hominins. These data on its own terms (22, 23). These two contributions substantially point to a complex relationship between environmental change improve the view through an important existing window into the and dietary adaptation in the mammal communities that are paleobiology of P. boisei.
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