Friday, May 27, 2011

Culinary trends in an extinct hominid

A few weeks ago I discussed a recent paper that analyzed the carbon and oxygen isotope ratios from Australopithecus boisei molars (Cerling et al. 2011). The major finding here was that an enlarged sample (n=24 more) corroborated earlier isotopic (van der Merwe et al. 2008) and tooth wear evidence (Ungar et al. 2008) that A. boisei probably did not subsist on as much hard foods as previously thought. Although this strange hominid probably ate mostly grass/aquatic tubers, some researchers think it may have looked something like this:
Left, A. boisei reconstructed skull, from McCollum (1999, Fig. 1). Right, artist's reconstruction of what the individual on the left may have looked like during life.
But looking at the numbers I'm wondering if the carbon isotopes reveal anything more about this curious hominid. If we plot boisei's carbon 13 values against the fossils' estimated ages, there's a small hint of a temporal trend, of increasing carbon 13 levels over time (more C4 plant consumption). Fitting a line to these data does indicate an increasing C4 component over time, but the slope of the line is not significantly different from zero. The early, high value could be an outlier (not eating the same stuff as his/her peers?), although the lowest carbon 13 value of all that would support this trend is also much lower than the other values; it could be a more anomalous one. So while it's tempting to hypothesize dietary change over time in A. boisei, at the moment it looks like you can't reject the hypothesis that diet is consistent throughout the Pleistocene until the A. boisei's demise.  Supporting dietary stasis, Ungar and colleagues (2008) reported similar molar tooth wear in specimens from 2.27-1.4 million years ago.
In addition, Cerling and colleagues sampled at least one of each of the cheek teeth. Because teeth form in the jaws in a sequence (not all at the exact same time), the isotopic signatures from given teeth represent the dietary intake of carbon at various different points in an individual's childhood. In the figure below I lumped upper and lower teeth together; the un-numbered "M" indicates molars unassigned to a specific position.

The first molar crown starts to form right around birth, and note here that it's carbon 13 values are slightly higher than the other molars. The premolars and second molar start to form around the same time, so it is curious that each of these teeth show distinctly different ranges of carbon 13 levels. The sole P3 is also the lowest value (eating fewer C4 plants) in the entire sample, but the P4 has less negative values (eating more C4 plants). Not sure what's going on here, but maybe later analyses of more specimens will clarify the situation.
Our australopithecine ancestors and cousins have proven to be a rag-tag bunch of funny bipeds, and A. boisei has proven to be one of the weirder ones, in my opinion. Of course descriptions of Ardipithecus ramidus and Australopithecus sediba skeletons have been recent reminders that we have lots left to learn about Pleistocene hominids. For my part, I'm interested in working out the deal with the group of "robust" Australopithecus.

Cerling, T., Mbua, E., Kirera, F., Manthi, F., Grine, F., Leakey, M., Sponheimer, M., & Uno, K. (2011). Diet of Paranthropus boisei in the early Pleistocene of East Africa Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1104627108

McCollum, M. (1999). The Robust Australopithecine Face: A Morphogenetic Perspective Science, 284 (5412), 301-305 DOI: 10.1126/science.284.5412.301

Ungar PS, Grine FE, & Teaford MF (2008). Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PloS one, 3 (4) PMID: 18446200

van der Merwe NJ, Masao FT and Bamford MK. 2008. Isotopic evidence for contrasting diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. South African Journal of Science 104: 153-155

Monday, May 16, 2011

Good olde dentistrie

I'm reading up on mandibular rotation, which is the change in orientation of the mandibular corpus relative to the rest of the skull during growth (the corpus is the horizontal part of your jaw that holds up your teeth; check out the shape changes in the mandibles in the blog header). So far as I can tell, the original classic paper on the topic is by Bjork (1955). Growth was studied by implanting metal pins into the jaws, then seeing how they move across ontogeny via X-rays (which were once called "roentgenograms," neat-o!) Here's a picture of the procedure, from Bjork (1955):
HOLY GOD WHAT DID THAT KID DO TO DESERVE THIS?! And although there must be a third person there, it sorta looks like there's a three-handed dentist wielding a hammer, a nail, and a kid's face. No wonder so many people are afraid of the dentist.

BJORK A (1955). Facial growth in man, studied with the aid of metallic implants. Acta odontologica Scandinavica, 13 (1), 9-34 PMID: 14398173

Friday, May 13, 2011

Neandertal terminal biogeography

How late did Neandertals persist in the Late Pleistocene? Two papers out this week discuss the dates of the latest Neandertals in western Asia.

