Anthropology.net

I don’t know who this is worse for, the editors & reviewers over at Nature or the authors of the article who can’t tell the difference between crocodile teeth markings and stone tool modification, nor raise the possibility. The paper, “Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia,” very confidently proclaims unambiguous evidence for,

“stone-tool-inflicted marks on bones found during recent survey work in Dikika, Ethiopia, a research area close to Gona and Bouri. On the basis of low-power microscopic and environmental scanning electron microscope observations, these bones show unambiguous stone-tool cut marks for flesh removal and percussion marks for marrow access.”

Butchered by early humans or eaten by crocodiles? Image: David DeGusta

Given that the said rib fragment, DIK-55-2, came from a prehistoric lacustrine site. These markings could have been produced by crocodiles. Crocs, if you aren’t aware of (ahem editors and publishing group) are very abundant in the Rift Valley — both currently and prehistorically. On top of that, crocs like to eat meat and scavenge. Yes its true, they are carnivores. Australopithecines were at most ominivores, with wide based teeth useful in grinding tubers and nuts. Crocs have more meat shearing, bone crushing teeth than 3.39 million year old stone tools, which there are none of at the moment.

Given that there really isn’t an archaeological record for Australopithecine tools, I’ll take a gander and say crocs like to eat meat and scavenge more effectively than A. afarensis could make and use said tools to butcher a large ungulate. They have been on this Earth for roughly 197 million years more than hominins have and they are really good at what they do… Again, probably better than a species of hominins who did not live in the Stone Age. It is just as likely (if not more) that the markings were produced by crocodiles just given the ecological context.

Now just how different at cut marks from crocodile teeth marks? David DeGusta, from Stanford University, compared and contrasted the two different markings using images from Njau and Blumenchine (2006) paper titled, “A diagnosis of crocodile feeding traces on larger mammal bone, with fossil examples from the Plio-Pleistocene Olduvai Basin, Tanzania,” to those published in the current Nature article. I’ve inserted DeGusta’s image into this post on right for your own inspection. DeGusta was also on Science Friday, discussing this possibility, with one of the article’s authors, Zeresenay Alemseged. What do you think? Do they look completely different or similar? Seriously, I am asking you to comment. I’d like to know what you see.

Personally I don’t see much of a difference. I agree that stone tools marks are more V shaped, while croc teeth are more pitted/rounded. But take this into light: tool use, especially butchery, is a very human behavioral trait. In their search to attribute this human behavior to a primitive hominin species who roamed 800,000 years earlier, to the era of Australopithecus afarensis, without considering another possible explanation, the authors and editors of Nature were somewhat foolish.

Many paleoanthropologists are in this mad rush to claim their precious find is the most human of hominins, so as to etch their name into the textbooks in rewriting human evolution, that they sometimes forget about doing thorough comparative science. And many publications are in this mad rush to publish the most human of findings, that they sometimes forget about thoroughly editing scientific works. Think that could be the case? I sure do… Why should we settle on secondary evidence for Australopithecine stone tools when none have been found yet, and when another possibility hasn’t been extensively exhausted?

McPherron, S., Alemseged, Z., Marean, C., Wynn, J., Reed, D., Geraads, D., Bobe, R., & Béarat, H. (2010). Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia Nature, 466 (7308), 857-860 DOI: 10.1038/nature09248

A brand new study in the open access journal PLoS One reports on the results of an analysis of the microwear on the teeth of Paranthropus boisei (also known as Australopithecus boisei). The results contest what we’ve all along assumed was going on with the form and the function of these robust australopithecine teeth. I even repeated this assumption in a recent post, discussing the dietary implications of a Neandertal.

The results indicate that marks on the teeth of Paranthropus boisei do not correspond with the size and shape of its teeth. This observation suggests that structure of the skull, mandible, and teeth are not enough to infer dietary preferences. Furthermore, evolutionary adaptation for eating may have been based on scarcity rather than on an animal’s regular diet.

The paper, “Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei,” is authored by Peter Ungar, Frederick Grine, and Mark Teaford. Brian, from Laelaps, has written up a very excellent and thorough review of the research. You should check that out.

Why did the authors look at dental microwear on the surface of the teeth? They explain that dental microwear offers direct evidence of the mechanical properties of food item, and that makes a lot of sense. During mastication, the food an organism eats abrades with the surface of the teeth. The texture of the food may scratch and dent teeth, especially if really hard foods like nuts, roots, and tubers are chewed. Likewise, softer foods affect teeth less.

Based upon the size and morphology of Paranthropus boisei teeth, many previous researchers thought they ate tough foods. Hell, the type specimen for P. boisei has been nicknamed “Nutcracker Man.” They have saggital crests similar to gorillas They also have very large teeth with thick layers of enamel. Coupled with the massive jaws, many people thought that P. boisei consumed really dense, fibrous plants.

As I’ve said, the results of the microwear analysis indicates that P. boisei did not eat the hard abrasive foods that we thought they did. Using a confocal microscope, engineering software and scale-sensitive fractal analysis the authors analyzed the around 54 molars from seven specimens of P. boisei. These seven specimens came from different areas, Ethiopia, Kenya, and Tanzania, respectively. Also, these specimens cover over 1 million year of P. boisei existence. Any similarities or differences in the microwear on P. boisei teeth from these temporally and spatially different locations woulda screened out variations in diet reflected to time or environment.

For your viewing pleasure, the authors provided this photograph of detailing the degree of microwear of the teeth of the seven different specimens.

Here’s the legend: (A) KNM-CH 1, (B) KNM-ER 729, (C) KNM-ER 3230, (D) KNM-ER 3952, (E) KNM-WT 17400, (F) OH 5, (G) Omo L7A-125. Clearly you see scratches…

… But what Ungar et al. were looking for was the degree, complexity and directionality of wear textures. Again, hard, brittle foods like nuts and seeds tend to lead to more complex tooth profiles, while tough foods like leaves lead to more parallel scratches, which corresponds with directionality. Comparisons of the the dental microwear profiles of P. boisei to the microwear profiles of extant primates indicates they didn’t eat the same type of foods. Grey-cheeked mangabeys and brown capuchins sometimes rely on hard nuts or palm fronds, as do the mantled howling monkey and silvered leaf monkey. Also included in the comparison was microwear analysis on teeth of Australopithecus africanus.

The P. boisei teeth show light wear, suggesting that none of the individuals ate extremely hard or tough foods in the days leading up to death. The microwear patterns are more consistent with modern-day fruit-eating animals than with most modern-day primates. This conclusion is a fundamental shift in the way we think about the diets of early hominins.

Before you write off the anatomical and evolutionary mantra “form = function,” please consider this: I did not read any discussion on other possible causes for the lack of deep pitting from hard foods.

This is unfortunate. I wonder if the depth of the pits coulda have been buffed out over the millions years these fossils persisted? We know the fossilization process is a time intensive process, and many different weathering affects affect how surfaces of bone are preserved. Furthermore, many fossils are formed by being deposited in or around water sources. Water moves things, like rocks, wood, and bones. The abrasive edges of rocks are buffed down to smooth river stones over time. Couldn’t this have happened on a microscopic scale?

Yeah, I think it is certainly possible, even if the entire specimen, like OH 5, don’t show signs of massive weathering, we are talking about teeth that are very old. Teeth that were exposed could have been buffed out by other elements like wind, too. Suffice to say, many different things coulda happened to them to affect the degree of the abrasions. Comparing them to modern primate teeth is somewhat flawed because modern primate teeth haven’t been affected by the forces of nature for millions of years.

Ungar, P.S., Grine, F.E., Teaford, M.F., Petraglia, M. (2008). Dental Microwear and Diet of the Plio-Pleistocene Hominin Paranthropus boisei. PLoS ONE, 3(4), e2044. DOI: 10.1371/journal.pone.0002044

The discussion about dimorphism between the sexes in the genus Australopithecus has been an ongoing debate in the world of physical anthropology and paleoanthropology. For many people interested in human evolution, this is an interesting question because sexual dimorphism can explain early mating systems.

