Zachary Cofran has been dissertation blogging about his work on dental development in robust australopithecines: "Data, development and diets". An interesting look at how the skimpy pattern of data in the fossil record can guide how research is developed.
The robust australopithecines existed between 2.5 and 1.5 million years ago. At this station are skeletal remains from two kinds of robust australopithecine. You have already met Australopithecus robustus earlier in the semester. The new species for you here is Australopithecus boisei. This species had the largest molar and premolar teeth of any hominin ever to have existed.
A. boisei comes from East Africa, with remains found in Ethiopia, Kenya, and Tanzania. The most famous fossil is OH 5, from Olduvai Gorge, Tanzania, around 1.7 million years old. Other significant specimens here include KNM-ER 739, KNM-ER 732 and KNM-ER 406, from Koobi Fora, Kenya, around the same age.
The specimens of Australopithecus robustus here will be familiar to you. All are from South Africa, and they include SK 48 and SK 12, from Swartkrans, South Africa, around 1.7 million years old, and TM 1517 from Kromdraai, South Africa, around 1.8 million years old.
These species may be closely related, but there are some differences between them. Examine them closely with the following questions:
1. The defining features of the robust australopithecines are the large postcanine dentition and large jaw musculature. How do these two groups of fossils compare on those features?
2. Robust australopithecines also have a very reduced anterior dentition (incisors and canines). Which fossils show that morphology?
3. The premolars in these species have enlarged, at the extreme they become more like molars in their morphology. Which fossils have the most molar-like premolars? Is the trend the same in the upper and lower dentitions?
3. With such great robusticity of the jaws and teeth, there are potentially great differences between males and females. Are the differences here consistent with sexual dimorphism? Which fossils are male, and which are female?
The stations in this lab will introduce one of the best-known species of fossil hominins, evidence of bipedal locomotion early in our evolution, some basic anthropometric measurements, and the anatomy of the femur.
Walking upright is a basic feature of humanity, which sets our family apart from other primates. Our way of walking is supported by many changes in our skeletons, especially the legs and feet. Some features are such distinctive evidence of bipedality that finding only a fragment of a fossil bone that preserves them is enough to show the fossil is one of our relatives.
The region just north of Johannesburg, South Africa, is a formation of ancient limestone in which groundwater has formed numerous caves and sinkholes. Some of these caves are used by animals for cool shade, water, and minerals; some are used by leopards, or in ancient times, sabretooths. By accident and predation, the skeletons of animals fall or are dragged into these caves, including our relatives the hominins. After around 2 million years ago, the most common kind of hominin in these caves was a species we call Australopithecus robustus.
The word "robust" refers to size and strength. A. robustus was not very large in body size, but it had exceptionally large molar and premolar teeth, and a very large and thick mandible, or jawbone. The main muscles of the jaw, the temporalis muscles, were so large that they ran up the complete height of the skull to meet at the midline. The high ridge of bone where these muscles attached to the top of the skull is called the sagittal crest.
A. robustus is one of the best-represented species of early hominins. The first specimen to be found was TM 1517, a partial skeleton with cranial remains from Kromdraai, presently in the Cradle of Humankind World Heritage Site. The largest sample of A. robustus fossils come from Swartkrans, less than 3 km from Kromdraai. The iconic skull, SK 48, provides a good illustration of the anatomy of the cranium of A. robustus with its sagittal crest, large, thick cheekbones, and relatively large molar teeth.
The most obvious features that A. robustus shares with living people are related to locomotion. Human bipedality, or upright walking, caused many changes to the skeleton. A simple comparison of the distal end of the femur, the end nearest the knee, is enough to tell that A. robustus was bipedal like humans. Quadrupedal animals, who go on all fours, very rarely support their weight on one leg and do not have to balance their centers of mass over a single point. Their legs are typically oriented straight from the hip joint to the ground. Humans, in contrast, have to support their weight on one leg every time they take a step. To accomplish this, their legs must angle from the hip joint under the body's center of mass. The human knee angles very obviously at the distal femur, so that when the condyles of the femur rest flat on the tibia (or a table), the shaft of the bone angles markedly from vertical.
This angle is called the valgus angle, and is one of the easiest-to-see traces of bipedality in fossil hominins.
This week, Thure Cerling and colleagues report in PNAS [1] carbon stable isotope data from 24 specimens of Australopithecus boisei. This is a huge sample as fossil hominins go, and they give a very consistent picture about the diet of this most robust of the australopithecines. These 24 individuals got between 61 and 91 percent of their carbon from grasses.
My 2005 explainer on stable isotope chemistry and early hominin diets fills in the details about carbon-12, carbon-13 and their relationship to 3- and 4-carbon photosynthetic cycles. The salient aspect of the comparisons involving A. boisei here is that C4 plants, mostly grasses, incorporate relatively more carbon-13 than do other plants, and herbivores assimilate this carbon-13 into their bones and teeth.
