The brain of human animals

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This week I participated in the workshop, “Corpus: The Body in Parts,” organized by Jeremiah Garsha and Jess Beck at University College Dublin (UCD). I joined an interdisciplinary group of researchers from across UCD, as well as keynote speaker Aja Lans (Johns Hopkins University) and artist Liliane Puthod. Our motley crew spent a day discussing different body parts and their intersections across science, society, time, and space. It was fascinating to learn new things about the body and how various parts are viewed in other disciplines, and the discussions have given me a lot to think about for my own research and teaching.

My corporal contribution to the workshop was the brain, so here are some quick notes I put together as I was thinking through what I’d want to talk about that day.

Left side views of five fossil human skulls arranged from left to right. The top row shows the original fossils, while the bottom row shows the same skulls with bone rendered transparent and the brain-covering internal surface highlighted in solid pink.
Fossil crania and the brain endocasts of Homo naledi specimens LES1, DH1, DH2, DH3, and DH4 (reversed).

The brain is central to the human experience, which is what Anthropology is all about. Anthropologists generally align with one of four subfields respectively focusing on culture, language, material expression (archaeology), and biological variation. The brain is the part of the body that enables culture, language, and material expression of the lived experience.

A brain is a like a beautifully congealed soup of fats, sugars, protein, water, salt. This remarkable mixture receives information from the outside world, integrates it along with information about our internal states, and then sends signals to the rest of the body about what to do next. In the context of anthropology, this soup of sentience inside the skull provides each individual human with the ability to cooperate with others to solve novel problems.

Human language is a great example of this universal behavioral flexibility. Language writ large encompasses a set of mostly arbitrary symbols (e.g., sounds, gestures) allowing individuals to express infinite ideas to others. There are literally thousands of languages spoken around the planet today, each using different sounds and rules for how these sounds get constructed to convey meaning. Every person is born capable of learning any of these languages— even several of them—as long as they are immersed in a language from a young age. The network of areas in the brain that enable language are pretty much the same in everyone.

So, the brain contributes to cultural and linguistic variation in humans in that it provides the cognitive scaffolding necessary for cultural expression, but the brain certainly doesn’t ‘determine’ cultural differences across the globe. This is important from a historical perspective because in some of the intellectual precursors to modern anthropology, researchers assumed a strong biological determinism underlying human behavioral variation, leading to scientific racism and eugenics which unfortunately have enduring legacies into the present day. Rather, despite the great cultural diversity among humans today, we are remarkably biologically similar to one another the world over.

An enduring question is how (and why) our cultural brains evolved in the first place. Answering this question requires a comparative approach, examining how human brains are similar or different to brains of other animals, as well as how brains vary among members of the same species. One of the coolest things to come out of these comparisons is that there isn’t really anything totally ‘new’ about our brains, except for one surprisingly simple thing. The basic components and structures that are found in the human brain are shared among all animals with a backbone, meaning the basic recipe for brain soup is over 500 million years old. The wrinkly outer rind of the brain—the cerebral cortex—is shared across all mammals, meaning it originated over 200 million years ago. Within mammals, humans are a member of a diverse group called primates, and primate brains have some unique characteristics compared to other furry creatures. We’ve known for over 100 years that human brains are much larger than you’d expect for mammals or even primates of our body size; research in the past 20 years or so has shown that ‘under the hood,’ humans have basically just a primate brain scaled up to a large size.

You might be wondering, if our brains aren’t so different from those of other primates, why are we such weird animals capable of behaviors never seen in the hundreds of millions of years that brains have been around? The answer is that, on the one hand, there are emergent properties that arise simply from increasing brain size, and these properties may allow our brains to do things that other animals’ can’t (as far as we know). On the other hand, close study of other animals is showing that they’re actually a lot smarter than we often give them credit for.

Even though we’re seeing ways that humans are like other animals in both the brain and what the brain can do, we are nevertheless unusual in many ways that underscore the human condition, and we still need an explanation for how/why we evolved. Researchers have made many insights into these questions by comparing us with our closest living relatives: chimpanzees and bonobos. You wouldn’t guess it from looking, but these two apes share a more recent common ancestor with us humans than they do with other apes like gorillas and orangutans. This ancestor probably lived some time between 6–8 million years ago, and presumably its brain would have been lot like that of chimps/bonobos. How did we get from that to the big and powerful processor between our ears?

