New (old) Australopithecus anamensis cranium

The Fall semester here at Vassar kicks off next week, and so of course a new fossil discovery is published this week that threatens to upend my course plans and throw my syllabi into disarray. Haile-Selassie and colleagues report a very well-preserved hominin cranium, from the Woranso-Mille region of Ethiopia and dating to 3.8 million years ago. The new cranium shares features with Australopithecus anamensis, a species previously mainly known through jaws and teeth. The fossil is therefore really important since it puts a face to the species’ name, and it is the oldest relatively complete Australopithecus cranium known. When I showed a picture of the fossil to my wife, who is not a paleoanthropologist, all she said was that it looked like the face of a dog who got stung by a bee.

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The new A. anamensis fossil MRD-VP-1 (left), and a dog that lost a fight with a bee. Fossil photo from the Smithsonian‘s coverage.

The big buzz in many news stories about the fossil (for example, Nature, ScienceNews, etc.) is that it rewrites an evolutionary relationship early in human history, with Australopithecus anamensis no longer the ancestor of A. afarensis, but rather the two being contemporaries. That idea is based on a 3.9 million year old frontal bone attributed to A. afarensis from a site called Belohdelie, also in Ethiopia (Asfaw, 1987): basically, the new A. anamensis cranium reveals a hominin with a narrow frontal region of the brain, which lived 100,000 later than A. afarensis with a relatively expanded frontal region:

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Top views of the reconstructed A. anamensis cranium (left), and the Belohdelie frontal (center), and my crappy photoshopped overlay of Belohdelie on A. anamensis (right). Images not to scale.

The lede, “human evolutionary tree messier than thought,” is not terribly interesting or compelling since it seems to characterize most fossil discoveries over the past several years. And in this case I don’t know how well supported the argument is, since the trait in question (narrow frontal region of the braincase or “post-orbital constriction”) can vary dramatically within a single species. The image below is from the paper itself—compare the difference in “postorbital constriction index” (left graph) between the new A. anamensis cranium (MRD) and A. afarensis (in blue). Both sets of fossils fall within the range of chimpanzees (P. troglodytes), and note the great range of variation within gorillas (G. gorilla).

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Part of Figure 3 from the paper by Haile-Selassie and colleagues. On the top is a view from above of fossil humans: Sahelanthropus tchadensis, Ardipithecus ramidus, the new A. anamensis, A. afarensis, and A. africanus. Below the graphs show how species differ in narrowing of the frontal (left) and length of the skull (right).

What I find most interesting about the new find is the great front-to-back length of the cranium—check out how long and narrow the brain-case is of the fossil compared with the later hominins to the right. This is an interesting similarity with the much earlier (6 million years ago) Sahelanthropus tchadensis, which is the left-most fossil in the figure. It makes me really curious to see the brain endocast of A. anamensis and the Sahelanthropus cranium—what was brain shape like for these ancient animals, and what does that mean for the earliest stages of human brain evolution? The Sahelanthropus endocast was presented at a conference six years ago but remains unpublished. Haile-Selassie and colleagues report that they made a virtual reconstruction of the A. anamensis endocast, so hopefully we’ll get to pick its brain soon.

 

Worst year in review

As we’re wrapping up what may be the worst year in recent global memory, especially geopolitically, let’s take a moment to review some more positive things that came up at Lawnchair in 2016.

Headed home

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Alternate subtitle: Go West
This was a quiet year on the blog, with only 18 posts compared with the roughly thirty per year in 2014-2015. The major reason for the silence was that I moved from Kazakhstan back to the US to join the Anthropology Department at Vassar College in New York. With all the movement there was  less time to blog. Much of the second half of 2016 was spent setting up the Biological Anthropology Lab at Vassar, which will focus on “virtual” anthropology, including 3D surface scanning…

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Cast of early Homo cranium KNM-ER 1470 and 3D surface scan made in the lab using an Artec Spider.

… and 3D printing.

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gibbon endocast, created from a CT scan using Avizo software and printed on a Zortrax M200.

This first semester stateside I reworked my ‘Intro to Bio Anthro’ and ‘Race’ courses, which I think went pretty well being presented to an American audience for the first time. The latter class examines human biological variation, situating empirical observations in modern and historical social contexts. This is an especially important class today as 2016 saw a rise in nationalist and racist movements across the globe. Just yesterday Sarah Zhang published an essay in The Atlantic titled, “Will the Alt-right peddle a new kind of racist genetics?” It’s a great read, and I’m pleased to say that in the Race class this semester, we addressed all of the various social and scientific issues that came up in that piece. Admittedly though, I’m dismayed that this scary question has to be raised at this point in time, but it’s important for scholars to address and publicize given our society’s tragically short and selective memory.