The high ratio of grass-derived carbon in A. boisei is fundamentally different from all living and fossil apes, and it is far higher than the values found for other early hominins. The only other primate that comes close is the fossil giant gelada Theropithecus oswaldi, a savanna-living species.
What were these extinct species really eating? Was grass the food? For living geladas, grass consumption includes seeds -- a fact that led Clifford Jolly to suggest that early hominins might also have specialized on seeds [2]. Of course, humans today also specialize on grass seeds. We call them grains, eat them in bread and drink them in soda. And beer.
But what about A. boisei? The large, thick-enameled premolars and molars, with their low cusps, seem well suited to grinding small hard objects and resisting the resulting wear. But Cerling and colleagues devote a good chunk of their discussion to the description of molar wear in A. boisei and other early hominins. Their argument is that the teeth of A. boisei show no signs of "hard object" feeding:
Of perhaps greater moment than its potential specific simila- rities, the microwear of P. boisei molars, which shows remarkable uniformity over time from about 2.3 Ma to about < 1.4 Ma (9, 24), stands in stark contrast to the wear fabrics exhibited by primate hard-object consumers. Indeed, there is no evidence beyond the anecdotal [e.g., the broken left first permanent molar crown in the KNM-ER 729 P. boisei mandible (8) and the observation that a couple of P. boisei molars show antemortem enamel chipping (25)] that these food items were hard.
These observations are not new, but putting them together with the evidence of grass consumption makes it pretty clear that seed eating was not a predominant source of dietary carbon. The "Nutcracker Man" sobriquet, applied to A. boisei because of its powerful jaw mechanics, must be false. No significant hard object feeding, very low dietary carbon from trees and non-grassy (or sedgy) plants.
Instead, Cerling and colleagues propose that both A. boisei and other early hominins wore their teeth on the, well, grassy parts of grass.
P. boisei cheek teeth display notable gradients of gross wear, resulting in large, deeply excavated dentine exposures, and in this regard, they are similar to other australopith species (e.g., A. afarensis and A. africanus) that also possess low tooth cusps with thick enamel. Thus, like other australopiths, P. boisei undoubtedly had a diet that consisted of foods with abrasive qualities—the gross wear is as likely due to repetitive loading of phytolith-rich tough foods as exogenous grit. Thus, either grass or sedge consumption and/or exogenous grit might well have contributed to P. boisei’s notable wear gradient.
And:
Recent dental microwear studies suggest that the mechanical properties of A. afarensis (and A. anamensis) diets were nearly identical to those of P. boisei (9, 24, 40–42). If this is so, could it be that the australopith masticatory package represents an adaptation to C4 resources such as grasses or sedges? The similarity in dental microwear fabrics among the eastern African australopiths, all of which lack any evidence for hard-object food consumption (9, 24, 40–42), is consistent with the notion that their craniodental morphology could reflect “repetitive loading” rather than hard-object consumption (7, 8, 43).
Grit might get in from eating underground parts like rhizomes. Phytoliths are small, hard silicate structures in the green parts of plants, including the stems and leaves of grass.
Last year I wrote about carbon isotope analysis of two specimens of Australopithecus boisei, the famous OH 5 "Zinj" specimen, and the Peninj mandible. Both specimens show evidence of a high consumption of grass-derived carbon -- estimated at 77% and 81% grass-derived carbon, respectively. Those levels are characteristic of grazing animals. Cerling and colleagues show that these values are right in the middle of the range among specimens of A. boisei that cover a half million years in Kenya and Tanzania.
In the paper reporting the carbon stable isotopes of OH 5 and Peninj, van der Merwe and colleagues [3] suggested that A. boisei may have relied on papyrus as a staple. The culms and rhizomes of papyrus both have substantial nutritional content but are very fibrous and require much chewing and spitting out fiber at intervals. The hypothesis would imply that A. boisei relied on these foodstuffs for the majority of its calories.
Cerling and colleagues do not mention papyrus, and take a much more direct approach on grass-eating. But they do report data on oxygen stable isotopes from the specimens that may be relevant to the ecological context of grass (or sedge) consumption. Oxygen isotopes in bone and teeth reflect the pattern of water consumption by an animal. Oxygen-16 evaporates and transpires preferentially from leaves, so an animal living in an arid environment that gets most of its water from plants will be relatively enriched for the heavier oxygen-18. An animal that depends on drinking water from lakes or rivers will tend to have lower oxygen-18. A. boisei is almost as low in oxygen-18 composition as hippopotamus, suggesting they were strongly dependent on water sources.
A highly water-dependent grass-eating A. boisei is a very different picture of the biology of this robust species. The South African robust species, A robustus, is very different in this regard. These two species are often lumped together, but this is unfair in many ways to their distinctive anatomical patterns. Knowing that their dietary adaptations were very distinct, we should be more inclined to focus on the details where they differ.
