evolution

Hip new Australopithecus deyiremeda juveniles

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Header: "Australopithecus deyiremeda" but in a gold Harry Potter font, beneath which in the "Chalkduster" font is written, "And the Explosion of non-adult fossils"

Dr. Yohannes Haile-Selassie & colleagues just published some amazing fossils from around 3.4 million years ago, that convincingly link an unusual hominin foot fossil to an ancient human called Australopithecus deyiremeda.

In 2012, Haile-Selassie and team reported a foot fossil from Burtele, Ethiopia, revealing a bipedal creature (like a human) but with some grasping ability in the big toe (like all other primates). Then in 2015, the team presented some jaws and teeth from a nearby geological locality in the Burtele region, around which they designated a new hominin species, Australopithecus deyiremeda. The researchers hesitated to allocate the Burtele foot to this new species since they didn’t have similar fossils for comparison between the different fossil localities. But as the scientists have recently reported, jaws and teeth discovered from the foot site, including an incredible juvenile mandible, match those of Au. deyiremeda from the nearby Burtele sites. Now we can put a foot to the name.

The Burtele fossils help reveal the diversity of early hominins like Australopithecus and the contexts out of which our own genus Homo evolved. What caught my attention hiding among this amazing assemblage was a fossil that only gets a quick mention in the paper—the ischium bone from the hip of a juvenile deyiremeda:

Extended Data Figure 7 from Haile-Selassie et al. (2025). The BRT-VP-2/87 juvenile ischium (from the right side of the body), depicted in side (a), middle (b), and back (c) views.

The fossil, given the catalog number BRT-VP-2/87, represents a different individual from the juvenile jaw mentioned above. It nevertheless provides a great deal of information despite being a small fragment (less than 2 inches long). The authors observe that the body of the ischium that extends beneath the hip joint is quite long, similar to modern apes, fossil Ardipithecus ramidus, and australopiths. This contrasts with the ischium of modern and fossil Homo in which the bone projects less beyond the hip socket:

Right juvenile ischium bones, scaled to similar size and oriented in similar positions. The black line on each depicts the distance from the hip socket margin to the top of the ischial tuberosity (left modified from Scheuer & Black, 2000 Fig. 10.15)

The bottom of the ischium is called the “ischial tuberosity,” and is the attachment surface for the hamstrings muscles. Having a long ischium provides the hamstrings of apes and other arboreal primates with more powerful hip extension—very useful when climbing trees but it also limits how far back the thigh can extend away from the body (Kozma et al., 2018). The shorter ischium of humans, Homo naledi, and other members of our genus may make our hamstrings a little less powerful, but it also helps us fully extend our legs which is crucial to our efficient bipedal walking and running.

Pelvis growth and development in chimpanzees (top row) and humans (bottom row), all scaled to a similar vertical height. Notice the differences in both the relative length of the ischium (blue bracket) and orientation of the ischial tuberosities between chimps and humans, consistent across the growth period. Images modified from Huseynov et al. (2016 and 2017).

Based on studies of modern humans and other primates, we know that this configuration of bones and muscles is established before birth, so we can be confident that adult Au. deyiremeda would have had a similar anatomy to BRT-VP-2/73, albeit at an unknown, larger size. A hip well adapted for climbing is consistent with the Burtele foot with a grasping big toe.

As Haile-Selassie and colleagues note in the online supplementary information accompanying the paper, only immature fossils allow us to reconstruct the evolution of growth and development. But one of the major challenges of studying immature remains is determining their age or state of maturation, which is critical for understanding how much change occurs between, say, infancy and adulthood. The authors of this study note that the qualitative appearance of the BRT-VP-2/73 hip socket surface is like that of modern humans around 6 years of age, yet the fossil is much smaller and more similar in size to 3 year-old humans. My colleagues and I (2022) faced a similar challenge when analyzing a juvenile Homo naledi hip, and we also relied on qualitative comparisons of how the joint “looks” at different stages of development.

But I think we’re at a point now where we can try to quantify some of these tricky developing surfaces to help place immature fossils more precisely along a timeline of development. For example, Peter Stamos & Tim Weaver (2020) adapted a method for quantifying the topography of teeth, to measure the complex curvature of the developing surface of the knee. If these quantitative methods can distinguish different phases of development in large samples of humans and other primates (e.g., Stamos et al., 2025), they could then be extended to the immature hominin fossil record.