The fossil record provides the only direct evidence (itself rather indirect) of what past brains were like, in the form of endocasts: imprints that the brain makes on the inside of the skull bones during growth. The image I selected for my flash talk (above) shows the endocasts from five fossils of a species called Homo naledi that lived in South Africa around 300,000 years ago. You can see that some are fairly complete (like the one on the left), while others have really prominent impressions created by the outer surface of the brain (like the one second from the right side). All five of these endocasts leave something (or a lot) to be desired. The human fossil record so far provides us with literally hundreds of fossil endocasts at various states of completeness, providing small, blurry windows into brains in the human lineage over the past six million years. Although most are fragmentary, the phantoms of ancient minds, I think they tell a cool story about the human condition, and call for a truly integrative and anthropological approach to studying the brain.

What the fossil evidence seems to show, along with archaeological and modern behavioral research, is that the human brain evolved in order to cooperatively extract and allocate an immense amount of energy from the environment—and a flexibility to do this in almost any environment. Growing a big, human brain requires extensive time and calories; these in turn can only be afforded by cooperative foraging and parenting. So far as we can tell, humans surpass all other animals (or at least primates) in abstract thinking, considering the thoughts and mental states of others, and working with others toward shared goals. The brain as a body part and subject of study is great because we need to draw on diverse types of data and evidence to understand it, an alphabet of disciplines from Anatomy and Anthropology to Physics and Zoology.

Bonobos and the ultimate hidden truth of the world

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A recent study finds that bonobos are no less aggressive than chimpanzees, but levels of aggression also vary across different groups in either species.

Chimpanzees and bonobos are humans’ closest living relatives, their ancestors having diverged from ours around 6 million years ago, before our two evolutionary cousins started becoming separate species over a million years ago (Yoo et al., 2025). Chimpanzees have long been depicted as violent, with males especially “warlike” — see for instance the recent documentary Rise of the Warrior Apes (which is excellent, although it doesn’t pass the Bechdel Test). Early studies of bonobos, on the other hand, revealed different social dynamics: females dominate males, all individuals use sexual contact for establishing and maintaining social bonds, and individuals are generally less aggression compared to chimpanzees.

Because these two species are equally related to humans, they provided starkly different models for the possible ancestral conditions out of which humans evolved (Parish et al., 2000). People often look to the animal world to support their ideas of what constitues ‘human nature.’ If either chimpanzees or bonobos are selected as the proxy for our last shared ancestor millions of years ago, one could argue that humans are ‘naturally’ aggressive or peaceful.

However, reports of violent aggression among bonobos in the wild have emerged in recent years, including a likely case in which several females attacked (and probably killed) a male in their group (Pachevskaya et al., 2025). So, are chimpanzees and bonobos that different after all?

Yesterday, Emile Bryon & colleagues reported the results of a systematic comparison between chimpanzees and bonobos in terms of their aggression toward others. The researchers studied 22 groups of chimpanzees (9 groups) and bonobos (13 groups) living in zoos, examining who antagonized whom and whether this involved more of a threat or came to physical contact that could cause bodily harm. The comparison among apes living in captivity as opposed to the wild provides greater sample sizes and reduces some of the variance that could be attributed to habitat differences.

Bryon and team found that, as their article title succinctly reports, “chimpanzees are not more aggressive than bonobos but target sexes differently.” Their results support earlier observations from wild-living apes in that among chimpanzees males are extremely aggressive toward females, while among bonobos females direct more aggression toward males. Overall rates of aggression did not differ between the two species, however, and within each species different groups varied extensively; wild-living apes vary between groups in similar ways.

This variability between groups of the same species is really important. Some groups had more aggressive encounters than others, and both chimpanzees and bonobos had groups with lower rates than expected from the species-wide patterns. It is easy to overgeneralize and essentialize animals based on limited observations—in this case, that ‘bonobos make love while chimpanzees make war.’ In contrast, the study shows that there’s no one way to form a bonobo or chimpanzee society. Our closest living relatives are more flexible in their behavior than we tend to give them credit for.

