Lessons from limb lab (activity)

This semester I have added a lab component to my Introduction to Biological Anthropology class. Lab activities and assignments provide students with opportunities to gather data, to think about them in the context of various theories, and to learn about how to analyze them. This past week’s lab looked at limb proportions within our classroom, in the context of “Allen’s rule” – in colder climates, animals tend to have relatively shorter distal limbs (radius+ulna and tibia+fibula). Allen’s rule, along with “Bergmann’s rule,” describe ecogeographic variation in humans and other animals: body size (i.e., mass) and shape (i.e., limb proportions) tend to vary with climate, such that populations living in colder environments tend to have less surface area relative to body mass, as an adapation to retain body heat.

A simple and effective way to quantify the relative lengths of limbs is through ratios: in this case we examined the crural and brachial indices. Here I’ve plotted average human indices against latitude as reported in a recent paper by Helen Kurki and colleagues (2008):

Left: Crural index (tibia length/femur length) related to latitude, as a proxy for climate. Right: Brachial index (radius length/humerus length) plotted against latitude. Data from Kurki et al. (2008). Black=female, red=male. Dashed lines are  regression slopes for each sex, and solid lines indicate the 95% confidence limits of the regression lines.

Human limb proportions related to latitude, as a proxy for climate. Left: Crural index (tibia length/femur length). Right: Brachial index (radius length/humerus length). Higher indices mean relatively longer tibia or radius. Data from Kurki et al. (2008). Black=female, red=male. Dashed lines are least squares regression lines for each sex, and solid lines indicate the 95% confidence limits of the regression lines. How well will our class’s data fit these models?…

These plots are consistent with Allen’s rule – the distal limb segments become relatively shorter with increasing latitude. In lab, we test whether our limb proportions reflect this presumably ecogeographic pattern. Here in Astana we are at 51ºN latitude, so these regressions predict that our class should have crural indices between 0.82-0.84, and brachial indices between 0.72-0.79. How well does our class fit these model’s predictions?

Pretty terribly.

…Pretty terribly. Plots are same as above, except with our class’s data added at 51º N latitude (vertical lines). In each, the vertical lines span the class’s 95% range (black=females, red=males), with the dots marking each sex’s average. Kazakhstan is huge, and students could have grown up in latitudes from 42º-55º N, but even assuming students had all come from Shymkent their distal limbs still appear much longer than expected.

These plots show that students in the class have longer distal limbs than expected – both for our latitude, and for humans generally. The poor fit of my students’ limb proportions probably doesn’t mean they’re bad humans. Instead, we probably deviate because we compared apples to oranges: the Kurki data were dry long bones measured on an osteometric board, whereas I had my students do their best to palpate and measure the maximum lengths of their own bones beneath layers of fat, muscle, skin, and clothing. Our high indices probably reflect the underestimation of humerus and femur lengths, whose most proximal points that can be palapated (greater tubercle and trochanter, respectively) lie a bit lower than the respective heads, which would have been included in the Kurki measurements.

It was interesting to review these plots with students. Even though they’re fairly new at reading graphs like these, there was an audible gasp and bewildered muttering when their own data went up on the board. I myself was surprised at these results, but I’m happy with how the exercise went. This particular ‘study’ helps students learn about ecogeography, adaptation and human variation, as well as the importance of homology and comparing like with like.

eFfing Fossil Friday: Funky facial flanges #FFF

David Reich and colleagues announced in this week’s Nature the discovery of a new species of extinct mammal, Vintana sertichi, that lived in what is now Madagascar between 66-72 million years ago. The species is based on a very well-preserved cranium of an early gondwanatherian (if you want to impress your friends this weekend, gratuitously use the word “gondwanatherian”). I don’t know much about early mammals like this, but it sounds like it was a weird creature (see the Stony Brook press release). Just looking at it’s face there’s something that sticks out as strange:

Ventana sertichi cranium (Reich et al. 2014, Figure 1a). Left is a 3D CT reconstruction, right is a line drawing highlighting all the individual bones (so many cranial bones). The view is from the right side, so the nose is on the right, the eye is the big hollow in the middle, and the back of the skull is on the left. The jugal flanges are the downward projections.