Some cool insights could also be gained by applying older and established methods like landmark-based geometric morphometrics, even on quite fragmentary fossils. This approach could capture the development and orientation of the ischial tuberosity relative to the hip socket surface in fragments like BRT-VP-2/73, MLD 8, and Homo naledi fossils (depicted above) and compared with fossil adults. Researchers have also devised robust ways of quantifying size and shape changes during growth based on modern animals, and using these patterns to then ‘grow’ immature fossils to more developed states, for comparison with actual adult fossils (McNulty et al., 2006). Applying this approach to even just the small fossil sample of ischia described here could tell us a lot about how ancient animals moved at different periods in their lives. Someone just needs to park their ischial tuberosities in a chair and do it!

A growing fossil record of immature hominins, alongside technical advances in quantifying and comparing anatomy, mean that we are ready to learn much more about how our extinct ancestors and cousins grew into competent adults.

New course: “Is the Human Brain Special?”

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For the first time in many years, I’m offering a new advanced undergrad seminar here at Vassar. When I arrived here 8 years ago, I was mainly thinking about Homo naledi and ontogeny, so those were the foci of my seminars. But my research has begun looking more at brain evolution and especially the evidence from fossil endocasts, and there is a lot of literature I need to catch up on.

So I’ve invited students along for this brainstorm, using the question “Is the human brain special?” as a starting point to learn about how the beautifully congealed soup sloshing around inside our skull makes us such quirky animals. In the first half of the semester we’ll read up on brain anatomy and structure, and students will use some of the fossil endocast data I’ve accrued over the years to learn more about a given brain region and extinct hominin. In the second half of the semester we’ll read about the brains, behavior, and endocast fossils of very distant relatives — invertebrates, birds, whales, and dogs — that have been celebrated for their own ‘advanced intelligence.’ We’ll also read about how the evolution of our brains may have predisposed us to certain conditions like addiction and Alzheimer’s, and how brain science has been exploited toward racist and sexist ends (increasingly relevant in America today, sadly).

It will be a lot of work (I’m a very slow, distractible reader) but I’m excited to delve into this literature and see what insights our super sharp students here at Vassar come up with in discussions and projects. The course syllabus (ANTH 323) is available on my Teaching page — I’d be keen to hear suggestions for readings and assignments from folks who know more about brains than I do!

The hand of Homo naledi points to life before birth

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Homo naledi is one of my favorite extinct humans, in part because its impressive fossil record provides rare insights into patterns and process of growth and development. When researchers began recovering naledi fossils from Rising Star Cave 10 years ago, one of the coolest finds was this nearly complete hand skeleton. The individual bones were still articulated practically as they were in life so we know which bones belong to which fingers, allowing us grasp how dextrous this ancient human was. And since finger proportions are established before birth during embryonic development, we can see if Homo naledi bodies were assembled in ways more like us or other apes.

The “Hand 1” skeleton of Homo naledi, adapted from a figure by Kivell and colleagues (2015). Left shows the palm-side view while the middle shows the back of the hand. The inset (b) shows many of the palm and finger bones as they were found in situ in Rising Star Cave.

In a paper hot off the press (here), I teamed up with Dr. Tracy Kivell to analyze finger lengths of Homo naledi from the perspective of developmental biology. On the one hand, repeating structures such as teeth or the bones of a finger must be coordinated in their development, and scientists way smarter than me have come up with mathematical models predicting the relative sizes of these structures (for instance, teeth, digits, and more). On the other hand, the relative lengths of the second and fourth digits (pointer and ring fingers, respectively) are influenced by exposure to sex hormones during a narrow window in embryonic development: this ‘digit ratio’ tends to differ between mammalian males and females, and between primate species with different social systems.

So, Tracy and I examined the lengths of the three bones within the second digit (PP2, IP2, DP2) and of the first segment of the second and fourth digits (2P:4P) in Homo naledi, compared to published data for living and fossil primates (here and here). What did we find out?

Summary of our paper showing the finger segments analyzed (left), and graphs of the main results (right). The position of Homo naledi is highlighted by the blue star in each graph.