The animal kingdom and especially our ape cousins are frequently invoked in order to naturalize or justify violence and inequality in the human world, and I think this study illustrates that we shouldn’t overgeneralize about other animals or ourselves. Many people including Donald Trump use naïve evolutionary reasoning to argue that ‘might makes right,’ that being a selfish asshole is a natural state molded by our evolutionary history. But as the late, great David Graeber (2015) has argued (admittedly, about bureaucracies), “The ultimate hidden truth of the world is that it is something that we make, and could just as easily make differently.” This study by Bryon and colleagues suggests that the ability to choose or avoid conflict and aggression may well be the ancestral condition out of which humanity evolved—that despite the violence and warmongering making headlines today, there’s nothing in nature that says it has to be this way.

References

Bryon, E. et al. (2026). Chimpanzees are not more aggressive than bonobos but target sexes differently. Science Advances 12, eadz2433. doi:10.1126/sciadv.adz2433

Graeber, D. (2015). The Utopia of Rules: On Technology, Stupidity, and the Secret Joys of Bureaucracy. Melville House.

Parish, A. et al., (2000), The Other “Closest Living Relative”: How Bonobos (Pan paniscus) Challenge Traditional Assumptions about Females, Dominance, Intra- and Intersexual Interactions, and Hominid Evolution. Annals of the New York Academy of Sciences, 907: 97-113. https://doi.org/10.1111/j.1749-6632.2000.tb06618.x

Pashchevskaya, S et al. (2025) Coalitionary intra-group aggression by wild female bonobos. Current Biology 35, Issue 19, R912 – R913. https://doi.org/10.1016/j.cub.2025.08.010

Yoo, D., Rhie, A., Hebbar, P. et al. (2025) Complete sequencing of ape genomes. Nature 641, 401–418. https://doi.org/10.1038/s41586-025-08816-3

#FossilFriday: Handy habilis’ formidable forearms

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Homo habilis just got some long arms to go along with its dexterous hands. In a recent paper in the journal The Anatomical Record, Fred Grine and colleagues describe and analyze some spectacular fossils recovered near the town of Ileret in Kenya, dating to just over 2 million years ago. There were a few different kinds of human-like species inhabiting the planet around this time, but researchers were able to assign these bones to Homo habilis thanks to some chemical clues connecting them to a nearly complete set of teeth found a few meters away. This partial skeleton of a young adult individual is an incredible discovery, connected by clever scientific sleuthing, and provides important information about an early member of the human lineage.

You can see some great photos of these fossils (as well as a fantastic fossil foot of a different individual) in a 2015 press release from the Turkana Basin Institute. A more recent announcement from the Institut Català Paleontologia includes a photo showing the late great Bill Jungers and fossil maven Meave Leakey with the fossils, which helps show the actual size of the bones.

Ann Gibbons’ article about the discovery has a great quote from paleoanthropologist Stephanie Melillo (who discovered the Burtele foot fossil): “If you dressed up a Homo habilis individual in clothes and you saw her walking in the distance, would you do a double take? This study shows us that the answer is YES!”

Still from a scent of the 1982 movie ET, showing the eponymous ET wearing a wig, dress, bowler hat, shawl, jewelry
Artist’s depiction of Homo habilis dressed up in clothes and you see her walking in the distance (image source)

The reason we might react to seeing Homo habilis like Gertie glimpsing E.T., as this skeleton shows, is that this early human had longer arms (especially forearms) than most of us do today. Thickness of the bones also shows that they were probably quite strong as a result of experiencing lots of force from use during life. Long and strong hominin arms are typically interpreted as evidence that these ancient ancestors spent a good deal of time climbing trees.

These features have previously been documented in some of the few other partial skeletons attributed to Homo habilis, as Grine and colleagues note. Indeed, the new article does a deep dive into what is known (and unknown) about the bones and body of Homo habilis, and it also provides a thoughtful review of recent research cautioning against over-interpreting climbing behaviors from fossil remains.

