That observation, made by Jake Brooker and colleagues at Durham University, became the seed of a study just published in Proceedings of the Royal Society B.1 The question underneath it is simple enough to state and surprisingly hard to test: does touching each other before a fight over food actually stop the fight from happening?

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That’s roughly the shape of the pattern a team led by Odd Jacobson at the Max Planck Institute of Animal Behavior documented across fifteen years and about 30,000 hours of observation1 of 31 wild Pongo pygmaeus wurmbii mother-offspring pairs. It’s a strange thing to find in this particular species, because orangutans are supposed to be the antisocial great ape. Adult females spend most of their lives alone with a single dependent offspring, born once every 7.6 years on average, the longest interbirth interval of any land mammal. They don’t groom each other. They don’t build alliances. When neighboring mothers do cross paths, the encounters are brief, infrequent, and by most measures costly rather than beneficial to the mothers themselves.

So why would an animal built around solitude go out of its way to undo it, over and over, for years?

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That first idea, the “brain lag” hypothesis, had a rough two decades. In 1999,1 Robert Deaner and Charles Nunn ran the definitive test using a dataset of paired primate species, comparing how the ratio between brain and body mass changed against how long ago each pair had diverged from a common ancestor. Their logic was tidy. If body size changes first and brain size trails behind, then recently diverged species pairs should show brains that are too small for their bodies, while anciently diverged pairs should have had enough evolutionary time to correct the imbalance. Plot the residuals against divergence date and you should see a rising line.

They found nothing. No correlation for males, none for females, none when controlling for circadian patterns. The conclusion seemed airtight: brain and body size evolve together, in lockstep, under the same pressures. The idea of a lag quietly left the literature.

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The textbook story goes like this: human childbirth is uniquely difficult because evolution made two conflicting demands on our pelvis. Walking upright required a more compact, bowl-shaped pelvis. Growing a large brain required fitting a large-headed baby through it. The result is the obstetrical dilemma, a term introduced by Sherwood Washburn in 1960 and since reproduced in virtually every introductory physical anthropology course. Human babies are born neurologically helpless, the argument runs, because they have to exit before their skulls get too large to pass. Other primates, by contrast, face little or no obstruction at birth. Their birth canals are spacious relative to neonatal head size. The comparison makes Homo sapiens look like a species that evolution painted into a corner.

A new study in Nature Ecology & Evolution1 runs those numbers again, with better methods and a far wider sample, and finds that the story doesn’t hold.

The team, led by Nicole Torres-Tamayo and Lia Betti at UCL, analyzed pelvic morphology and neonatal head dimensions across 29 primate species using three-dimensional geometric morphometrics. The critical methodological move was abandoning the human-calibrated measurements that have shaped comparative anatomy since Adolph Schultz published his influential diagram of cephalopelvic proportions in 1949. Schultz’s approach measured the pelvic inlet the same way across all species, anchoring the anterior-posterior diameter between the top of the pubic symphysis and the sacral promontory. That works reasonably well for Homo sapiens, where bipedal adaptations have pulled the constraining sacral margin toward the inlet rim. In other primates, the sacrum sits higher relative to the pubis, meaning the actual narrowest point of the canal is lower down. Using the human landmark systematically overestimates available space in non-human species. The birth canal of a chimpanzee looks more capacious than it is. The same goes for monkeys.

The head measurement problem is equally significant. Schultz measured neonatal skulls in sinciput presentation, which means the forehead-to-occiput length forms one axis of the ellipse. Most primates don’t enter the birth canal that way. Geladas, baboons, squirrel monkeys, and macaques typically deliver face-first, with the neck extended, which presents a smaller cranial profile to the inlet. Homo sapiens delivers in occiput-anterior presentation, which similarly minimizes the dimensions that actually matter. When Torres-Tamayo and colleagues replaced sinciput measurements with species-appropriate face and occiput presentations, cephalopelvic mismatch decreased substantially across the board. But the correction fell disproportionately on species where the sinciput measurement was most unrealistic, making the old diagram’s claim of human exceptionalism look even more like an artifact of flawed calibration.

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A study published this month in Proceedings of the Royal Society B1 tries to answer exactly that question, and the answer is stranger than expected.