The first graph above compares the relative length of the first and last segments of the pointer finger across humans, apes, and fossil species. The dashed line shows where the data points are predicted to fall based on a theoretical model of development. There is a general separation between humans and the apes reflecting the fact that humans have a relatively long distal segment, which is important for precise grips when manipulating small objects. Fossil apes from millions of years ago and the 4.4 million year old hominin Ardipithecus are more like apes, while Homo naledi and more recent hominins are more like modern humans. Because both humans and apes fall close to the model predictions, this means the theoretical model does a good job of explaining how fingers develop. Because humans and apes differ from one another, this suggests a subtle ‘tweak’ to embryonic development may underlie the evolution of a precision grip in the human lineage, and that it occurred between the appearance of Ardipithecus and Homo.

The second graph compares the ‘digit ratio’ of the pointer and ring fingers from a handful of fossils with published ratios for humans and the other apes. Importantly, the digit ratio is high in gibbons (Hylobates) which usually form monogamous pair bonds, while the great apes (Pongo, Gorilla, Pan) are characterized by greater aggression and mating competition and have correspondingly lower digit ratios. Ever the bad primates, humans fall in between these two extremes. Most fossil apes and hominins have digit ratios within the range of overlap between the ape and human ratios, but Homo naledi has the highest ratio of all fossil hominins known, just above the human average. It has previously been suggested that humans’ higher ratio compared to earlier hominins may result from natural selection favoring less aggression and more cooperation recently in our evolution. If we can really extrapolate from digit proportions to behavior, this could mean Homo naledi was also less aggressive. This is consistent with the absence of healed skull fractures in the vast cranial sample (such skull injuries are common in much of the rest of the human fossil record).

You can see the amazing articulated Homo naledi hand skeleton for yourself on Morphosource. Its completeness reveals how handy Homo naledi was 300,000 years ago, and it can even shed light on the evolution of growth and development (and possibly social behavior) in the human lineage.

New decade, new syllabi

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We just kicked off the Spring semester here at Vassar College, and so I’ve got some freshly-updated bio-anthro syllabi hot off the press. This semester, I’m doing my annual introductory class (Anth 120, “Human Origins”), a resurrected seminar (Anth 305: “Human Evo-Devo”), and a second stab at a new methods module (Anth 211: “Virtual Anthropology”).

Anth 120 is similar to previous versions, although this year I’ve taken out a reading/lecture on Paleolithic technology, replaced with articles scrutinizing evolutionary psychology. We’ll see how it goes.

The other two classes are greatly overhauled from previous versions. Anth 211, “Virtual Anthropology,” is my first contribution to a new curricular initiative here at Vassar, which are called “intensives.” Anth 211 is kind of a hybrid between a regular class and an independent study, giving students experience with computer-based, “virtual” methods used in biological anthropology and related fields.  In the first half of the semester, students will get to try out some of these methods and see what kinds of research questions they’re used for. In the 2nd half of the term, students do their own Virtual Anthropology study drawing on the materials in my HEAD Lab, and then present a research poster at the end of the year. I debuted this intensive last Fall, and based on that experience I’m providing a bit more training and have more activities for students this Spring. If last semester’s projects are at all predictive, we should have some fun projects in store this year.

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Anth 305 is a fossil-focused examination of the roles of growth and development in human evolution, and this year’s version is also highly modified from the last time I taught it over two years ago. In that first version, course content was patterned along the skeleton, e.g., one week looked at evolution and development of teeth, next week the spine, etc. Such a bauplan might work for building bodies, but it wasn’t the best for teaching. So this year, we’re spending the first few weeks on the fossil record of human evolution, getting acquainted with the curious characters of our deep past. From there, we go over skeletal / developmental biology, before delving into special evo-devo topics like “morphological integration” and “heterochrony” for the rest of the semester. We’ll also read lots of old, “classic” papers along the way.

Syllabi for these, and other classes, can be found on the teaching page of the site, if you want to learn more.

New anthropology syllabi for 2017

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This Fall I’m teaching three courses at Vassar, two in Anthropology and one in Environmental Studies. Syllabi are posted to my Teaching page in case anyone wants to use them – here are the highlights:

Anth 235: Central Asian Prehistory

Anth 235 site map

I taught this for the first time last Spring, so the Fall syllabus is updated based on how things went in the first go around. This time, students will get more more in depth with the fossil hominins and less on the lithics on the early side. On the more recent end, there are now readings expressly concerned with sites of the Bactrian-Margiana Archaeological Complex, as well as archaeology of both the Tarim and Pazyryk mummies.