I won’t get into that just yet, but I will overview some of the key points raised in the last 25 years with a specific emphasis on recent back and forth brouhah from two different camps. To start, in all honesty, I don’t know who really sparked the debate about investigating sexual dimorphism in early hominids, but I think it is safe to guess that it all happened after Lucy was discovered in the mid ’70′s.

One of the first traceable accounts of the debate started with one of my undergraduate professors. She introduced us to the questions she raised in the ’80′s about whether the different species of australopithecines are truly separate species or representatives of extremes in sexual dimorphism. At the time, and even now, a probability of this magnitude did not come without controversy. It got a lot of people talking about robust australopithecines being representatives of the male sex, whereas the more gracile ones are females. This consideration has been called going ‘beyond the orthodoxy’ by some.

Since then, a very thorough compilation of the debate was synthesized by Frederick Grine in the late ’80′s, who by the way, just recently put out a new revision of the text, “Evolutionary History of the “Robust” Australopithecines.” The debate has been going on still, but the general consensus fell in line with those made by Henry McHenry in 1992: Australopithecus afarensis was more dimorophic than modern day humans.

Much of the problem with the debate was that there just are not that many samples of australopithecines to reconstruct a good, statistically valid, understanding about the amount of variation in a population. That’s one of the biggest shortcomings in anthropology… the fact that we’re trying to extract as much information as possible from limited number samples. It is no fault of anthropology, that’s the nature of the research… When one finds a fossil, that’s basically all they find, a single fossil. We’d all like to find complete skeletons, but that’s rarely the case.

Furthermore, Australopithecus fossils do not come out from the same stratigraphic layer nor the same location. The fossils that do come out from the same layer and location, are most often not attributable to the same individuals (i.e. not from one definite skeleton). Nor are the fossils consistent in quality and composition of the physical fossil. In other words, the fossil record of Australopithecus, one of the more exhaustive collections in hominin species, is still not complete enough.

Enter the discovery of the A.L. 333 locality in Hadar, Ethiopia. What was unearthed at A.L. 333, is what people call the “First Family” of Australopithecus, an abundant but fragmentary collection of fossils from the same stratigraphic layer. The discovery was made from the same area where Lucy was found, and in the mid ’70′s, but it wasn’t published until the early ’80′s. The assemblage contained over 200 fragments of fossil bone with a minimum number of individuals at 9 and a maximum number at 22. Since they were all concentrated in the same stratigraphic layer and in close proximity, many consider the site a simultaneous death assemblage — basically a catastrophic event killed at least 9 early hominins in one fell swoop. This discovery seemed fulfill of the biggest shortcomings in analyzing Australopithecus, having a comparatively large collection of fossils from the same time, come from the same locality.

Fast forward to August of 2003, when PNAS published a paper that analyzed the dimorphism present in A.L. 333 collection. The title basically tells it all, “Sexual dimorphism in Australopithecus afarensis was similar to that of modern humans.” The authors made about 30 different measurements of bones from A.L. 333 and compared them to Lucy as well as the Makapansgat humeri. Unfortunately, because 9 complete skeletons weren’t found, the fossils used in the analysis are not all one anatomical element… rather, some are humeri, some are radii, others are femora, etc. The used the measurements from these different anatomical elements to make an estimation on the femoral head diameter, which is based off the best preserved Australopithecus afarensis femoral head, Lucy’s. This is called the template method, when a certain element is not represented in all the samples, so another element’s measurements that is present in the sample, is used to make a relative estimation about the missing element. To all you who live and breath statistics, this maybe a big bad faux pas — but really what else are you to do, make no analysis at all?

The authors of this PNAS paper extend their conclusion from the lack of dimorphism present in Australopithecus afarensis collection in A.L.333, to a discussion about a potential monogamous behavior in Australopithecus afarensis. In humans, the lack of a large dimorphism between males and females, is linked to monogamy. Why? Since human babies are born at a less developed stage and require more help and care to raise the baby, the males have lost their dimorphic stature while focusing on raising the baby. Without a large stature they aren’t able to harem up females that more dimorphic species like silverback gorillas have done. I’m not to sure I understand this linkage because it seems like one of those, “we don’t have empirical evidence but this story makes sense,” sorta situations.