Bottom line: A. boisei represents a highly distinctive dietary pattern, not present in any living ape, that no longer exists. At least the giant gelada, T. oswaldi, may also have exploited similar resources. Some grass resources, including papyrus corms and rhizomes, have high caloric and nutritional value, but require adaptations to deal with the fibrous content.
Re: "Tartar control and Neandertal plant use".
In your review of the study on Neadertals and grain in dental calculi, you wrote the following:
"The remains of starch grains and phytoliths tell us about diet breadth but not the proportions of different foods. They do note that nitrogen stable isotopes are most informative about protein-rich food sources, so that a substantial consumption of starchy plants such as grains and USOs might be hidden by isotope analysis."
I also read this study, and I was curious about this comment in the discussion, as this is way outside my field of expertise. I was wondering if you could write a blog post commenting in more detail about what isotope data can and cannot tell us about the proportion of foods or food groups eaten by pre-historic populations, or if you have already done so, if you could direct me there.
Many thanks for writing! You've been doing some nice work there.
I have a long essay on the stable isotopes and diet:
http://johnhawks.net/weblog/reviews/early_hominids/diet/stable_isotopes_...
And two that deal more extensively with Neandertals and nitrogen isotopes:
http://johnhawks.net/weblog/fossils/neandertal/neandertal_mammoth_diet_2...
http://johnhawks.net/weblog/fossils/neandertal/neandertal_fish_drucker_2...
And here's one about cave bears:
http://johnhawks.net/weblog/reviews/behavior/non-primate/ursus/cave_bear...
But all these are out of date in some respects. I've since had several conversations about the nitrogen isotopes. One thing that elevates 15N is breastfeeding, so the time of enamel formation relative to weaning makes a big difference. In more recent populations, the isotopes are often employed to give a picture of the place of birth relative to where the bones were found. A more comprehensive review is in order, but I'm not sure what the next find will be in hominins.
Anyway, I hope that helps, and thanks again for the kind words!
Ann Gibbons reports on the AAPA meetings with a story about all the Homo erectus pelvis and stature papers ("Human ancestor caught in the midst of a makeover," subscription required). Research on the proportions of early Homo was the main event of the meetings, and Gibbons really caught the highlights of the story.
I wrote about body size in Homo erectus a few months ago, and much of the story follows from the basics I outlined there ("The changing height of Homo erectus"). But there I emphasized that the estimated adult height of KNM-WT 15000 was an outlier in a relatively small body size distribution.
What I didn't anticipate is that some interesting work might come along to question the tall adult stature estimate for that skeleton. Gibbons describes the work of Ronda Graves and colleagues, presented at the meetings:
Using intermediate growth rates, graduate student Ronda Graves of Stony Brook University in New York state calculated that Nariokotome Boy would have had less time than originally predicted to reach his adult height when he died. She estimated at the meeting that he would have reached 163 cm in height and 56 kg in weight as an adult—"shorter and wider" than previously thought.
This seems very short, at least when I first saw it. On reflection, Ohman and colleagues (2002) had provided a stature estimate at death of KNM-WT 15000, as only 147 cm, and they suggested it might have been as short as 141 cm. That's an awful lot shorter than had previously been estimated on the basis of regressions.
If Graves and colleagues are right about the lack of a human-like growth spurt, an additional 20 cm (8 inches) wouldn't be unusually small for an adult stature. Those stature estimates would put KNM-WT 15000 between the 50th and 90th percentiles for American 10-year-old boys, or between the 25th and 75th percentiles for 11-year-olds. By contrast, an adult stature of 163 would be around the 3rd percentile for adult American men. The assumptions about growth totally determine the outcome for adult height.
The credibility of the growth assumptions can only be tested by looking at other adult and juvenile remains. There is much more to say on this topic, but I'll point out one relevant comparison: The estimated stature of the adult skeleton from Dmanisi, including the complete D4167 femur and D3901 tibia, is between 145 and 166 cm. Graves' KNM-WT 15000 stature estimate is right within this range.
Meanwhile, there was a lot of disagreement about hips.
[Scott] Simpson and Linda Spurlock of the Cleveland Museum of Natural History realigned the pieces of Nariokotome Boy's pelvis, guided by a female H. erectus pelvis from Gona, Ethiopia, that Simpson reported 2 years ago (Science, 14 November 2008, p. 1089). They found that the widest measure from side to side on the boy's pelvis is 255 to 260 millimeters rather than 225 to 230 mm. This would give the boy an adult hip breadth of 295 to 301 mm rather than the 266 mm originally proposed, and would match those of the short, wide-hipped female from Gona, whose pelvic breadth was 288 mm. "H. erectus was not simply a small-brained modern human," says Simpson.
Simpson's reconstruction seemed reasonable, and it's actually not that big a difference -- roughly an inch and a half (3 cm) in bi-iliac breadth. The main differences were in the overall shape of the pelvis, being shorter with a more flaring iliac blade.