Anth 305: Human Evolutionary Developmental Biology

cropped-zevodevo1.jpg

This is a seminar version of the first class I ever made on my own, previously taught at the University of Michigan and Nazarbayev University. There have been lots of new discoveries and I’ve published more on this topic since the last time I taught the class. I’m  also excited to see how this class goes as a seminar in which students contribute more to discussion, rather than me rambling on about osteoblasts, morphological integration, and the like.

Enst 187: A Prehistoric Perspective on Climate Change

climatesummit

This is a 100% brand spankin new class, that uses the climate-denialist argument, “But climate has always been changing,” as a basis for comparing the past and the present. In this First-year Writing Seminar, we’ll compare arguments for defining the “Anthropocene,” examine how climate change may have impacted human evolution, and study archaeological evidence for how climate change has impacted different prehistoric societies.

Historical contingency and an herbivorous calamity

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This post was chosen as an Editor's Selection for ResearchBlogging.org

A while ago I asked, “What the hell was Australopithecus boisei doing?” To recap: there’s this weird side branch of human evolution that was dubbed “Australopithecus boisei” and lived in Eastern Africa from around 2.3 – 1.4 million years ago. They lived right alongside our ancestors, early Homo. Humans from around the world today are not as diverse as racists would have you think (really!), so you’d be totally blown away by the diversity of the early Pleistocene. Since 1959 when A. boisei (then Zinjanthropus boisei) was first discovered, people noticed its massive molar and premolar teeth, thick and powerful jaws, and muscle markings indicative of diabolical chewing power. ‘Probably subsisted on a diet of low-quality, hard to chew foods,’ people reasoned.

But a few years ago, this picture changed: evidence from toothwear and the chemical composition of teeth suggested A. boisei was actually eating grass or sedges (see the referred post or a nice recent review by Julia Lee-Thorp for more info). Such a diet is totally at odds with what people had hypothesized based on the size of the chewing muscles and teeth.

Colobus molars, good for shearing apart leaves. (image: http://bit.ly/xefm6t)

I was discussing this last point with a colleague the other day, who could not believe A. boisei ate grasses or the like: Many animals known to eat grass or leaves tend have molars with high crowns with slicing edges for shearing apart a mouthful of vegetation (above), but A. boisei molars are large and low-cusped, becoming fairly flat with wear (below).

Australopithecus boisei specimen KNM-ER 15930 (Leakey & Walker 1988, Figure 8)
But, it occurred to me, maybe high-crowned, shearing molars simply were not an ‘option’ in the evolution of Australopithecus boisei (see note below**). Natural selection is a powerful force of evolution, but it is limited because it can work only with existing variation: it does the best it can with what it’s got. The earliest surefire hominins, Australopithecus anamensis and afarensis, certainly did not have ‘cresty’ molars with pointy cusps, and neither did many late Miocene apes, for that matter. Rather, the ancestors of A. boisei had fairly low bulbous molar cusps, and that’s some serious evolutionary baggage for a hominid hoping to corner the grass and sedge market.
So we can draw up the following hypothesis for the evolution of A. boisei: as the early members of the species moved into a niche of eating grass/sedges, rather than evolve cresty teeth, they increased the size and enamel thickness of their ancestors’ molars to better-withstand their diet. Perhaps this was the ‘easiest’ solution to adapting teeth to a crappy diet (maybe some developmental constraint?). Or perhaps there’s another, yet unidentified food responsible for the species’ curiously high-C4 diet … who knows? Nota bene: this isn’t necessarily what I think happened, it’s just a hypothesis consistent with current evidence about A. boisei‘s anatomy and diet.
If Life on Earth has taught us anything, it’s that there are many ways to do the same thing. What’s more, evolution is highly constrained by pre-existing biology and historical circumstance. Australopithecus boisei may have been ‘a victim of its times,’ forced into an herbivorous niche for which it was ill-equipped.
READ MORE!
Leakey RE, & Walker A (1988). New Australopithecus boisei specimens from east and west Lake Turkana, Kenya. American Journal of Physical Anthropology, 76 (1), 1-24 PMID: 3136654
Lee-Thorp, J. (2011). The demise of “Nutcracker Man” Proceedings of the National Academy of Sciences, 108 (23), 9319-9320 DOI: 10.1073/pnas.1105808108
*Edited 07 Nov 2015
** This blog post was written in 2012. In 2016, Peter Ungar and Leslea Hlusko wrote more on this idea of boisei being constrained in its dental evolution here.