These two conclusions irked the camp that want to think Australopithecus afarensis was largely dimporhic. And the most outspoken voiced their opinion in a 2005 issues of the Journal of Human Evolution. The paper, “Sexual dimorphism in Australopithecus afarensis revisited: How strong is the case for a human-like pattern of dimorphism?“John Hawks covered this two years ago, so I won’t rehash the points too much. In a nut shell, the authors of this slam the A. L. 333 study on several points, the primary one being that the A. L. 333 specimens represent males. They also raise questions about whether or not skeletal measurements, the only thing that can be analyzed from fossil remains, correlate with sexual dimorphism as well as if skeletal dimorphism can be used to predict social or behavioral features of primates. The authors were especially irked with prediction of monogamy due to the lack of dimorphism in the A.L. 333 samples.

The A.L 333 study offered up a quick and terse response that same year. Here’s the paper, “The case is unchanged and remains robust: Australopithecus afarensis exhibits only moderate skeletal dimorphism. A reply to Plavcan et al. (2005).” The authors address how skeletal dimorphism is only one aspect of size dimorphism, and how it can never tell us the total body size dimorphism by itself. About the monogamy claim, they clarify that no living primate model provides a basis to construct for A. afarensis sexual behavior, and therefore it is necessary to construct one. That’s why they merely raise the possibility that A. afarensis wasn’t exclusively, definitively polygamous, it coulda been monogamous as well as polygamous.

We’ve heard little about this debate since this 2005 cat fight. That all changed when this paper came out late last year in the American Journal of Physical Anthropology, “Strong postcranial size dimorphism in Australopithecus afarensis: Results from two new resampling methods for multivariate data sets with missing data.” The authors of this new paper offer up a really promising new methods to take out inaccuracies of estimating a missing measurement from another element. Furthermore, they take a multivariate analysis, which means more than one measurement which brings in more data that can be compared and scrutinized. They ultimately figure out that A. afarensis was as dimorphic as gorillas and orangutans.

I read this paper and man, I must admit, I have no idea what their two methods they are. I got so frustrated in making sense of out it. They claim their methods can compensate for inconsistencies using template method, but it just sings to me as an abstracted, obfuscated template method. Furthermore, if you wanna make sense of the method, maybe perhaps crunch out the numbers yourself, the authors provide you with this mind-numbing array of equations that I’ve uploaded to your right. I’m sorry if I can’t provide a more intelligible critique but really what are they trying to do — make their data so unapproachable that it can never be validated nor refuted? I’ve been told by many different professionals that if the data and the methods seems too complex, if they bring derivatives and calculus into to the fold, then something is fishy.

So the latest word is that A. afarensis dimorphism is similar to gorillas and orangutans — who’s male and female skeletons look like completely different species to the untrained eye! The jury is still out, there definitely seems to be two camps that are continually hashing this out, and until we find more complete comparable specimens, we’ll be reading this sorta stuff for a long time.

I do want to make a comment about where I stand on the issue. Like many others, I want to believe that A. afarensis was dimorphic… at least comparable to the levels seen in chimpanzee and bonobos. Why? Because, they are one of the earliest hominins we know of and have a lot of specimens of. Furthermore, from what we know of A. afarensis birth canal size and brain size, they weren’t as undeveloped as humans. Rather, they were born at similar developmental stages seen in chimpanzees. The argument that males lost dimorphism to help raise the kids doesn’t apply when we consider this fact. Now, the evidence from A.L. 333 tells us otherwise, and I trust that evidence because it comes from one stratigraphic layer and time. Furthermore, I trust the simple straight forward analysis on the A.L. 333. Not only does it come from a reputable source, Owen Lovejoy, perhaps one of the more preeminent biological anthropologists out there, but it also is followable — the statistical analysis, the data, the methods are clear and thorough.

Gordon, A.D., Green, D.J., Richmond, B.G. (2008). Strong postcranial size dimorphism in Australopithecus afarensis: Results from two new resampling methods for multivariate data sets with missing data. American Journal of Physical Anthropology, 135(3), 311-328. DOI: 10.1002/ajpa.20745