Gibbons describes the disputation that happened after Chris Ruff's presentation. Ruff has suggested that the Gona pelvis may not represent Homo -- that its broad proportions and small acetabula (hip sockets) suggest it may have belonged to an australopithecine (presumably, A. boisei).
Much of the disagreement comes down to the estimation of femur head diameter from acetabulum breadth -- Ruff (2010) gave an estimate of 32.6 mm, Simpson and colleagues estimated between 35 and 36 mm, based on a different method. What you would want is enough acetabula of both genera to be able to examine their variation directly. We don't have such a sample; what we have are a few acetabula and several femur heads. We have the additional problem that living people seem to have a different relation of femur head and acetabulum diameters than in other anthropoids, and it's not obvious which should be applied to early hominins.
I guess (in the relative absence of data) that this acetabulum diameter of the Gona pelvis was in the zone of overlap between Homo and Australopithecus. There's no question that later Homo -- say after 1 million years ago -- is substantially larger in acetabulum diameter, from every specimen so far described. But there are occasional small specimens of Homo even in the Middle Pleistocene. At 1.15 million years old, the Gona specimen is more than 300,000 years later than the last known occurrence of Australopithecus. The femur head that would fit the Gona acetabulum would be smaller than KNM-ER 1472 or D4167 from Dmanisi, both around 40 mm. At least one australopithecine femur head (AL 333-3) is that large, so the femur head diameter distributions do overlap. The STW 431 acetabulum diameter is a sliver larger than that of the Gona pelvis (Ruff 2010 makes it 3 mm bigger, but other workers have given a smaller estimate). SK 3155 may well be Homo and has a smaller acetabulum.
Of course, if we go as far as SK 3155, we have to consider the topic of the Malapa innominate. Can we tell small-bodied Homo from Australopithecus on the basis of pelvic morphology? Several people writing about the Gona pelvis have made it sound like a bigger version of Lucy's. But that's not really true. The australopithecine-like appearance comes from its breadth and consequent features, including the long pubes and flaring anterior ilia. The rest? Maybe there's something here for a clever anatomist.
UPDATE (2010-04-27): I have some e-mail about the last occurrence of A. boisei, which I wrote above was more than 300,000 years older than the Gona pelvis.
The most potent counterargument is Swartkrans Member 1, which has uranium-lead dates around 830,000 years ago, and has been placed by many workers around a million years ago. I actually hadn't been thinking of South Africa. But it is relevant, as the East African record between 1.4 and a million years ago may not be strong enough to argue that the last occurrence of A. boisei is really very close to the extinction time.
Meanwhile, there is OH 36, an ulna from Olduvai Gorge that may represent A. boisei. Since it's (obviously) not cranial, and is quite large and robust compared to postcranial remains that are associated with A. boisei, I've always been very skeptical of that assessment. If there's one feature of the ulna that actually has some phylogenetic importance in the Early Pleistocene, I figure it's size.
But given the current question about body size, that reason for skepticism may have receded in importance. On the other hand, OH 36 seems to represent a substantially bigger individual than the Gona pelvis, so maybe introducing robust australopithecines into the mix doesn't help anything.
Several things puzzle me. Even into Member 1 times, Swartkrans is dominated by A. robustus, with very little Homo. In East Africa, A. boisei is never quite so predominant in the hominin assemblage as the case in South Africa, but was nevertheless very common up to 1.5 million years ago. Did it persist much later? Was it cryptic from the point of view of the fossil record? Are the Swartkrans dates older than we think?
Gibbons A. 2010. Human ancestor caught in the midst of a makeover. Science 328:413. doi:10.1126/science.328.5977.413
Ohman JC, Wood C, Wood B, Crompton RH, Günther MM, Yu L, Savage R, Wang W. 2002. Stature-at-death of KNM-WT 15000. Hum Evol 17:129-141. doi:10.1007/BF02436366
Ruff C. 2010. Body size and body shape in early hominins -- implications of the Gona pelvis. J Hum Evol (in press) doi:10.1016/j.jhevol.20 09.10.0 03
I've followed the literature on early hominid diets from the beginning of the weblog. In 2005 I discussed Peter Ungar's analyses of dental occlusal morphology in A. afarensis versus Homo, concluding:
The contrast between Homo and A. afarensis is in the same direction as the contrast in occlusal morphology between primarily meat-eating carnivores like felids and canids as opposed to more omnivorous carnivores like bears. Another observation is that meat is a major food resource of chimpanzees, although this is hardly a fallback resource. Indeed, if meat eating was indeed an important component of the behavioral repertoire of early Homo, it probably is not fair to assert that the difference in diet between Homo and Australopithecus was primarily a difference in fallback resources. It may be true that australopithecines and early Homo overlapped in their food resources, particularly in plant species consumed. But considering the likely effectiveness of early humans as predators, I think it likely that the fallback foods of early humans--when hunting was ineffective--may well have been the preferred foods of australopithecines. And when australopithecines were forced to abandon their preferred foods by early humans, they were forced to fall back upon resources that either were common or were difficult for early Homo to exploit. The disappearance of early small-bodied Homo by around 1.6 million years ago, and the ultimate extinction of the robust australopithecines after a progressive increase in their molar sizes (Wood and Lieberman 2001) indicate that this fallback strategy could not be maintained in the face of increased hunting effectiveness by large-bodied Homo.
