A Plethora of Pelvis Papers

Part 1 – The Ilium

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The pelvis is the coolest skeletal element. I might be slightly biased, given that I wrote my dissertation on it. But probably not – it is, objectively, the coolest.

Why is the pelvis so cool? Because it can tell us a lot about how a primate walks around and gives birth, while simultaneously being super complicated to try to figure out.

Recently, two special issues of the scientific journal The Anatomical Record were published focusing exclusively on the pelvis. It was like your gift-receiving holiday of choice for pelvis nerds like me. (And, really, there can never be a true plethora of pelvis papers; the more pelvis papers, the better!) I’m finally getting around to reading them, so I figured I’d do a short series of posts on some of the ones that particularly interested me, starting with one on the ilium.

But first, a quick primer on the pelvis:

The pelvis is made up of two innominates (hipbones) and the sacrum/coccyx (tailbone). The two hipbones are themselves made up of three bones each (the ilium, ischium, and pubis) that fuse within the socket of the hip joint (called the acetabulum, which is Latin for “little vinegar cup”) around ages 16-18.

male pelvis
A complete male pelvis (Gray 1918)

Anthropologists really dig the pelvis because ours is highly modified for walking on two feet (bipedalism), so it looks very different from the pelvis of our closest living relative, the chimpanzee.

schultz primate torsos
The trunk skeletons of a macaque, gibbon, chimpanzee, and human (left to right) (Schultz 1950).

The trend in paleoanthropology recently has been to think of our last common ancestor (LCA) with chimpanzees as being more chimp-like than human-like (though there are some who have argued against this, like the team that discovered Ardipithecus ramidus). So what might this mean for the anatomy of the pelvis of the LCA? Was it more chimp-like or more human-like, and how can we test this?

Hammond and Almecija set out to answer these questions in their contribution to the May special issue (“Lower Ilium Evolution in Apes and Hominins”). They focused on the lower ilium because it varies in length between primate species and the variation has been suggested to be related to differences in how different species move around. They used a combination of measurements, statistics, and tree-building programs to look at variation in lower ilium height within and between species, tried to reconstruct the pelvic anatomy of progressively older LCAs (including the chimp-human LCA and the LCA of all of the living apes), and then compared those reconstructions to some of the predictions that the Ardipithecus team made about the evolution of the pelvis when they published that fossil.

What they found (based on a really large sample of pelvic measurements from 58 humans, 112 great apes, 61 gibbons/siamangs, 95 Old World monkeys, 33 New World monkeys, and 8 fossils), was that the variation they saw in lower ilium height was not purely size-related, which suggests that there might be functional or evolutionary reasons behind it. They also found that gorillas have ilia that might resemble the primitive condition for all hominoids (apes + hominins) and that the chimp-human LCA probably had a shorter lower ilium than living chimpanzees, as had been suggested by the Ardipithecus team. What this means is that living chimpanzees and orangutans may have both independently evolved long lower ilia, which complicates our use of parsimony when building evolutionary trees; sometimes shared features don’t come from a common ancestor, but evolve (via similar pathways, from similar structures) in two related taxa due to similar pressures.

So what’s the take-home message? Well, a lot of people have suggested that there is a characteristic “ape-like” long lower ilium that is somehow functionally related to their locomotion, but that doesn’t seem to actually be the case. The innominate is a complicated bone and it’s not just how a primate gets around that influences it.

Also worth taking home: the pelvis is super cool and so are fossil apes.

If you dig the pelvis, stay tuned! This is the first post in what will be a short series on the pelvis. (Maybe short. Maybe not. Much like the evolutionary history of the lower ilium.)

Disclaimer: I have met/know the authors of this paper. And I’d be just as excited about it even if I didn’t because the lower ilium needs all the love it can get.

Reference
Hammond, A.S. and Almécija, S. (2017). Lower ilium evolution in apes and hominins. The Anatomical Record, 300(5), 828-844.

Who Were the Oldest Homo sapiens?

Guest Blogger: Rene Studer-Halbach

Jebel Irhoud 1
The cranium of Jebel Irhoud 1, the original specimen discovered by miners in 1961. Modified from Bruner & Pearson (2012)

On June 8 a team of researchers headed by Jean-Jacques Hublin published a pair of papers describing a new set of fossils excavated from Jebel Irhoud, Morocco. The authors argue that these new discoveries are the earliest known Homo sapiens found anywhere in the world. This leads naturally to two simple questions: was this individual a human, and did it really live roughly 315,000 years ago?

To answer the first question, Hublin et al. used digitized 3D landmarks (or, a consistent set of points on all of the skulls) to statistically analyze the shape of the Jebel Irhoud specimens and compare them to a set of other hominin fossils. This allows you to compare shape differences independent of size differences. This analysis suggests that these specimens are more similar to Homo sapiens than any other species. That being said, this method is far from conclusive. Several of the major features that we use to identify Homo sapiens in the fossil record, including a vertical forehead, globular braincase, and protruding chin, are absent from the Moroccan fossils. Are these Homo sapiens because they are more similar to us than anything else, or do we need to rely on the presence of those specific traits to define the species? If they are humans, then we need to update our definition of what it means to be a human, morphologically. Even if not, it’s clearly something extremely human-like living in a time and place where we never expected to find one.

The second question has its own set of complications. The team (Richter et al.) used thermoluminescence dating of artifacts and electron spin resonance (ESR) dating of teeth to arrive at the date of the fossils. Thermoluminescence and ESR dating both measure radiation exposure (or accumulated dose) to determine the age of an artifact or fossil. The ESR dating suggested a date of 252 – 318 ka, but with a p-value that was not low enough to be statistically significant. In and of itself, that would be a tenuous basis for such an extraordinary claim, but the thermoluminescence dating of burned artifacts found in association with those fossils revealed a date of roughly 315 ka for the geological layer as a whole. This was repeated many times over. It’s not perfect, but the date seems reasonably secure.

What does this all mean? Why has this been reported everywhere, from social media to TV news? Most of the coverage has focused on the date. These may be the earliest members of our species ever discovered. That’s cool, and especially since it pushed back the first appearance date so far, from ~200,000 to ~315,000 years ago. But I think that misses the most interesting aspect of this discovery. It makes us reconsider what it means to be human in an evolutionary sense.

As the authors note in the title of their article, this find makes the case for a pan-African evolution of Homo sapiens. Whatever these individuals were, they were different from us, that much is clear, but they were more similar than anything else we’ve found outside of Homo sapiens. Did the traits that we use to define ourselves evolve piecemeal, across Africa? The discoverers of these new fossils suggest as much, arguing that the clear delineations between archaic and modern Homo sapiens no longer apply. It might be that these specimens represent a bridge between those two groups. If so, what we call them is largely a question of what definition you like to use for a species. That’s a question for another time, and maybe one that’s best to answer by looking at other species, where the stakes don’t seem so high.

One way you could characterize the last several decades of research in human evolution is to say that our understanding has changed from a linear evolution to a bushy one. We’ve filled out the tree a little more, and we see more of the branches and evolutionary dead-ends in our lineage. These finds are doing the same thing, but for the evolution of our own species, regardless of what they’re called. Hopefully this will inspire a new set of excavations across Africa, looking for more fossils to confound us and upend our expectations.

References
Hublin, J. J., Ben-Ncer, A., Bailey, S. E., Freidline, S. E., Neubauer, S., Skinner, M. M., Bergmann, I., Le Cabec, A., Benazzi, S., Harvati, K. & Gunz, P. (2017). New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens. Nature, 546(7657), 289-292.

Richter, D., Grün, R., Joannes-Boyau, R., Steele, T.E., Amani, F., Rué, M., Fernandes, P., Raynal, J.P., Geraads, D., Ben-Ncer, A. & Hublin, J.J. (2017). The age of the hominin fossils from Jebel Irhoud, Morocco, and the origins of the Middle Stone Age. Nature, 546(7657), 293-296.

Rene Studer-Halbach is a PhD candidate in the Department of Anthropology at Rutgers University. He works on ecological niche modeling and community structure in South African Plio-Pleistocene primates.  

Academic Family Trees

You may have noticed from literally all of the preceding posts that evolutionary anthropologists are into family trees. Who is related to what and how? Is Homo naledi the weird cousin at the family reunion or your great-great-great-great-grandhominin? The interest doesn’t stop at the relationships between fossil taxa; anthropologists are also into their own family trees – their academic family trees, that is.

A couple of years ago, some anthropologists from the University of Texas started the Academic Phylogeny of Physical Anthropology (physanthphylogeny.org) with the goal of tracing advisor-advisee relationships in our field. The tree now includes 2036 people (including me!) from 163 institutions and goes back to some of anthropology’s biggest names, like Louis Leakey, Earnest Hooton, and Franz Boas, to name a few. (Hooton has the most descendants, by far.)

But some of the folks on the tree also have some more unusual “ancestors” – people who weren’t anthropologists at all (like Nobel Prize winning biologist Nikolaas Tinbergen). I’m one of those people; my earliest ancestor to make it onto the tree is Dr. Glenn Jepsen, the first person to be appointed Sinclair Professor of Vertebrate Paleontology at Princeton University. He also served as the Curator of Vertebrate Paleontology and the Director of Princeton’s natural history museum. He worked on Paleocene/Eocene fossil mammals from South Dakota and Wyoming, including preparing and describing the earliest known definitive fossil bat Icaronycteris index.

icaronycteris index
Icaronycteris index Jepsen, 1966 – From the Yale Peabody Museum of Natural History Collections website

That is one good-looking fossil bat. Anyway, what got me started writing this post is that, when I’m not shouting into the internet science void, I work as a collections technician at the New Jersey State Museum under the Curator of Natural History – who actually knew Jepsen! As Jepsen ran Princeton’s (now defunct) natural history museum and it was right down the road from the NJSM, there was naturally communication back and forth between Jepsen and various museum-affiliated people, some of which is still stored at the NJSM. Earlier this week, I found this amusing letter to him in a drawer of old correspondence:

Jepsen letter snip

“…and even the physical anthropologists,” indeed! Apparently we’re a tough crowd. Guess some things don’t change!

Thanks for reading! (And definitely check out physanthphylogeny.org)