Eugene Melamud and colleagues at Calico Life Sciences, CleMetric, and the University of Wisconsin-Madison compiled lifespan records for 39 captive primate species from the Primate Aging Database, a decades-long accumulation of demographic data from zoos, research centers, and conservation sites across the United States and Europe. The dataset covers more than 12,000 individual animals, from lesser bushbabies to western lowland gorillas, with enough deaths recorded across most species to fit meaningful mortality models.

The framework they used goes back to 1825. Benjamin Gompertz, working from English mortality tables, noticed that the probability of dying doesn’t increase linearly with age — it accelerates exponentially. Starting around the time of sexual maturity, your risk of dying roughly doubles every several years. Gompertz captured this with a two-parameter model. One parameter, called the baseline hazard, describes how likely you are to die right at the onset of adulthood. The other, the aging rate, describes how quickly that probability climbs as you get older.

These two parameters are mathematically distinct and, it turns out, evolutionarily distinct as well.

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That is not a coincidence or a superficial resemblance. According to a new study in Communications Biology,1 the rhythmic structure underlying laughter has been present and conserved across all five lineages of extant great apes, including Homo sapiens, for at least 15 million years. The paper, led by Chiara De Gregorio, Marina Davila-Ross, and Adriano Lameira, is the first to conduct a comparative rhythm analysis of laughter across the complete set of living hominids: Pongo pygmaeus, Gorilla gorilla, Pan paniscus, Pan troglodytes, and Homo sapiens.

What they found is that every species laughs isochronously. The intervals between successive laugh bursts fall into a regular, evenly timed pattern, a 1:1 ratio between consecutive intervals. The “ha-ha-ha-ha” is not just phonologically similar across species. The beat is, at its core, the same.

An adolescent gorilla being tickled on his foot. The gorilla responds with laughter, pulls his foot away, but then presents his foot in order to be tickled some more. Credit: Dr. Marina Davila Ross

What Isochrony Reveals

Isochrony is a specific claim about timing. It means the intervals between sound onsets are metrically regular, not just roughly similar. Finding it in laughter across all five hominid lineages implies it was already present in the common ancestor from which orangutans diverged roughly 15 million years ago. The trait did not appear in chimps and then spread to humans, or emerge independently in multiple lineages. It was there from the beginning.

The team recorded 140 laughter bouts from 17 individuals across the five species, all young animals observed during playful interactions with familiar humans, in both tickling and spontaneous social play contexts. From these recordings, they extracted inter-onset intervals and computed what are called rhythmic ratios: the relative duration of one interval compared to the next. A ratio clustering around 0.5 indicates isochrony. Across all species, that’s what they found.

Bonobo laugh. Credit: Dr. Marina Davila Ross

Chimpanzee laugh. Credit: Dr. Marina Davila Ross

Gorilla Laugh. Credit: Dr. Marina Davila Ross

Orangutan Laugh. Credit: Dr. Marina Davila Ross

But the isochrony was not uniform across contexts. During tickling, laughter was highly regular. During social play, it deviated significantly. This is not surprising once you think about the mechanics: rough-and-tumble play involves torsion, compression, and impact on the thorax, which disrupts the respiratory cycle and with it the regular timing of vocalizations. Tickling laughter, produced without that physical interference, gave the researchers a cleaner window into the underlying rhythmic structure. It also gave them a methodological insight. If you want to study the evolution of the phonatory-respiratory system, tickling is the more stable condition.

The acceleration of laughter tempo across hominid evolution was also more clearly visible in tickling laughter than in play laughter. When the team modeled laughter tempo as a function of phylogenetic distance from Homo sapiens, the trend was statistically significant for tickling but not for play. The noise introduced by physical activity in social play effectively masked the evolutionary signal.

The Human Departure

Here is where humans become genuinely distinct, though not in the way that discontinuity narratives usually frame it.

Laughter has gotten faster across hominid evolution. It has also become more variable in its timing, with Homo sapiensshowing the highest coefficient of variation in inter-onset intervals of any species in the study. That variability is not random sloppiness. Research on human laughter perception suggests that variable timing is perceived as more socially and emotionally positive than rigid, stereotyped timing. A laugh that shifts and accelerates and varies across its duration reads differently from one that ticks along like a metronome. Humans have developed, it seems, not just faster laughter but more expressive laughter in the purely temporal sense.

