Things around the blog have been a bit slow with BS&M on its summer hiatus (and me teaching an intensive summer human osteology course), but new anthro papers continue to come out!
What I’ve been reading:
Chimpanzee super strength!
Matthew O’Neill and colleagues tested the claim that chimpanzees are “super strong” relative to modern humans using a combination of actual chimpanzee muscle samples and computer modeling. Spoiler alert – they’re only about 1.35 times stronger than we are, and the reason for this has to do with both muscle fiber type and fiber length. Chimps have more “fast fibers” than we do, along with longer fibers, which the authors suggest make their muscles capable of greater maximum force output and power than ours. This might be beneficial for a large-bodied, arboreal primate. But not all arboreal primates have skeletal muscle dominated by fast fibers; O’Neill et al. also point out that the slow loris has, like we do, muscle that is mostly made up of slow fibers. And, based on their comparisons to other mammals, the authors suggest that our slow, short muscle fibers likely evolved within the hominin lineage, making them a unique characteristic of our group.
So what this means from an evolutionary perspective is that sometime over the last 7-8 million years, potentially coinciding with our shift toward obligate (full-time) upright bipedalism, the architecture of our muscles changed along with our skeleton. This is super cool because soft tissue anatomy isn’t preserved in the fossil record (except in certain rare, extreme conditions, and never in hominins) and this gives us a way to potentially investigate it. I also have some purely self-serving questions/ideas about how this relates to my own research interests, but I think I’ll stay quiet about them for the time being.
In other Anthro News: if you’re in the area and haven’t been, check out the Philadelphia Zoo. They’ve got some very cool primates (omg, red-shanked douc langur) and the Zoo360 Animal Exploration Trails are awesome. The family of gibbons was hanging out in one when I was there and watching the baby do its hilarious little bipedal run up close was incredible.
Reference O’Neill, M. C., Umberger, B. R., Holowka, N. B., Larson, S. G., & Reiser, P. J. (2017). Chimpanzee super strength and human skeletal muscle evolution. Proceedings of the National Academy of Sciences, 201619071.
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.
On May 9th, Lee Berger and colleagues published three new papers on Homo naledi, the most recent South African hominin fossil find to make media waves. The original H. naledi fossil material was discovered in 2013 by two cavers working with Berger; it comes from the Dinaledi Chamber of the Rising Star cave system, from which it takes its species name (Berger et al. 2015). The first H. naledi discovery was remarkable because there are at least 15 individuals (from juveniles to adults of both sexes) represented in the assemblage and there are often multiple copies of each skeletal element present, which allows paleoanthropologists to look at variation within the species, and to see how it grew and developed. In total, there are about 1550 hominin bones and teeth in the assemblage – the largest single species assemblage found anywhere in Africa (Berger et al. 2015).
The three new articles covered the discovery of additional skeletal material (Hawks et al. 2017) and the age of the fossils (which had been a major source of speculation) (Dirks et al. 2017), and proposed a hypothesis for understanding Pleistocene hominin diversity in subequatorial Africa as part of a larger mammalian biogeographic pattern (Berger et al. 2017).
The new fossil material comes from a second chamber within the Rising Star system, the Lesedi Chamber, and represents at least three individuals (though in actuality the material likely comes from more than three individuals, based on where the bones were recovered from within the Chamber). The most spectacular of these remains is a relatively complete skull with associated skeletal material; the researchers have named this individual Neo. The new material looks a lot like the previously discovered H. naledi bones and teeth, and also includes both adult and juvenile material. The major thing that differentiates the Lesedi Chamber finds from the Dinaledi Chamber finds is that the Lesedi Chamber also contains animal skeletal material (Hawks et al. 2017).
The paper on the age of the fossils used several different methods (including dating geological features of the cave itself as well as directly dating some of the fossil teeth) to produce an age range for the material of 236,000-335,000 years old (Dirks et al. 2017). This means that the material is later Middle Pleistocene in age, much younger than would have been predicted based on looking at features of the skull and skeleton (like brain size).
In a previous paper, Hawks and Berger (2016) discussed what three different hypothesized ages (including a scenario that does match the new date) for the original H. naledi material would mean for its place in human evolution, and they follow up on this in the third new paper – now that we have a date, what does it mean? The date is younger than the first appearance of Homo erectus around 1.8 million years ago. As H. erectus is generally thought to be part of the lineage that is directly ancestral to us, this would seem to preclude H. naledi as a member of our direct line, unless it represents a sister group to our species that preserves a lot of the primitive morphology of a shared ancestor. This interpretation bumps H. erectus to a side branch of our family tree. A different type of analysis of the features of various species of Homo suggests instead a more bushy view of earliest Homo – whatever that ancestor was split into a number of species, each having only some of the ancestor’s features. What we can say is that H. naledi is likely only part of a branch that originated earlier in time, with the authors going so far as to suggest that we might already have fossils from earlier on this branch that we have not recognized due to their fragmentary nature (Berger et al. 2017).
Regardless of where Homo naledi ends up on our family tree, it’s still an incredible discovery that contributes to our understanding of the human fossil record. It provides a window into a time period in our history from which we don’t have great data and underscores two important ideas: first, that for most of our evolutionary history, Homo sapiens was not the only hominin species on the planet; and second, that there are still spectacular fossils waiting to be discovered.