Pinhasi and colleagues (2011) stress the importance of directly dating Late Pleistocene human-ish fossils. There are numerous techniques used to estimate the ages of the fun stuff we find underground. For fairly old fossils like australopithecines, perhaps the most reliable radiometric method is Argon-Argon, though this requires the fossils to be relatable to volcanic sediments whose argon levels can be measured. The point is that dates of burial are often not estimated from the fossil materials themselves, but rather the sediments and such surrounding the fossil of interest. But younger fossils (than say 50,000) preserve some of the bone's original carbon -allowing age estimates of the fossils themselves by radiocarbon dating.

Pinhasi and colleagues note that while seven separate Neandertal specimens from across Europe and western Asia have been directly dated to be younger than 36 thousand years, these dates may be underestimates. In other words, Neandertals may not have lived after 40 thousand years. To this end, these researchers directly re-dated the infant Neandertal from Mezmaiskaya Cave in Russia, and estimate the poor lad to have died around 42-44 thousand years ago. The authors predict that future direct redating of other Neandertals will show Neandertals to have disappeared by 40 thousand years ago, and that they would have overlapped in time with more modern-looking humans either minimally or not at all. If only there were more information on the latest dates for Middle Paleolithic people!

Lucky me, in tomorrow's Science, Ludovic Slimak and colleagues report on Mousterian tools dating to 32-34 thousand years ago, from the site of Byzovaya Cave "in the western foothills of the Polar Urals" (Slimak et al. 2011: 841). "POLAR!" The site is way further north than any site with Neandertal bones like Mezmaiskaya and Okladnikov, which is pretty impressive. But, there are no human remains associated with the tools, so we don't know who made them. To what extent do these finds address Pinhasi's and others' contention of no Neandertals after 40 thousand years ago?

Slimak and colleagues carbon-dated animal bones that were butchered with the Mousterian tools, which were allegedly made only by Neandertals. There is a major problem with the wide-held assumption that Mousterian (Middle Paleolithic) tools were made only by Neandertals, whereas Upper Paleolithic industries beginning with the Aurignacian were made only by humans. This goes along with people's wont to make a connection between stone tool 'culture' and biologically determined, phylogenetically significant behavioral capacities. But of course, we know biology doesn't determine behavior, and so there's no reason to assume [Mousterian:Neandertal::Aurignacian:'Modern' Human]. Where Mousterian remains have been associated with diagnostic skeletal remains, they are Neandertal. But the Aurignacian, so far as I know, is not associated with diagnostic fossils - we can't say for certain who made it. Plus we know Neandertals were doing something kooky, yet logical in some sort of cognitively complex way, with bird feathers in Italy 44 thousand years ago (Peresani et al. 2011). So the Byzovaya stone tools may demonstrate a late, northern holdout of Neandertals, but then they could simply mean that the new technology either hadn't arrived or hadn't been successful in the far reaches of sub-Artic Pleistocene humanity.

If the latter is the case and Pinhasi & team's hypothesis that Neandertals didn't coexist in time and space (or did only minimally) holds, then the old assumption of Mousterian = Neandertal becomes dubious for other sites with Mousterian tools but no diagnostic fossils. This would also beg the question of the role of modern humans in the Neandertal demise - did the Neandertals disappear and open a niche for other groups of people ('moderns')?

So how were Neandertal populations distributed through space and time in their latest days? I dunno! But for the moment I suppose I'd be surprised if no fossils with Neandertal morphology turn out to be younger than 40 thousand years as suggested by Pinhasi and co. But then I could be wrong.
Hoffmann, A., Hublin, J., Hüls, M., & Terberger, T. (2011). The Homo aurignaciensis hauseri from Combe-Capelle – A Mesolithic burial Journal of Human Evolution DOI:10.1016/j.jhevol.2011.03.001

Peresani, M., Fiore, I., Gala, M., Romandini, M., & Tagliacozzo, A. (2011). Late Neandertals and the intentional removal of feathers as evidenced from bird bone taphonomy at Fumane Cave 44 ky B.P., Italy Proceedings of the National Academy of Sciences, 108 (10), 3888-3893 DOI:10.1073/pnas.1016212108

Pinhasi R, Higham TF, Golovanova LV, & Doronichev VB (2011). Revised age of late Neanderthal occupation and the end of the Middle Paleolithic in the northern Caucasus.Proceedings of the National Academy of Sciences of the United States of America PMID:21555570

Slimak, L., Svendsen, J., Mangerud, J., Plisson, H., Heggen, H., Brugere, A., & Pavlov, P. (2011). Late Mousterian Persistence near the Arctic Circle Science, 332 (6031), 841-845 DOI:10.1126/science.1203866

Monday, May 9, 2011

If imitation is the sincerest form of flattery

Life as we know it has taken some strange courses. Of all the things an animal could do with its time, pretending to be an ant is apparently pretty popular. According to a review article in the latest Current Biology, there are probably over 2000 abhorrent species of myrmecomorphs (ant impersonators), including spiders, caterpillars, mites, beetles, and other types of arthropod biodiversity I'm not familiar with, that have come to resemble ants in some form or another.