The concept of "fallback foods" has captured a large mindshare in explaining early hominid diets. The idea is that a species may depend on preferred, staple foods for most of the year, but adopt less preferred, "fallback" foods when their staple is not available -- for instance, during the dry season.
What can fallback foods explain about early hominids? For one thing, they could explain the difference between robust and non-robust australopithecines. We know from isotope data (reviewed in this 2005 post about Matt Sponheimer's work) that A. africanus and A. robustus had similar fractions of C3 and C4 plant source foods in their diets. Across the year, they may have eaten roughly the same mix of foods. A 2005 paper by Greg Laden and Richard Wrangham (discussed here) explored the idea of underground storage organs of plants, or tubers, as fallback foods for australopithecines. Later studies of isotope data using laser ablation of small segments of the enamel (discussed here) showed that diet proportions may have substantially varied across the time that teeth were developing -- possibly concordant with the idea of seasonal or longer-period fallback foods. An earlier analysis of dental microwear in the two hominids by Scott and colleagues (discussed here) came to a similar result: there was great variability in wear properties, especially within A. robustus, although the average in the two species showed a possibly greater fraction of brittle, hard foods consumed by the robust australopithecines.
So I've written about the topic a lot, and followed it closely.
Now, Peter Ungar, Frederick Grine and Mark Teaford have examined the wear properties of the molars of Australopithecus (Paranthropus) boisei. They find that -- unlike A. robustus -- none of the seven specimens showed any evidence of having eaten hard or brittle foods:
Comparisons with the extant baseline series suggest that none of the Paranthropus boisei individuals examined consumed extremely hard or extremely tough foods in the days before death. All of these specimens lacked the extremes of Asfc evinced by Lophocebus albigena and especially Cebus apella, both known to consume hard, brittle foods. Paranthropus boisei molars also lacked the extremes of epLsar seen in Trachypithecus cristata and Alouatta palliata, both known to consume tough leaves and stems. The P. boisei individuals examined evidently avoided such metabolically challenging foods, at least in the days before death. This is notably consistent with Walker's [23] early assertion that P. boisei microwear patterns resemble those of living frugivores, and differ from those of living grazers, leaf browsers, and bone feeders.
Comparisons with the South African hominins suggest that while Paranthropus boisei may have consumed foods with similar ranges of toughness as those eaten by Australopithecus africanus, the eastern African "robust" hominin did not eat harder and brittler foods than the South African "gracile" form. Further, the patterns for P. boisei and P. robustus are very different. Paranthropus robustus likely ate foods that were on average much harder and less tough than P. boisei. The differences in both central tendencies and ranges of variation suggest different feeding strategies, and by implication, that the two species of Paranthropus probably had markedly different diets or foraging strategies (Ungar et al. 2008, italics lost).
That is very interesting that A. robustus and A. boisei are so different in their microwear patterns. It makes me wonder whether there may have been substantial habitat variation in the use of hard foods -- maybe the extant A. robustus sample, mainly drawn from a small area of South Africa, had access to some food items that were rare or absent across the larger East African range of A. boisei. But if some A. boisei populations had also depended on such hard resources some of the time, you might expect that we would have found one, or at least a bit more variability. Yet the sampled specimens, drawn from a distance from Ethiopia to Tanzania and well over a half million years of time, are pretty uniform in their microwear, showing some variability in the anisotropy dimension (here, high values have mostly parallel striations, attributed to fibrous food consumption).
So we can return to the question: the major hominid competitor of A. boisei was Homo. Both lineages appeared in the period around 2.5 million years ago, and remained sympatric throughout the next million years. Some of the dynamics of that interaction must have involved diet (considering the different dietary adaptations of the two). We can speculate that A. boisei didn't get much meat, which would then be an important difference. But what else was A. boisei eating?
Meanwhile, the data are still consistent with the idea of fallback foods in A. robustus as a driver of dental morphology, but the story for A. boisei now seems less clear. With only seven specimens, there is almost certainly not enough data to test the hypothesis -- which after all predicts that the use of hard brittle foods may be rare. But that's not positive evidence either. Is there some other food that might explain the hyperrobust craniodental morphology?
Ungar PS, Grine FE, Teaford MF (2008) Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE 3(4): e2044. doi:10.1371/journal.pone.0002044
Sponheimer and colleagues (2006, link) zapped some Swartkrans teeth with lasers to measure their 13C content. I wrote quite a bit here last year about australopithecine diets, including a long review of isotopic evidence for australopithecine diets.