Vintana sertichi cranium (Reich et al. 2014, Figure 1a). On the left is a 3D CT reconstruction, and on the right is a line drawing highlighting all the individual bones (so many cranial bones). The view is from the right side, so the nose is to the right, the eye socket is the shadowy hollow in the middle, and the back of the skull is on the left. The jugal flanges are the downward projections.

Jutting downward from the sides of the jaw are ‘jugal flanges,’ projections of bone on the homologs of human cheeks. Projections of like these usually serve as muscle attachment sites, and the size of the projection generally reflects the size of the muscle. These facial flanges anchor the masseter muscle, a major chewing muscle that helps close the jaw. The size of this flange in Vintana suggests its chomp packed a punch. A debilitating bite. A face not even a mother could love (so now they’re extinct).

Vintana‘s bony tear-catchers caught my eye because most primates I’ve seen have, you know, less heinous faces. Scouring the internet, big jugal flanges are a fairly rare sight, but can apparently be found in glyptodonts (giant, armadillo-like mammals that lived tens of thousands of years ago) and various sloths. The closest thing I’ve seen to this gross bony flange in Primates are on the zygomatic bones of some extinct, baboon-like animals, such as Dinopithecus ingens:

Fragmentary skull, viewed from the top, of Papio (a.k.a. Dinopithecus) ingens, from Swartkrans, South Africa. Photo credit: CalPhotos.

Fragmentary skull, viewed from the top, of Papio (a.k.a. Dinopithecus) ingens, from Swartkrans, South Africa. As a punishment for its zygomatic excess, its face was confiscated. Photo credit: CalPhotos.

and Theropithecus brumpti

Theropithecus brumpti from the Omo basin. Photo credit: CalPhotos.

Theropithecus brumpti from the Omo Basin, Ethiopia. Photo credit: CalPhotos.

So some primates dabbled in jugal flangery like Vintana, but Natural Selection was having none of it. Anyway, Vintana overcame this craniofacial adversity with characteristic Mesozoic moxie, and is an important piece in the puzzle of mammal evolution. It will be interesting to see what other mammalian surprises the Mesozoic has in store for paleontologists.

Can ‘ape-like’ actually be ‘human-like’?

I’m reading up on life history in Homo erectus for a few projects I’m working on, and something’s just caught my eye. A 2012 issue of Current Anthropology presents a series of papers from the 2011 symposium, “Human Biology and the Origins of Homo.” This issue is full of great stuff, and to top it all off, it can be accessed online for free! (here’s the JSTOR link)

Gary Schwartz has a paper here recounting what is known (or as he stresses, what is still largely unknown) about growth and life history in early Homo. Dental evidence accumulated over the past 30 years has pointed to a rapid (ape-like) life cycle for fossil hominins, in comparison with a slow, long and drawn out human pattern. But much of the evidence against a human-like pattern is somewhat indirect. For instance, Holly Smith (1991) has shown that there’s a pretty tight relationship between brain size and age at first molar (M1) eruption in Primates:

M1 crancap

Fig. 1 from Schwartz (2012). “Bivariate plot of ln M1 emergence age in months (y) versus ln cranial capacity in cubic centimeters (x) for a sample of anthropoids.” The hominins and humans are the open shapes, to which I’ve visually fitted the red line.

It’s a very high correlation (r=0.98). This means that armed with simply an animal’s cranial capacity, which is fairly easy to estimate given complete enough fossils, one can estimate with a bit of confidence its likely age range for M1 emergence. With brain sizes between apes’ and ours, fossil hominins can be estimated to have erupted their M1s at younger ages than us. Many subsequent studies of tooth formation, based on the microscopic remnants of tooth development, have supported these inferences. So presumably, faster, ape-like dental development could be extrapolated to mean ape-like body growth rates and other aspects of life history as well.

But although this is a tight relationship, there are deviations. As Schwartz notes in the article, and others have noted before, high correlations found when examining large interspecific groups (e.g., primates as a whole) often break down when the focus is on smaller groups of more closely related species (e.g., just apes). Based on the relationship figured above, humans are expected to erupt M1 around 7 years of age, but nearly all humans erupt M1 closer to 6 years (hence the open diamond for humans is below the regression line). What hominins appear to share in common with humans is a younger age at M1 eruption than expected for primates of their brain sizes (the red line I’ve added to the figure).

Hominins’ faster dental development and eruption may be ape-like in absolute terms, but eruption ages may be human-like when their brain size is taken to account. As with many life history variables, the significance of this similarity (if anything) is difficult to ascertain.

eFfing Fossil Friday: Frozen Femur

A 45,000 year old human femur from Siberia provides new information about genetic mutation rates and modern human origins. As Quiaomei Fu and colleagues report in this week’s issue of Nature, this seemingly simple leg bone carries so much information, not because of its gross anatomy, but because of the ancient DNA it preserves.

The femur wasn’t discovered by paleontologists, but by an artist/historian looking for fossils around the Irtysh River. The bone came from from a site called Ust’-Ishim, only some 650 km north of the snowy capital where I work in Kazakhstan:

Ust'-Ishim

The site in question, Ust’-Ishim is marked by the yellow star. The red and blue sites to the southeast are other Upper Paleolithic sites. Okladnikov (3) and Denisova (4) have also yielded fossils preserving ancient DNA. Modified from Fu et al. figure 1.

The bone was directly radiocarbon dated to around 45,000 years ago. With a fairly precise age of the bone, Fu et al. could estimate the rate at which genetic mutations arise, by counting the number of new mutations in recent humans that aren’t shared by the Ust’-Ishim femur. This led to an estimate of around 0.43×10−9  new mutations per site per year. This is a relatively low rate compared to estimates based on geologically older fossils, but consistent with more recent estimates that directly compare parents and offspring.

The Ust’-Ishim individual had levels of Neandertal ancestry comparable to living Eurasians (~2.3% of the genome), but there is no evidence of any Denisovan ancestry. Because this individual lived closer to the date of modern-Neandertal admixture, the Neandertal segments of its genome are longer than in modern people (recombination over generations breaks these regions apart into shorter segments). Knowing about recombination rates, Fu et al. could infer that admixture between Neandertal and modern human populations occurred between 50-60,000 years ago.

This eFfing Friday fossil provides more tantalizing evidence for DNA-bearing human fossils just across the Kazakhstan border. With Ust’-Ishim to the north, Denisova and Okladnikov caves to the east, and Teshik Tash to the south, my colleagues and I are very keen to find similar sites here on the KZ side.

Reference: Fu et al. 2014. Genome sequence of a 45,000-year-old modern human from Siberia. Nature 514: 445–449. doi:10.1038/nature13810.

Osteology everywhere: Pollicem verte(b)r(a)e [Latin puns are hard]

I just got back from the meetings of the European Society for the Study of Human Evolution in Florence. As you can guess, bones and genes and anatomy and apes and biomechanics and energetics and everything were on everyone’s minds. Even in the midst of an unseasonal surprise typhoon of lunch time ice:

Ambush of hail.

Aw hail no.

Along the way, I passed a gift shop window and this book cover immediately caught my eye:helert

No, it’s not an ancient Roman gladiator’s helmet. It’s clearly a lumbar vertebra, probably of some quadruped. We’re looking down onto the top (or front of it) from the cranial view. The body or centrum is the rounded part toward the bottom of the picture, the short transverse processes jutting off to the sides. The spinous process, pointing toward the top, is even thick and blunt distally as is characteristic of lumbar verts. Here’s a comparison:

Middle lumbar vertebrae, from the cranial view (modified from Figs. 3-4 of Moyà-Solà et al., 2004). 0=modern baboon, A=Proconsul nyanzae (KNM-MW 13142-J)(B) P. catalaunicus (IPS-21350.59). (C) Cast of Morotopithecus bishopi (UPM 67.28) from Moroto (Uganda). (D) D. laietanus (IPS-18000) from Can Llobateres (Spain). (E) Pongo pygmaeus

Middle lumbar vertebrae of various Miocene apes (A-D) in cranial view (modified from Figs. 3-4 of Moyà-Solà et al., 2004). 0=modern baboon, A=Proconsul nyanzae (KNM-MW 13142-J), B=Pierolapithecus catalaunicus (IPS-21350.59), C=Morotopithecus bishopi (UPM 67.28), D=Hispanopithecus laietanus (IPS-18000), and E= modern orangutan.