It's interesting how and why different life forms have come to p-ant-omime. For example, in the picture above, (Maderspacher & Stensmyr 2011, Fig. 3) on the left side is the crab spider (Aphantochilus rogersi) mimicking ant species in the genus Cephalotes - which the spider comes upon unawares and then feeds upon (getting pwned on the right side of the photo). If imitation is the sincerest form of flattery, then mimicry must be the most malevolent means of creepy.

Or here's a treehopper (Cyphonia clavata, an insect and not a spider like above) that doesn't just disguise itself as an ant, but rather has a whole ant-shaped appendage bursting from its back in a disgusting perversion of alien birth in the Alien series (Maderspacher & Stensmyr 2011, Fig. 1). It is quite remarkable that a surprisingly common yearning to be perceived as an ant has resulted in convergent evolution of an ant-ish figure in myriad of nature's more disgusting creations, not to mention in ants themselves.

Florian Maderspacher & Marcus Stensmyr (2011). Myrmecomorphomania Current Biology, 21 (9) : R291-293. doi:10.1016/j.cub.2011.04.006

The universe is too damn big

I made the unfortunate discovery last weekend that Google Earth has not just badass satellite photos of our home planet, but also really badass maps of places we're ready to colonize: The Moon, Mars, and The Stars. Clearly I'm never going to get any real work done ever again.

Everything extraterrestrial has blown my mind for as long as I can remember. I have always been bothered by the fact that there are things in the universe of such superlative scale that I cannot imagine the amount of matter involved. [Warning the following information comes from WolframAlpha and Wikipedia, so I could be off on some figures]
On the one hand, life is small. Cells can be small enough such that a human body is comprised of an estimated 10 trillion of them. The DNA basepairs within cells are small enough that there are 3 billion in each cell's nucleus (the small green bits within the DNA helix in the picture). That's tiny. (image: Molecular Expressions)

But then on the other hand, there are things whose sheer size is just stupefying. Earth's diameter averages around 7900 miles. We are located on average about 93 million miles from the Sun, which itself is over 864,000 miles in diameter. Outside our solar system, the closest star to us is Proxima Centauri, which is 4.2 light years away from the Sun. 4.2 light years means it takes light 4.2 years to travel the mere 25 trillion miles between the two stars. At a much grander scale, we are part of the Milky Way galaxy, which is an estimated 590 quadrillion miles across (quadrillion = 1 million billion).

Wikipedia tells me there are an estimated 200 billion additional galaxies vying against ours for the title of "Sweetest Galaxy." But galaxies are gregarious - the Milky Way is part of the "Local Group" (which I'm sure is like the cool kids' table as far as galaxies go), comprised of around 30 other galaxies, and spanning some 58 quintillion miles (or in other words, 58 billion billion miles). More infuriating, these groups and clusters of galaxies themselves aggregate into "superclusters," the most colossal cosmic conglomerates in the universe (though these aren't bound together by gravity, so I guess they shouldn't really count in a way). Our Local Group is part of the larger Virgo supercluster, which is an estimated 110 million light years across, or 65 quintillion miles.

The image at the right contains two galaxies from the Hercules Galaxy Cluster who were best buds, until they came to blows and literally collided with one another (image: Could anything possibly be more epic than a galaxy fight? No.

So our bodies are comprised of countless bits of small matter, myriad micrometric molecules executing our basic vital functions. At the opposite end of the size spectrum, galaxy clusters are made up of a stupid sum of stars, more numerous and more massive objects than the scads of cells in our bodies. Baffling.

Saturday, May 7, 2011

What the hell was Australopithecus boisei doing?

A little over 2 million years ago there a major divergence of hominids, leading on the one hand to our earliest ancestors in the genus Homo, and on the other hand to a group of 'robust' australopithecines, the latter group a failed evolutionary experiment in being human. In our ancestors, parts of the skull associated with chewing began to get smaller and more delicate, while the robust australopithecines increased the sizes of their crushin'-teeth and chewin'-muscle attachments.
A face not even a mother could love, so now they're extinct (from McCollum 1999 Fig. 1). Note the very tall face, flaring cheeks, and massive lower jaw which would have facilitated wicked-pisser chewing power.