With respect to dietary differences between A. africanus and A. robustus (the two species with any substantial isotopic sampling), there are four essential observations:
One hypothesis for the difference in Sr/Ca ratios is exploitation of underground tubers (warthogs and mole rats have elevated Sr/Ca similar to A. africanus). A mix of C4 foods has been proposed to solve the grass-eating problem, including seeds, rhizomes, insects, lizards, and herbivore meat. But these don't really solve the postcanine tooth conundrum, and while they may both be true; neither is really testable.
OK, so does the new laser ablation study solve any problems? First, let's read a bit about what exactly it is, and why it might be useful. Ann Gibbons has written a ScienceNOW article:
[A] team of American and British researchers studied the teeth of four individuals of Paranthropus robustus (also known as Australopithecus robustus) from the Swartkrans Cave in South Africa. The team scanned the teeth with a sensitive laser, which did not destroy the teeth but etched them lightly enough to free carbon gases long trapped in the enamel. Because different plants absorb atmospheric carbon dioxide differently, the researchers were able to see what types of vegetation the hominids ate based on the ratio of carbon isotopes in their teeth.
An accompanying perspective by Stanley Ambrose explains:
In contrast to conventional methods, the laser ablation technique used by Sponheimer et al. barely penetrates the enamel surface of an area of less than 0.5 mm2 and is thus nearly nondestructive (2). Laser ablation also avoids the problem of time averaging in large drilled grooves. Moreover, perikymata can be counted, providing a good estimate of the minimum time interval sampled and of the duration of tooth formation.
The Paranthropus teeth studied by Sponheimer et al. show interesting patterns of seasonal variation in diet and climate. All have the isotopic composition of mixed feeders, and two show at least ca. 40% variation in the proportions of C3- and C4-based resources over 1 year. One individual had a predominantly C3-based diet and foraged in a cooler, more humid environment; it may have formed its tooth in a very wet year. The others ate more C4-based foods in a warmer, drier environment. Their average carbon-isotope ratios are similar to those of adaptively versatile savanna baboons (2). Analyses of seasonal variation in teeth of modern and fossil baboons and of other hominin species are necessary to evaluate dietary specialization in Paranthropus and niche overlap with other hominin species.
Back to me. There are two possibilities. First, the differences between 13C values for different samples might be sampling the actual dietary variability of single A. robustus individuals over the course of their tooth development (in this paper, sampled over a course of a couple hundred days).
Or second, they may just be sampling noise.
The paper presents comparative data to suggest that this is actual variability in diet and not isotopic noise. They sampled some steenbok teeth from Swartkrans with the same technique. Steenbok are consistent C3 browsers; their diet doesn't vary much in its 13C proportion over time. And the samples from the steenbok teeth didn't show very much variation across different sampling zones from the same tooth. Hence, it looks like the samples from different perikymata actually may give a consistent picture of dietary 13C composition over time.
Compared to the steenbok, the A. robustus samples show great heterogeneity in 13C content. This heterogeneity is manifested when looking at multiple samples from the same tooth, and it is also manifested when looking at different individuals. So far, that would seem to indicate dietary heterogeneity -- the A. robustus individuals ate a different mix of foods over time, and different individuals ate different foods.
On the basis of the magnitude of difference (particularly within the single specimen SKX 5939), Sponheimer et al. propose that some individuals must have gone from a diet predominantly composed of C3 foods to one predominantly C4 within the span of two years (estimated 644 days).
Here's how their paper concludes:
A dental microwear study of the earlier (3.0 to 3.7 Ma) hominin Australopithecus afarensis found no evidence that its diet changed over time or in different habitats (20). In contrast, stable carbon isotope (3, 4) and dental microwear texture analyses (1) of the slightly younger (3.0 to 2.4 Ma) hominin A. africanus demonstrated that its diet was far more variable. This suggests the possibility that a major increase in hominin dietary breadth was broadly coincident with the onset of increasing African continental aridity and seasonality after 3 Ma (21, 22) and only shortly antedated the first probable members of the genera Homo and Paranthropus (23-25) and the earliest stone tools (26). The undoubted toolmaker Homo is thought to have been a dietary generalist that consumed novel foods such as large ungulate meat and tubers that are abundant in savanna environments (27-30). Paranthropus, in contrast, with its extremely large and flat cheek teeth, thick enamel, robust mandible, and heavily buttressed facial architecture, is often portrayed as a dietary specialist (27-29). Further, it has been argued that this specialization contributed to its extinction when confronted with increasingly dry and seasonal environments later in the Pleistocene, whereas Homo's generalist adaptation was crucial for its success (28, 29). Our results suggest that Paranthropus had an extremely flexible diet, which may indicate that its derived masticatory morphology signals an increase, rather than a decrease, in its potential foods. Thus, other biological, social, or cultural differences may be needed to explain the different fates of Homo and Paranthropus (31).
We have lots of other reasons to believe that robust australopithecines were not dietary specialists, as pointed out by Wood and Strait (2004). Robust australopithecines had broad geographic ranges, were able to disperse over long distances, and persisted despite substantial climatic and environmental changes. The evidence for dietary differences across the lifespan is certainly consistent with this.
It does, however, make for an interesting conundrum: if australopithecines were selected on the basis of their ability to find different foods over the course of years, that suggests a strong role for social learning of more food types and broader geographic ranges. But if this was the path taken by robust australopithecines, what was the path taken by Homo?
Ambrose SH. 2006. A tool for all seasons. Science 314:930-931. DOI link
Gibbons A. 2006. Not just nuts and berries for these hominids. ScienceNOW 9 Nov. Full text
Sponheimer M, Passey BH, de Ruiter DJ, Guatelli-Steinberg D, Cerling TE, Lee-Thorp JA. 2006. Isotopic evidence for dietary variability in the early hominin Paranthropus robustus. Science 314:980-982. DOI link
Wood B, Strait D. 2004. Patterns of resource use in early Homo and Paranthropus. J Hum Evol 46:119-162. DOI link
Adam Brumm and colleagues (2006) describe the stone artifacts from the Mata Menge archaeological site on Flores. This site is one of several described by Morwood et al. (1998, 1999) dating to the Lower-Middle Pleistocene boundary. This paper places the date for the artifacts between 880,000 and 800,000 years ago.
Mata Menge and other contemporary sites (there are three with stone artifacts dating to before 700,000 years ago) do not preserve any hominid bones, and there is no evidence that H. floresiensis was there making these tools. But the paper is being interpreted in the context of H. floresiensis -- because one scenario has these hominids evolving in situ from earlier Homo erectus, which presumably made these early tools.
"Small-brained or not, Homo floresiensis was capable of making stone tools, and therefore the standard story of the relationship between brain size and behavioral complexity in human evolution may be less straightforward than currently assumed," said the team's leader, Adam Brumm of the Australian National University in Canberra.
Until now it was thought that the larger the brain, the smarter the hominid. Brumm said his findings suggest that may not be the case.
"The causal relationship between brain size and the complexity of tool behavior in humans is assumed, not demonstrated," Brumm said. "Until now stone tools have only been found in association with large and relatively large-brained hominids, but Homo floresiensis changes that, forcing us to rethink the way we associate big brain with sophisticated behavior."
A Nature News article by Michael Hopkin also reviews the find and the recent Flores flap.
The paper itself is relatively short and most consists of description of the 500-some-odd artifact assemblage from Mata Menge. The comparison with the tools from Liang Bua take up relatively little space. That's too bad, because a long description of the Liang Bua artifacts would be very welcome. But that apparently must wait. In the meantime, here is the concluding paragraph from the paper:
The stone artefact assemblages from Mata Menge and the Pleistocene levels of Liang Bua are remarkably similar. We still do not know the species identity of the Mata Menge knappers, as no associated hominin remains have been recovered so far, but the age of the site clearly precludes modern humans. At Liang Bua, however, the skeletal remains of at least nine individuals are represented in finds from the Pleistocene levels, and all diagnostic elements are of H. floresiensis. The most parsimonious explanation for this is that the stone artefacts from Mata Menge and Liang Bua represent a continuous technology made by the same hominin lineage. Pronouncements that H. floresiensis lacked the brain size necessary to make stone artefacts are therefore based on preconceptions rather than actual evidence.
I think that logic bears repeating:
Item 1 is certainly true. Item 3 is circular -- since it is the existence of H. floresiensis that the tools are supposed to demonstrate, or at least support.
In any event, the mere presence of H. floresiensis (assuming it is real) cannot demonstrate that the species made the tools. We can compare Liang Bua to Swartkrans, which preserved many specimens of Australopithecus robustus and only a small number attributable to Homo. Despite the lopsided proportions of the fossils, there has been no resolution to the question of who made the Swartkrans tools -- far from it, actually. Most of us assume that most of the tools were made by Homo, although it is not possible to exclude the possibility that A. robustus made some of them. But really, this is only a preconception about the abilities of Homo and A. robustus. And, hey, it is only a preconception that prevents us from saying that the tools were made by baboons, which were very abundant in the cave as well.
So the presence of H. floresiensis is relevant only if modern humans were not possible makers of the Liang Bua tools. A bit more on that below.
The key question beneath the conclusion of the paper is whether item 2 is accurate -- are the Mata Menge tools really similar to the Liang Bua tools? This is where the paper seems weakest to me. It is really a stretch to claim that these assemblages were linked by any kind of tradition.
Of course, it would be a stretch in any one place to claim a single tradition spanning 700,000 years or more -- that is a transfer of information across some 35,000 generations. Sure, the Acheulean was maintained for this amount of time or more, but it's not obvious that the Acheulean comprises a single tradition, or that there was much information transfer at all. If the Liang Bua and Mata Menge tools were no more similar than two Acheulean sites of the same age separation, then I would say there was no evidence of a tradition linking them. So even if the tools looked fairly similar, there still might not be a compelling case for descent or isolation based on the artifactual similarities.
But all that presupposes that the tools are similar. Here, I just don't see it. There are two pieces of evidence that supposedly show similarity between the Mata Menge and Liang Bua tools. The first is a similar type of "perforator" -- in other words, a pointy tool that is flaked bifacially to accentuate the point. If you get Nature, you can look at the figure comparing these "perforators" from Liang Bua with the earlier "perforators" from Mata Menge. Go ahead, look. Other than the fact that they both are pointy, I don't see the similarity. The Liang Bua examples are extensively shaped by bifacial flaking; only one of the Mata Menge examples even looks like there was any attempt to shape the "point" unifacially.
Now, consider that these "similar" tools were taken from a sample of over 500 for Mata Menge and a sample of over 3000 from Liang Bua.
The other proposed similarity is this:
For instance, both assemblages show an emphasis on the use of volcanic/metavolcanic fluvial cobbles as raw materials, along with the transportation of flake blanks for use as cores. Core reduction strategies at Mata Menge and Liang Bua are also very similar, with special emphasis on freehand reduction of cores both bifacially and radially. In fact, small, invasively reduced radial cores from the two sites are virtually indistinguishable. In addition, single platform cores, multiplatform cores, cores with 'burination' scars from the production of elongated flakes, 'truncated' flakes and cores indicating anvil-supported percussion and 'perforators' occur in both assemblages (Fig. 4). The maximum dimensions of flake scars on Mata Menge and Liang Bua cores are also very similar (Fig. 5; see also Supplementary Information). This is notable given that Liang Bua cores were more often on flakes, whereas Mata Menge cores were predominantly cobbles and hence tend to be bigger.
"Occur in both assemblages" could link any stone tool assemblages in the world.
Morwood et al. (2004) picture some of the more "advanced" tools from Liang Bua, including blades and microblades (which they suggest were hafted). If these occurred in both assemblages, they might be on to something. But really there are no special similarities between Mata Menge and Liang Bua.
What about those modern humans? They were in Australia by 50,000 years ago. They have to have been passing by Flores by that time, or earlier. The early Liang Bua tools are supposed to be as old as 95,000 years. Maybe modern humans weren't there then, but it's not yet clear from the dates so far presented that any of the artifacts are that old, either. Dates in excess of 70,000 years for modern humans would be credible, and might well explain the site. Whatever is the case, it seems very unlikely that the earliest date for modern humans on Flores could be as recent as this:
In contrast, the first skeletal evidence currently available for modern humans on the island, at Liang Bua around 10.5 kyr bp, is associated with various changes and additions to the stone artefact record, including an increased emphasis on the use of chert and the appearance of new stone artefact types (for example, edge-glossed flakes, grinding stones), as well as the first evidence for symbolic behaviour, such as personal ornaments (for example, beads), pigments and formal disposal of the dead.
So an increased use of chert and deaccentuation of the local volcanic stone is characteristic of the non-floresiensis tools, according to this paper.
But the "perforators" from Liang Bua pictured in the paper are on chert!
Some observations: A cultural transition within modern humans in the region around 10,000 years ago would not be surprising. Cultural transitions also occurred in Australia shortly after that time, possibly related to an influx of new people from Indonesia. That this transition might be associated with greater symbolic behavior also is sensible within the Australasian context, where a Holocene transition to greater symbolic behavior occurred.
Brumm A, Aziz F, van den Burgh GD, Morwood MJ, Moore MW, Kurniawan I, Hobbs DR, Fullagar R. 2006. Early stone technology on Flores and its implications for Homo floresiensis. Nature 441:624-628. DOI link
Hopkin M. 2006. Old tools shed light on hobbit origins. Nature 441:559. DOI link
Morwood MJ, O'Sullivan PB, Aziz F, Raza A. 1998. Fission-track ages of stone tools and fossils on the east Indonesian island of Flores. Nature 392:173-176.
Morwood MJ, Aziz F, Nasruddin, Hobbs DR, O'Sullivan PB, Raza A. 1999. Archaeological and palaeontological research in central Flores, east Indonesia: results of fieldwork, 1997-1998. Antiquity 73:273-286.
Morwood MJ, Soejono RP, Roberts RG, Sutikna T, Turney CSM, Westaway KE, Rink WJ, Zhou J-X, van den Burgh GD, Due RA, Hobbs DR, Moore MW, Bird MI, Fifield LK. 2004. Archaeology and age of a new hominin from Flores in eastern Indonesia. Nature 431:1087-1091.