Modern apes use an upright posture more frequently than living monkeys, who are quadrupedal. An anatomical correlate of these postures is the position of the transverse processes. Compare the baboon (0 in the figure above) with the orangutan (E). In the monkey the transverse processes come off the sides of the centrum (below the horizontal line), while in the orangutan the processes come off the pedicle further back. In your lumbars the transverse processes arise a little bit more toward the back than in the orangutan.

This is a pretty characteristic pattern, meaning that we can reconstruct the habitual posture of an animal based on a single bone – even just part of a single bone as in the case of Hispanopithecus (D, above). Proconsul nyanzae (A), dating to around 19 million years ago and therefore one of the earliest apes, has a monkey-like lumbar vert; the rest of its skeleton is monkey-like and so we think many of the earliest apes moved around like modern monkeys. In contrast, Morotopithecus bishopi (C), at 20.6 million years ago, is also one of the earliest apes but has a more modern-ape-like lumbar. And so with Pierolapithecus and Hispanopithecus.

The vertebra gracing the cover of our gift shop book is clearly more monkey-like, presumably from a simian who long ago walked on all fours across the blood-soaked floors of a cacophonous Colosseum.

My ESHE poster is Gona blow your mind

I’m in Italy for the annual meeting of the European Society for the Study of Human Evolution. It’s been a great conference, seeing interesting talks (check out #eshe2014 on Twitter), meeting old friends and meeting new ones, and enjoying excellent food and espresso. Here’s the poster I presented yesterday (download pdf):Screen Shot 2014-09-20 at 9.33.38 AM

It’s a follow-up to posts here and here. The long and short of it is, there was a substantial amount of body size variation (i.e., between males and females) in Homo erectus, on par with levels seen in modern day gorillas. This is interesting because H. erectus brain size (and brain size growth) would have required massive amounts of energy, so some have hypothesized a cooperative breeding strategy; sexually dimorphic species generally do not engage in such cooperative behavior. So I suggest that body size variation in H. erectus is an ecological strategy, with small female body size reducing the metabolic burden on mothers.

Osteology Everywhere: Literally

Last weekend, Kazakhstan celebrated Constitution Day. Rather than stick around for the festivities in the florid Capital city, some friends and I ventured out West to Mangystau, to the deserts flanking the Caspian Sea. Although much of the area is sprawling, barren desert, it’s geologically much more interesting than my home here in the White Tomb.

2014-08-30 17.31.29

A perfect camping spot here on Mars.

The purpose of the trip was ostensibly holiday, but in landscapes such as this my field training kicks in. While I made sure to take in the scenic views, my gaze was mostly directed downward, as on survey, in search for bones, lithics and other signs of paleontological promise.

One thing about Life is that it teems. I don’t mean the obvious, ubiquitous microbes or infinitesimal infestations on all our faces. Even the big stuff can thrive, even in seemingly inhospitable places.

This little buddy wants nothing to do with everything.

This curmudgeon puts the ‘turd’ in ‘turtle.’

Some buddies on their way to work.

These buddies are on their way to an important meeting at the office.

But what goes up must come down, the only promise is The End. As a result of this shared fate, many of the landscapes we encountered were literally littered with the bony remnants previous denizens. Sun-scorched and bleached, the calling cards of Tetrapods stuck out like sore thumbs among the dirt and scrub.

Hip off the old block.

Hip off the old block.

A horse doing its best impression of SK 46.

A horse doing a good impression of SK 46.

Mangystau boasts an embarrassment of turtle bones and shells.

Mangystau boasts an embarrassment of turtle bones and shells.

A small, noble beast.

Alas, this was a noble little buddy.

I will admit I have no idea what animal this comes from, but I would guess some small mammal. If you know, please tell!

I will admit I have no idea what animal this comes from, but I would guess some small mammal. If you know, please tell me.

But this surface smorgasbord of bones will not translate into a future fossil festival. Sitting on the surface, bones like these are likely to be scattered, trampled, disturbed by anthropology nerds. Most will not get the chance to sink into the Earth, soak up leaching minerals, and lie in wait for paleontologists of the future. In desert landscapes such as in Mangystau, ‘osteology everywhere’ is an ephemeral description.