For more fossil fun, the article’s supporting online material includes “3D manipulative files” of the original specimens, so anyone can have a look at the bones in 3D using Microsoft Word:

Two-panels showing a Microsoft Word window (left panel) with a 3D model of a fossil, beneath which is written "SOM Figure 9. 3D manipulative file of shaft of right acetabulum"; and an internet browser screenshot (right panel) depicting the "Supporting Information" section from this website: https://anatomypubs.onlinelibrary.wiley.com/doi/full/10.1002/ar.70100

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.

Race is absolutely a human invention

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Trump and his administration are actively dismantling our economy and democratic institutions. One of the most recent parts of this assault is an executive order issued last week, “Restoring Truth and Sanity to American History.” The title itself is Orwellian doublespeak since the order describes the rejection of truth and science, while it in fact aims to whitewash American history.

An entire section of the order is devoted to, “Saving our Smithsonian” Institution, the national complex of museums, education outreach programs, research facilities, and a large Zoo. The order singles out an exhibit at the American Art Museum because it, “promotes the view that race is not a biological reality but a social construct … ‘a human invention.'”

As noted in the New York Times, the exhibit displays many quotations from the Statement on Race & Racism published by the American Association of Biological Anthropologists a few years ago. The statement explains, “Humans are not divided biologically into distinct continental types or racial genetic clusters,” which I think gets at the fundamental misconception most Americans have about race. Whether uninformed or outright racist and malicious, many people conceive of race as an invisible, unchanging essence that determines an individual’s capacities and behaviors. In the olden days race was thought of as ancestral and ‘in the blood,’ but in the genomic age people began attributing racial essence to DNA. All of these biological data—from blood to whole genomes—for at least the past fifty fucking years have shown that, again, “Humans are not divided biologically into distinct continental types or racial genetic clusters.”

Now, some folks who call themselves “race realists” (you can’t spell “race realist” without “racist”) might point to scientific research about human genetic variation with graphs showing humans partitioned into statistically-inferred clusters corresponding roughly with geography. The problem that tends to arise from here is the over-interpretation this within-species variation. As Lewontin showed back in 1972, and subsequent studies have consistently confirmed with more and more data, the amount of genetic variation that distinguishes different populations is but a small proportion (less than 15%) of the overall variation within our species. What’s more, because this variation is scattered throughout all of our DNA, most of it should be “neutral” with regard to evolution, with little or no effect on how likely an individual is to survive or reproduce. Simply put, humans across the planet are more genetically similar than different, and the limited genetic differences between populations probably doesn’t really influence how they behave or what they are capable of. Even though geneticists have argued this for years, many Americans are still quick to over-interpret the biological significance of these minuscule genetic differences, often tragically so.

To the contrary, race as many people think of it today is a recent historical concept – a “Fatal Invention” as Dr. Dorothy Roberts explains in her 2012 book. This is the consensus among experts in both the natural and social sciences. Yet Trump’s executive order specifically rejects this well established knowledge that race is in fact “a human invention,” as the order quotes from the Smithsonian art exhibit. This is one of the many purposes of rejecting the science and claiming that race is a biological reality — it serves to naturalize social differences and social inequality. If you maintain that people’s qualities are genetically determined and that groups differ fundamentally in their inherited genetics, then you have justification for avoiding social interventions to racial (and other kinds of social) inequality. As Dr. Michael Blakey explained back in 1999, “Race is essentially a means of defining ethnic and social status groups as biological entities. … In a racist or White Supremacist society, such as the United States, this … will often become the basis for decisions about the allocation of social resources and the solutions to social problems.”

Trump and Musk are both known to harbor unscientific and racist views about genetics, and both have been associated with far right and often white supremacist groups. The recent executive order claims that by communicating the actual science of human variation and the history of racism in this country, the Smithsonian is “under the influence of a divisive, race-centered ideology.” But with this administration, every accusation is a confession. They are actively dismantling our institutions and efforts that aim to address and repair the damage from centuries of racism, in order to advance their own white supremacist agenda (see for example here, here, and here).