Weirder, there is a South African form (Australopithecus robustus) and an East African form (A. boisei, the figure here looks like it's based off this species) of robust australopithecine. These two may have inherited their robust adaptations from a common ancestor, or they may be unrelated lineages that evolved these features in parallel. A boisei has been referred to as 'hyper-robust,' its face and teeth are generally larger than those of A. robustus.

For a while it's been supposed that these 'robust' chewing adaptations in our weird, extinct evolutionary cousins (every family has those, right?) reflected a diet of hard objects requiring powerful crushing and grinding - things like hard fruits, seeds, Italian bread, etc. But a few years ago Peter Ungar and others (2008) examined the microscopic wear patterns on the surfaces of molar teeth of A. boisei and noted that they lacked the characteristic pits of a hard-object feeder. A. robustus on the other hand does have wear patterns more like an animal that ate hard foods. Why such a difference? Why the hell wasn't boisei behaving robustly?

Also in 2008 Nikolaas van der Merwe and colleagues analyzed the carbon isotopes preserved in the teeth of A. boisei and some other fossils. Briefly, plants utilize two isotopes of carbon (C12 and C13), but 'prefer' the lighter-weight C12. Some groups of plants like grasses have thrived because they're less picky and can get by just as well with C13. Different kinds of plants, then, incorporate different amounts of these two carbon isotopes into their tissues, then when animals eat it, these isotopes get incorporated into the animal's developing tissues, including tooth enamel. So by looking at the relative amounts of carbon in teeth, researchers can get a rough idea of whether an animal was eating more of the C13-loving or C13-loathing plants (or the animals eating the plants). van der Merwe and others found A. boisei to have a way higher percentage of the plants that don't discriminate against C13 as much, possibly things like grass, sedges or terrestrial flowering plants. GRASS?!

Last week, Thure Cerling and colleagues expanded on the earlier study led by van der Merwe, including a larger set of boisei specimens spanning 500 thousand years of the species' existence. Lo and behold, Cerling and others got similar results: the isotopic signature in A boisei is similar to grass-feeding pigs and horses in its habitat - was the badass "hyper robust" A boisei just a hominid version of a horse? Now, the silica in grass make it extremely wearing on tooth enamel, and while A. boisei had crazy thick molar enamel, I would be a little surprised if the boisei dentition could withstand a lifetime of a grassy diet. Nevertheless, boisei's diet clearly differed from robustus, based on both dental wear and carbon isotopes.

This raises interesting questions about the evolution of the robust group. Does their shared 'robust' morphology reflect common ancestry, with the subtle differences the result of their divergent diets? Or do the subtle differences indicate that they evolved separately but their diets for whatever reasons resulted in similar mechanical loading on their jaws and faces? It should also be noted that while the dates for South African cave sites are always a bit uncertain, it is possible that A. robustus persisted alongside genus Homo until around 1 million years ago, whereas the fossil record for A. boisei craps out around 1.4 million years ago - was A. boisei too specialized on crappy grass, resulting in its evolutionary demise?
A horse-ish, human-ish hominid? Australopithecus boisei, rest in peace. 2.1 - 1.4 mya.

Cerling TE, Mbua E, Kirera FM, Manthi FK, Grine FE, Leakey MG, Sponheimer M, & Uno KT (2011). Diet of Paranthropus boisei in the early Pleistocene of East Africa. Proceedings of the National Academy of Sciences of the United States of America PMID: 21536914

McCollum, M. (1999). The Robust Australopithecine Face: A Morphogenetic Perspective Science, 284 (5412), 301-305 DOI: 10.1126/science.284.5412.301

Ungar PS, Grine FE, & Teaford MF (2008). Dental microwear and diet of the Plio-Pleistocene hominin Paranthropus boisei. PloS one, 3 (4) PMID: 18446200

van der Merwe NJ, Masao FT, & Bamford MK (2008). Isotopic evidence for contrasting diets of early hominins Homo habilis and Australopithecus boisei of Tanzania. South African Journal of Science 104: 153-155

Sunday, May 1, 2011

The descent

I begin my prelims tomorrow. I have two days to write a ton of brief essays showing what all I've learned - or failed to - about biological anthropology since coming to graduate school.

In the course of studying, I realized I'd amassed a collection of doodles and scribbles, plenty paper put to less than optimal use: