You guessed it, we had another guest speaker this Friday instead of our regularly scheduled BS&M meeting. No complaints here, though. This week’s speaker is a giant in the field of paleoanthropology: Dr. Bernard Wood of the Center for the Advanced Study of Human Paleobiology at George Washington University. Dr. Wood stopped by Rutgers to give us his thoughts on the last ~50 years worth of work on the origins of the genus Homo.
Instead of just writing a summary of his talk, I’m trying my hand at Storifying my tweets from the event. Check out the thread and let me know what you think about this format!
Next week, alas, still no BS&M (!), but check back here for ANOTHER defense update! Stan Kivai will be defending his dissertation on the effects of mechanical and nutritional properties on foraging in juvenile Tana River mangabeys. Fingers crossed that the snake is small!
Last week, I instigated my fellow BS&Mers into veering a little off the beaten path, and convinced everyone to discuss a recent Ecography paper that took a metacommunity approach to studying shape variation in rodents. Metacommunities are sets of communities linked by the dispersal of more than one species. Community scale studies are largely lacking across evolutionary anthropology, and given that many extant primates (and probably fossil hominins) fit nicely in the metacommunity framework, this seemed like a good opportunity to explore the potential applications of this approach to anthropological questions.
The paper was met with mixed reviews within our group. Part of the problem was that not everyone was familiar with some of the metrics and analyses (and jargon) utilized in the metacommunity framework. Namely, community weighted means (CWM), principal coordinates of neighborhood matrices (PCNM), principal coordinates of phylogenetic structure (PCPS), and redundancy analysis (RDA). Admittedly, the paper was not written in the most accessible way. For anyone that might be interested in reading this paper, see the table at the end of this post for my best attempts at explaining these (at least for our group) commonly unfamiliar concepts.
One talking point amongst the BS&Mers was whether we actually learn anything new by zooming out to the metacommunity level. A few people (mostly from the B&S contingent of our group) argued that we don’t necessarily need the added complexity of communities to study how environment influences shape variation, and that this is easily done at the species level. I personally believe that looking at variation in community level indices (like CWM) allows us to look at very large scale evolutionary relationships that we might miss at the species level. By scaling up to metacommunities, we are acknowledging (after accounting for phylogeny) that all members of the ecological community are subject to the same environmental variables, and therefore the same selection pressures. Understanding how the mean trait value for the entire community varies with the environment gives us a starting point when trying to understand function.
Another interesting talking point amongst the BS&M crowd was whether or not the paper should have been rooted in some hypotheses. Some of us expressed dissatisfaction at how function was implied throughout the paper, yet there were no hypotheses given for how shape should vary with respect to environmental variables. I actually think that in this case, not having explicit expectations was a bit refreshing. I certainly wouldn’t have considered some of the traits that the authors found to vary significantly with environment.
One thought I was left with after reading this paper was that I wish the authors had included community weighted variance (CWV) in their analysis. Having a measure of variance would at least tell us if a particular trait is worth considering in the first place. It would have also been nice to have incorporated a model averaged phylogeny (sensu 10ktrees).
BS&M ended with a very productive brainstorming session about how to extend this framework to primatological and anthropological questions. I think it would be interesting to use remotely sensed measures of canopy structure to see how morphology varies across primate metacommunities. I also think it would be important to use this framework to uncover how neutral and niche processes shape primate communities. Others proposed extending this framework to studying how various measures of social organization may affect primate trait evolution at the metacommunity level. And of course, we couldn’t resist discussing how we might extend this framework to studying trait evolution across fossil hominin communities. A fair bit of the hominin discussion was how to surmount the palimpsest nature of the hominin record.
Overall the BS&M crew seemed receptive to the approach presented in the paper and acknowledged the importance of the complexity inherent at the metacommunity level.
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.
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.
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.
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 Homosapiens 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.
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.
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:
“…and even the physical anthropologists,” indeed! Apparently we’re a tough crowd. Guess some things don’t change!
Hominin herpes, European apes, and a fossilized spine
Sometimes a lot of cool stuff happens between BS&M meetings. In an effort to keep up with the constant flow of science and to tide you over until our next discussion, we’re going to try to post mid-week mini-blogs and links to what we’re reading during the week.