How small apes grow big canine teeth

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Gibbons are sometimes referred to as “lesser apes” since they’re the smaller-bodied cousins of “great apes” like us humans, chimpanzees, gorillas, and orangutans. But what they lack in body mass they make up for in taxonomic diversity, with roughly 20 species distributed across four genus groups (Kim et al., 2011). And while male great apes (except humans) have large canine teeth, both sexes in gibbons have large maxillary canines — flashy weaponry for defending territory.

Pointy canine teeth peeking out from the upper and lower jaws of an adult female gibbon cared for at the International Primate Protection League (source)

My research has generally focused on brains and growth throughout human evolution, but I started looking at gibbons a few years ago when the COVID-19 pandemic put research travel on hold. Inspired by Julia Zichello’s 2018 article about gibbon models for understanding hominin evolution and appreciating that “overlooked small apes need more attention,” I had the opportunity to CT scan a unique skeletal collection of white-handed gibbons (Hylobates lar), which was sadly harvested from the forests of Thailand back in the late 1930s. Previous research on skull growth in gibbons has mostly used small samples compiled from different species (and sometimes even different genera). In contrast, this CT dataset includes many individuals at each stage of maturation from late infancy through adulthood, effectively representing a single population at a point in time. So with this larger cross-sectional sample of a single species, we can better understand how gibbon brains and faces grow. And because permanent teeth form in a long, continuous sequence throughout the growth period, an individual’s state of dental development can serve as a marker of where they are along the maturation process.

In a paper hot off the press, Julia Boughner and I analyzed dental development in this unique sample (article here). One of the coolest things we found was that gibbons’ large upper canine teeth are among the first to begin but last to finish tooth formation. In fact, the large canines growing inside relatively small faces may inhibit growth of one of the neighboring incisor teeth until the face has grown to create enough space for it. And while most teeth developing within the jaw begin emerging into the mouth once there’s enough room for them, gibbons’ gargantuan upper canines are forced out of hiding as they outgrow their bony crypts (check out the right-most jaw in the second row below).

Cross-sectional representation of tooth formation in white-handed gibbons, starting with the youngest in the top left and ending with the oldest in the bottom right. The first permanent tooth to form and emerge, M1, is highlighted along with the canine “C.”

In addition to characterizing ‘normal’ dental development, we also observed several developmental anomalies and pathologies in the sample. Our observations corroborate previous research showing that tooth formation generally proceeds ‘as scheduled’ despite various other disturbances to development.

It remains to be seen whether early development of the canine at the cost of delayed incisor formation is a pattern unique among all the apes, since most other studies of ape tooth formation have examined the lower jaw while our study focused on the upper jaws. But the canine-incisor tradeoff that we identified sets the stage for subsequent study of skull growth in this sample, as it highlights the many factors and functions that must be coordinated during growth.

While we have several projects planned with this unique dataset, we have also published the tooth formation data that we analyzed, and the original micro-CT scans themselves will be published to the online repository Morphosource.org soon, once a few more projects are finished.

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.

Krapina endocast update (open data & code)

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In the Summer of 2019 I worked with some great Vassar undergrads to make virtual endocasts and generate new brain size estimates for the Neandertals from the site of Krapina, which we then published in 2021 (discussed in this blog post). The virtual approach to endocast reconstruction uses 3D landmark-based geometric morphometrics methods, and so in the spirit of open science we also published all the landmark data used for the study (as well as a bunch of other fossil human brain size estimates) in the Zenodo repository (here).

Neandertal fossil specimens Krapina 3 (purple/green) and Krapina 6 (yellow/red) with preserved landmarks and virtually reconstructed endocasts.

Something major and global happened around that time — who can even remember what? — and so I never got around to posting R code to accompany the study. So, I’ve finally gotten around to adding some very basic code to the Zenodo entry (better late than never). The code simply reads in the landmarks, estimates missing data for fossils, and does some very basic shape analysis and visualization. It’s doesn’t get into all the nuts and bolts of our study, but it should be enough to help folks check our data or get started with shape analysis in R.

R code includes ways to visualize the landmark data. Left: Principal components analysis graph of endocast shape for humans (red) and Neandertals (blue). Right: Triangle meshes of the average human and Neandertal endocast shapes, viewed from the right, bottom, and back.