So the first semester went well, and next semester I’ll be teaching a seminar focused on Homo naledi and a mid-level course on the prehistory of Central Asia. The Homo naledi class will be lots of fun, as we’ll used 3D printouts of H. naledi and other hominin species to address questions in human evolution. The Central Asia class will be good prep for when I return to Kazakhstan next summer to continue the hunt for human fossils in the country.

Osteology is still everywhere

A recurring segment over the years has been “Osteology Everywhere,” in which I recount how something I’ve seen out and about reminds me of a certain bone or fossil. Five of the blog 18 posts this year were OAs, and four of these were fossiliferous: I saw …

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Anatomy terminology hidden in 3D block letters,

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Hominin canines in Kazakhstani baursaki cakes,

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The Ardipithecus ramidus ilium in Almaty,

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Homo naledi juvenile femur head in nutmeg,

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And a Homo erectus cranium on a Bangkok sidewalk. As I’m teaching a fossil-focused seminar next semester, OA will probably become increasingly about fossils, and I’ll probably get my students involved in the fun as well.

New discoveries and enduring questions

The most-read post on the blog this year was about the recovery of the oldest human Nuclear DNA, from the 450,000 year old Sima de los Huesos fossils. My 2013 prediction that nuclear DNA would conflict with mtDNA by showing these hominins to be closer to Neandertals than Denisovans was shown to be correct.

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These results are significant in part because they demonstrate one way that new insights can be gained from fossils that have been known for years. But more intriguingly, the ability of researchers to extract DNA from exceedingly old fossils suggests that this is only the tip of the iceberg.

The other major discoveries I covered this year were the capuchin monkeys who made stone tools and the possibility that living humans and extinct Neandertals share a common pattern of brain development.

Pride & Predator

An unrelated image from 2016 that makes me laugh.

The comparison between monkey-made and anthropogenic stone tools drives home the now dated fact that humans aren’t the only rock-modifiers. But the significance for the evolution of human tool use is less clear cut – what are the parallels (if any) in the motivation and modification of rocks between hominins and capuchins, who haven’t shared a common ancestor for tens of millions of years? I’m sure we’ll hear more on that in the coming years.

In the case of whether Neandertal brain development is like that of humans, I pointed out that new study’s results differ from previous research probably because of differences samples and methods. The only way to reconcile this issue is for the two teams of researchers, one based in Zurich and the other in Leipzig, to come together or for a third party to try their hand at the analysis. Maybe we’ll see this in 2017, maybe not.

There were other cool things in 2016 that I just didn’t get around to writing about, such as the publication of new Laetoli footprints with accompanying free 3D scans, new papers on Homo naledi that are in press in the Journal of Human Evolution, and new analysis of old Lucy (Australopithecus afarensis) fossils suggesting that she spent a lifetime climbing trees but may have sucked at it. But here’s hoping that 2017 tops 2016, on the blog, in the fossil record, and basically on Earth in general.

Australopithecus africanus–a highly strained face?

A cool paper came out today [1] in which researchers used finite element analysis to test hypotheses about the biomechanics of chewing in Australopithecus africanus, a South African hominin that lived probably 3-2 million years ago. Finite element analysis is a technique adapted from engineering in which one creates a virtual model of a structure (here, CT data to make a composite cranium of two fossils, Sts 5 and Sts 52), gives the structure mechanical properties (here, the properties of bone), apply various forces of various direction and magnitude, and then can see how the structure is affected by these loads. It’s an interesting technique that has only recently become popular in physical anthropology, probably as the technology becomes better and cheaper, even though Chris Ruff recommended the technique twenty years ago [2].

The authors used this technique to see if the bony facial structure of A. africanus was adapted specially withstand special bite-loads, specifically at the premolars. For example, A. africanus‘s ‘anterior nasal pillars’—bony struts on either side of the nasal aperture—have long been thought to be buttresses against bite forces, but this had yet to be demonstrated. The authors tried different loading patterns—molars only, premolars only, and molars + premolars–to see how strains were distributed across the cranium.

Regardless of the loading regime, the strains through most of the face were the same. However, when just the premolars were loaded, strains increased noticeably on the nasal margins, compared to the other loading patterns. Thus, the results were consistent with the hypothesis that the face of A. africanus, specifically its anterior nasal pillars, are special adaptations for high-strain bite forces concentrated on the premolars. The results also indicate that the A. africanus cranium was adapted to such a high-force diet, rather than a high-volume (i.e. chewing lots of food) diet, because overall strains were the same regardless of whether the only the molars or all the posterior teth were loaded.