More telling still is context sensitivity. Of all five species, only humans modulated laughter tempo according to context. When tickled, children in the study laughed faster than during social play. In the non-human apes, no such modulation was detected. This matters because it suggests an active, flexible vocal control that can respond to the social and emotional demands of a situation, rather than simply outputting a fixed motor pattern. The laughter you produce during tickling, where the rhythm is fast and regular, differs measurably from laughter in social play, and humans alone show that modulation.

This connects to a broader argument the study makes carefully. Human speech requires extraordinary fine-grained control over the timing and sequencing of vocalizations. That kind of control does not appear from nowhere. What laughter’s evolutionary record suggests is a long, gradual accumulation of rhythmic flexibility across the hominid lineage. Orangutans have isochronous laughter but little context sensitivity and relatively slow, consistent tempo. Moving along the phylogenetic tree toward the human branch, the rhythm becomes faster, more variable, and more responsive to context. Humans are at the far end of that trajectory, not standing apart from it.

Lameira and colleagues have been making versions of this argument for some time in relation to other orangutan vocalizations, including recursive vocal motifs and evidence of metered rhythm in Pongo song. What this study adds is comparative depth: every living hominid lineage, analyzed together for the first time, showing a coherent directional pattern across 15 million years of vocal evolution.

The study is appropriately cautious about sample sizes. Between two and four individuals per non-human species is a real limitation, and the authors acknowledge that future work with larger samples will refine species-level estimates of variability. The broader phylogenetic pattern is robust enough to survive that caveat, but species-by-species claims should be read with that constraint in mind.

What the data do not support is the older picture, still implicitly present in some accounts of language evolution, of a dramatic threshold crossing. Under that view, the crucial changes in vocal control were essentially a Homo sapiensdevelopment, qualitatively different from anything that came before. Laughter’s rhythm suggests something less abrupt: a long, incremental refinement of the same capacities that every great ape already possessed in some degree, with the human branch extending a trend rather than initiating one.

Sound does not fossilize. You cannot dig up a vocal tract from two million years ago and play it back. What you can do is listen to what all living hominids share, and trace the variations in what they do differently. The rhythm in an orangutan’s laughter during a tickling session is carrying information about a common ancestor that most people have never thought about. It is not a quaint analog of human behavior. It is a data point from 15 million years ago, still audible.

Further Reading

• Davila-Ross, M. J., Owren, M., & Zimmermann, E. Reconstructing the evolution of laughter in great apes and humans. Current Biology 19, 1106–1111 (2009). https://doi.org/10.1016/j.cub.2009.05.028

• De Gregorio, C., & Lameira, A. R. Twice times two: dual mechanism for double rhythmic meter in orangutans and the evolution of human song. iScience 29, 114273 (2026). https://doi.org/10.1016/j.isci.2026.114273

This is the adversarial context in which chimpanzees across Africa have developed what researchers call ant-dipping: the use of stick tools to harvest army ants, load them onto the stick, and pull them off with the free hand before eating. The technique varies across populations. Some communities use the pull-through method, sliding the loaded tool through a cupped hand. Others bring the stick directly to the mouth. Some use long tools; some use short

ones. Some communities in forest sites use a digging stick to widen the nest entrance before inserting the dipping tool, turning it into a two-stage tool set. A few populations, notably the Pan troglodytes of Taï Forest in Côte d’Ivoire, don’t use tools at all for certain ant species, simply grabbing brood by hand.

What’s driving all this variation? Ecology, culture, or some combination? That question is harder to answer than it sounds, because almost all the detailed work on ant-dipping has been done in forest habitats. Savanna populations have been underexamined. And chimpanzees are the only non-human ape that lives in open, hot, seasonally dry savannas at all — the kind of landscape that early hominins also occupied. If you want to understand what tool-using behavior looks like under those ecological pressures, savanna chimps are the only comparison available.