This week, Google alerted me to another instance of possible between-species hanky-panky in the fossil record. In a new analysis, Underdown and colleagues attempted to figure out the most likely pathway through which humans got genital herpes (HSV2) from the ancestors of chimpanzees. Yes, you read that right. The closest relative of human HSV2 is not HSV1 (oral herpes), but ChHV1 (the chimpanzee version of herpes). The authors suggest that these two viral lineages split from one another between 1.4 and 3 million years ago, and that either Homo habilis got “proto-HSV2” from the ancestor of modern chimps and gave it to Paranthropus boisei, who then passed it on to Homo erectus, or P. boisei got it directly from the ancestor of modern chimps and transmitted it to H. erectus. (H. erectus is generally considered directly ancestral to Homo sapiens, which is why the virus only has to make it to that species to end up in us.)
Before things get too weird, I want to point out that the authors don’t think that the interspecific hanky-panky went down between either H. habilis or P. boisei and a member of the population of ancestral chimps. They suggest that hunting or scavenging meat from infected chimpanzees would have likely been enough to pass the virus on to one of the hominins, probably via chimp blood coming into contact with an open wound during the butchery process. Once “proto-HSV2” made it into H. habilis or P. boisei, however…
Anyway. HSV2 now joins HPV (from Neanderthals) and body lice (from some archaic form of Homo) as evidence of ~close~ contact between humans and our hominin cousins (Reed et al. 2004, Pimenoff et al. 2017). The coolest thing about all of this research is that it’s not based directly on fossils or on ancient DNA; you can use things like the evolution of viruses to tell us about our own evolution. Awesome.
Read on for links to what we’ve been nerding out over this week and the references for the herpes paper. Note: the featured photo is the OH5 cranium of Paranthropus boisei (credit: efossils.org)
This past Friday, our journal club took on “The affinities of Homo floresiensis based on phylogenetic analyses of cranial, dental, and postcranial characters” (Argue et al. 2017). Essentially, Argue and colleagues attempted to figure out what other hominin species H. floresiensis (often called the Hobbit) was most closely related to, using statistical tree-building methods.
Since it was published in 2004 by Brown et al., H. floresiensis has been a bit of a mystery. Much like Homo naledi, there’s been a lot of discussion about where it belongs in the human family tree because its anatomy was A) weird and B) totally unexpected for its age (somewhere between 100-60 thousand years old) and its geographic location (on Flores, a small Indonesian island). The Hobbit was very short in stature, with a very small brain (in the range of orangutans, chimpanzees, and the much-older australopithecines), large teeth for its size, primitive-looking wrist bones, and disproportionately large feet relative to its height and leg length (hence its nickname of the Hobbit). Its discovery on Flores was a surprise because the other hominins that have been found in Indonesia (like Homo erectus from Java) were older and had larger brains (and we generally think brain size in the human lineage has increased over time – but last week’s chat about H. naledi also brought this up as a problematic assumption).
In their new article, Argue et al. set out to test two hypotheses: either the Hobbit is a late survivor from an earlier primitive hominin lineage, or it is a dwarfed descendent of H. erectus. They also commented on another controversial claim that’s been made about the Hobbit – that it is simply a modern human with a genetic/developmental pathology. They tested their hypotheses by applying two tree-building methods to a large sample of characters (particular features or measurements of the skeleton) from the skull, teeth, and postcranial (below the head) skeleton. One method (parsimony) attempts to build the shortest possible tree (one that requires the fewest changes in traits to get from species to species), while the other method uses probability to figure out which trees are most likely to occur (given a particular model of evolution).
When you set out to do a project like this, you’re forced to make some choices as a paleoanthropologist. If you have isolated postcranial bones from a hominin site where you’ve previously found fossils of more than one hominin species from the same time, how do you decide which body belongs with which head? You also confront the issue that not all researchers agree on which specimens belong in which species. And, as always, the fossil record is incomplete; you don’t have all of the characters for all of the species. To account for these potential problems, Argue et al. tested their two hypotheses with several different data sets – for example, they did one test where they considered all of the potential postcranial skeletal material that’s been called Homo habilis to actually be H. habilis and another where they excluded the questionable material.
What Argue et al. found was that their two different hypothesis testing methods and various different data sets produced broadly similar results in support of the first hypothesis: the Hobbit either shared a common ancestor with Homo habilis or is the sister group to the grouping of Homo habilis/Homo erectus/Homo ergaster/Homo sapiens. They are able to reject the hypothesis that the Hobbit is a dwarfed H. erectus (and reject a number of other species as possible close relatives). What this suggests is that (as was proposed for Homo naledi in last week’s papers) there is a long ghost lineage (unknown ancestors) for the Hobbit dating back more than 1.75 million years that is still waiting to be found. Ghost lineages – so hot right now.