This last result leaves me perplexed as to the significance of a major trend in hominin evolution (except with the inception of Homo): postcanine megadontia. On the whole, through hominin evolution the molars and premolars get bigger and bigger (again, not really in Homo)—this is extreme in A. boisei, one of the latest surviving, “robust” australopiths. Presumably, this tendency toward megadonty increased the amount of food that could be ingested at a given time. But, the paper suggests that premolar loading was, if nothing else, an important selective factor in A. africanus, and presumably later hominins–this explains molarization of premolars (again, most noticeably in A. boisei). But it does not explain why molars should also enlarge, unless molar and premolar size are integrated, that is, under the same genetic and developmental control(s). Hey, that’s a cool hypothesis.… Also, I suppose the results don’t refute the possibility that molar size increased in order to accommodate higher volume of food, but rather they simply indicate that the face was not specially adapted to withstand such a diet.

It would also have been interesting to see how the cranium performed under more loading regimes, expecially of the anterior teeth. What really distinguishes A. africanus from its offshoot A. robustus (and especially A. boisei, to whom I don’t think africanus is ancestral) is its retention of large canines and incisors. How do the crania of these taxa differ in their ability to withstand different dental loading patterns? The DNH 7 skull [3], of a very small female from the S. African site of Drimolen, is probably complete enough to be subject to a similar study.

Anyway, it’s a cool paper that really shows off the promise of ‘virtual anthropology.’

Suggested Reading
1 David S. Strait, Gerhard W. Weber, Simon Neubauer et al., Proceedings of the National Academy of Sciences, USA (2009).
2 C. B. Ruff, Folia Primatologica 53 (1-4), 142 (1989).
3 A. W. Keyser, S. Afr. J. Sci. 96 (4), 189 (2000).

Old bones, new methods

A paper has come out that examines dinosaurs’ sense of smell by using computed tomography (CT) to measure the size of the relative size of dinosaurs’ olfactory bulbs, part of the brain that deals with the sense of smell. This is one example of the really interesting paleontological questions that can be addressed with modern imaging techniques (hint into my NSF proposal…).

The olfactory bulb is a part of the brain that deals with smell. Quite simply, the larger the olfactory bulb, the greater an organism’s reliance upon smelling. As a group, primates rely less on scent than many other mammals, and so their olfactory bulbs are relatively small. Humans have a greatly reduced bulb compared to other primates. Prosimians, the most primitive primates, have much larger bulbs than other primates. Thus, scent plays a larger role for their ecology, possibly due to the fact that many are nocturnal (there’s less light at night, but smells always abound).

This dinosaur study used this logic to infer that Tyrannosaurus rex (probably the ‘sexiest,’ most over hyped dinosaur ever) had a very strong sense of smell. This has important implications about T. rex–was it nocturnal (like all those scenes in Jurassic Park…)? Did scent play an important social role (like modern scent-marking mammals including prosimians)?

Also cool was how the team examined olfactory bulb and brain size. The importance of an scent can be inferred based on an animal’s olfactory bulb size relative to the size of the entire brain. The study used endocasts (naturally preserved impressions of surfaces) of brains and CT scans of skulls to estimate these sizes. This illustrates the usefulness of computed tomography in paleontology. It can be difficult to study certain aspects of fossils non-invasively. By CT scanning a fossil, a digital 3D image of it is created, and this allows researchers to examine all the surfaces (including interior) of the bone, with much better accuracy and resolution than X-rays. In this way, researchers can create ‘virtual’ endocasts, among other things. CT scanning also makes bone (and other material) of different densities distinguishable, so that fossil teeth and the insides of bones can be examined without having to damage them.

CT data are becoming very important not just in medical imaging, but also biological anthropology (Dana knows lots!). Many anthropologists, including the team I met in Vienna this summer, are using CT data to reconstruct fragmentary fossils, uncover tooth shapes in fossil hominins, study brain evolution with ‘virtual’ endocasts, and many other things. This is a very exciting time for anthropology and paleontology, as modern medical imaging techniques have made it possible to address (and ask) research questions that were not possible in the past. In fact, my current NSF graduate research fellowship proposal is seeks to develop a new method for studying cranial variation using CT scans. If I get it (fingers crossed), I’ll keep you all updated with how the research goes. Here’s hoping!

Reference
Zelinitsky D, Therrien T and Koboyashi Y. Olfactory acuity in theropods: paleobiological and evolutionary implications. Proceedings of the Royal Academy B. Corrected proof, in press.