Original article
Cofran Z, Boone M, Petticord M. 2021. Virtually estimated endocranial volumes of the Krapina Neandertals. American Journal of Physical Anthropology 174: 117–128. (link)

What do brain endocasts tell us?

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What makes the human brain special, and how did it change throughout our evolutionary history? One way to answer this question by comparing actual brains or MRI scans of living animals. But only fossils can show what changed and when over the past several million years, and sadly brains are basically an elaborately congealed soup that doesn’t stay fresh upon death, so they never fossilize (well, almost never). Happily, though, bones can preserve for millions of years, and they are literally molded by their soft and squishy surroundings. As the brain grows, it pushes outward against the inner surface of the skull, which can save the scars of the submerged cerebrum: nerds like me call these impressions an “endocast.”

Endocasts of Homo naledi (pink) and Homo erectus (yellow). Fossils are viewed from the left side and are variably preserved.

Nicole Labra and Antoine Balzeau have led a cool study, hot off the press, examining what such endocasts can tell us about the underlying brain anatomy. Importantly, they show how difficult it is to clearly and consistently identify many brainy boundaries. This is very salient in “paleoneurology,” the study of brain evolution especially based off endocasts: the problem probably best illustrated by the nearly century-long debate about the natural endcoast of the “Taung child” fossil (Australopithecus africanus).

Labra & colleagues used a clever approach to address this paleontological and epistemological problem. They first generated an endocast directly associated with its brain from an MRI scan of a living human, allowing them see precisely where specific brain grooves (“sulci”) lay relative to the endocast surface. They then asked a bunch of researchers—myself included—to try to identify sulci on the endocast, and then looked at how our responses compared to both one another’s and to the actual, known sulcus positions.

Figure 1 from Labra et al. (in press) showing how the brain and endocast were obtained and analyzed.

Their analysis showed that we varied quite a bit in our identifications on the endocast. As Emiliano Bruner (who also participated) discusses in his blog post, we tended to identify the stronger impressions toward the bottom and sides of the endocast better and more consistently. Some of this variability and uncertainty among researchers is due to the faintness and incompleteness of many brain impressions, and some due to biased expectations about where a given sulcus “should” be based on our previous experiences and published references.

When Antoine Balzeau first contacted me about this project, I was just beginning to dabble in paleoneurology, learning some brain anatomy for the first time for a description of an old Australopithecus endocast called “MLD 3.” I initially thought MLD 3 would be a quick and simple study—boy was I spectacularly disappointed!

Figure 3 from Cofran et al. 2023, comparing two different chimpanzee brains, and two corresponding interpretations of the MLD 3 endocast.

Probably reflecting observer bias and desire for definitive results, we initially interpreted the endocast impressions on MLD 3 as representing a ‘human-like’ anatomy that is super rare in living chimpanzees (namely the “LS” depicted in the right half of the figure above). The researchers who peer-reviewed the first draft of our paper, though, suggested we be more cautious in our interpretations; one reviewer outright disagreed with us in support of a more ‘ape-like’ interpretation (left half of the figure above). The review process alone underscored the subjectivity and uncertainty in analyzing endocasts. In the end we presented both interpretations, and I honestly don’t know which (if either) is most likely to be correct. So the study by Labra and colleagues provides a nice empirical illustration of this cranial conundrum.

Fortunately, researchers are developing methods to help identify brain structures on endocasts. Amélie Beaudet, Jean Dumoncel, and Edwin de Jager among others have done some really impressive work looking at variability in both brains (for instance here) and endocasts (for instance here). By using computer-based 3D data and methods, these researchers have shown where many brain sulci tend to be located (see here). By developing a better understanding of variation in where sulci sit on an endocast, we can have a better idea of which sulci might be represented on fossil endocasts, which in turn can tell us about the brains of our extinct relatives. Edwin and Amélie presented a very cool new analysis of Australopithecus/Paranthropus boisei endocasts, building off this digital approach, at the recent ESHE conference. And as noted in our MLD 3 paper, I think machine learning and other ‘artificial intelligence’ approaches could also help us identify ambiguous features from frustrating fossil fragments.