Evolution: What it is and why humans aren’t immune to it

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An alternate title for this post could be “BigThink Too Big For Own Britches.”

Physicist Michio Kaku (via John Hawks via Pharyngula) has re-brought my attention to the fact that a great deal of people (smart people like Kaku included) misunderstand the mechanics of biological evolution. Quite simply, evolution is change in a gene pool over time. This pool could be an entire species or a small population within that species.

There are a number of ways evolution can happen. A mutation is a new genetic variant that arises in an individual, which can then be spread to later generations when that individual reproduces. A single strand of human DNA is like a string of some 3 billion letters. When a person replicates their DNA for it to be passed on to their offspring (meiosis), having to reproduce such a long strand ensures that a mistake is made at least once in a while. Hence mutations increase variation in a gene pool.
But the frequencies of genes in a population can change, that is they may become more or less common within the gene pool. This could happen by genetic drift, which is the random loss of genes. If a gene is neither adaptive nor harmful, it could simply be lost over time due to sheer chance. In contrast to mutation, drift reduces genetic variation.
If genes are adaptive or harmful, their frequency in a gene pool becomes subject to natural selection. If a gene (or set of genes) is adaptive, that means the possessor of those genes will be more likely to survive and reproduce than others, i.e., the individual will be more likely to pass on these genes. Over time, the adaptive genes will increase in frequency in a population. Conversely, genes that lower the likelihood of surviving and reproducing will become less frequent in subsequent generations. Either of these scenarios means selection is reducing genetic variation. But sometimes different forms of a gene can be adaptive in different situations or combinations, so selection will act to maintain both of these in the gene pool. So in contrast to mutation and drift, selection can reduce or maintain genetic variation.
Finally, gene flow refers to genes being introduced into a gene pool from another source. This could occur when someone from one population reproduces with an individual from another population, and so new genes may enter one of the groups. Like mutation, this will increase genetic variation in a gene pool.
Common misconceptions
It may seem counterintuitive, but evolution does not equate with progress. This is a common misconception, probably due to the social ideologies under which evolutionary theory developed. Because of selection, evolution often means that a population becomes better-suited to its environment over time, which seems like progress. But as we’ve seen above, not all evolution is selection; mutation and drift are fairly random processes of evolution that don’t necessarily bear on adaptation. In addition, environments and circumstances change, so that even if something evolved in a place where it was adaptive, it may be harmful in a new context. For example, as the earliest humans lost their body hair, they probably evolved to have darker skin: adaptive in the tropics where humans originated. But later, when early humans moved into more northerly latitudes with less ultraviolet exposure from the sun, the dark skin that was adaptive for a hairless human in a tropical environment came to hinder the body’s vitamin D synthesis: maladaptive!
Also contra popular opinion, individuals do not evolve, populations do. Trojan brand condoms recently had an ad campaign in which they encouraged men to “evolve” by using Trojan condoms when having promiscuous sex. This is in line with the incorrect idea above that ‘evolving’ means ‘becoming better’ or ‘more sophisticated.’ Of course, condoms may actually help a population to evolve: those who use condoms to prevent pregnancy are ensuring they do not pass on their genes. And if there’s any genetic predisposition to make one more likely to use condoms (and there’s not), these genes would certainly become less common in future generations. [I am NOT encouraging people not to use protection, by the way]
So this brings us to a final point: the main misconception expressed in Dr. Kaku’s video is that humans are not evolving. Technology and urbanization, he tells us, have circumvented natural selection on human features (well, the “gross” or visible ones). This is very wrong and shortsighted. In fact, this is one of the bases of the eugenics movement of the early 20th century. Eugenicists thought, ‘Nature is no longer ensuring some people don’t pass on their genes, so we ought to do it ourselves for the good of humankind.’ This first thought, about the insufficiency of Nature, is echoed by Dr. Kaku (though surely he does not think the second).
Simply put, HUMANS ARE STILL EVOLVING. Remember, not all evolution = natural selection. The genetic composition of humankind is still subject to the random forces of mutation and drift. In fact, because the human population size has increased exponentially of late, the fact that there are way more people than ever means that there are more mutations entering the population, and at a faster rate, than ever! But selection is still at work, too. There are still diseases that kill people before they can pass on their genes. There are still environmental situations – even in ‘civilized’ places! – that prevent people from passing on their genes.
We humans are still evolving because we are still subject to the forces of evolution, and we always will be. Now what physicist could’ve told you that?!