A study published in Scientific Reports1 in 2026, led by Andreu Sánchez-Megías and R. Adriana Hernandez-Aguilar at the University of Barcelona and the Jane Goodall Institute Spain, provides the first detailed account of ant-dipping in a savanna population. The site is Dindefelo, a community nature reserve in southeastern Senegal, covering about 140 square kilometers along the Guinea border. The climate is strongly seasonal, dry for roughly six months, with a mean annual temperature around 28.5°C. Evergreen vegetation — the moist, sheltered habitat army ants require — makes up less than twelve and a half percent of the landscape. Ants are scarce there. One of the lowest densities reported for the genus anywhere on the continent.

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There is a groove in the fossil record. A small, tapered notch, worn into the root surface of a tooth between two neighboring teeth. Researchers have been finding these marks in ancient human dentitions for over a century, and for most of that time the prevailing explanation has been a tidy story: our ancestors were picking their teeth. Sticks, fibers, bone fragments, whatever was at hand. One paper a couple of decades ago called it the oldest hominid habit. The grooves show up in specimens more than two million years old, in Neanderthal dentitions, in medieval European samples. The narrative held.

A new study published in the American Journal of Biological Anthropology1 hasn’t destroyed that narrative, but it has made it considerably harder to defend. What Towle, Krueger, and colleagues found, after examining 531 individuals from 27 wild primate species, is that these grooves are not unique to Homo. Wild apes and monkeys can develop them too. That’s the complication. The more striking finding, though, is the absence: not a single primate in the sample showed abfraction lesions, those deep, wedge-shaped notches at the gumline that are among the most common dental problems treated in modern clinics. Whatever is causing them, it appears to be something specifically wrong with how we live now.

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The setup is elegant in its simplicity. Each individual has an incentive to extract as much as possible. But if everyone does that at once, the resource collapses and everyone loses. Garrett Hardin’s famous 1968 formulation framed this as a structural inevitability, though researchers since Elinor Ostrom have documented in detail how humans develop norms, rules, and communication strategies to sidestep it. What no one had tested, until now, was how chimpanzees perform at this problem when grouped in more than pairs.

The answer, published in Communications Psychology1 by Kirsten Sutherland, Daniel Haun, and Alejandro Sánchez-Amaro, is surprising in two respects. First: chimpanzee quartets significantly outperformed dyads at keeping the resource open. Second: the mechanism behind that success was not what the researchers expected.

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That’s strange. Gorilla and Homo sapiens diverged from a common ancestor somewhere between eight and ten million years ago, and in that time we have done radically different things with our hands. Gorillas press the dorsal surfaces of their middle finger bones into the ground and walk on them. Humans flake stone, tie knots, and suture wounds. The behavioral gap is enormous. The carpal morphology, in those two bones at least, is not.

A new study published in Proceedings of the Royal Society B1 takes this observation and scales it up. Led by Laura Hunter at the University of Chicago, with co-authors Matthew Tocheri, Caley Orr, Biren Patel, and Zeresenay Alemseged, the analysis covers more than two thousand specimens from living primates and 55 fossil hominin carpals, using a mathematical approach called spherical harmonics to capture the three-dimensional geometry of each bone in full. Three machine learning methods then classified fossil wrist bones against the extant comparative sample. The result is the most comprehensive quantitative treatment of carpal morphology across anthropoids yet attempted, and it produces a finding that cuts against a popular alternative to the knuckle-walking ancestor hypothesis: the modern human wrist didn’t escape its African ape past. It grew out of it.

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The research, published in Biology Letters1 by Cyril Grueter, Xiaoguang Qi, and Stefan Lüpold, examined sexual size dimorphism across up to 146 primate species and found that the factor most strongly associated with how different males and females are in body mass is not the mating system. It’s something more spatial: how much a group’s home range overlaps with the ranges of its neighbors.

That’s a meaningful distinction. Mating system, the typical proxy researchers use to infer the strength of sexual selection, describes the competitive dynamics happening inside a group. A polygynous species, where one or a few males monopolize multiple females, is presumed to have intense male-male competition and therefore strong selection for large male bodies. Monogamy, by contrast, implies less competition, less dimorphism. The logic is appealing. The data, once phylogenetic relationships are properly accounted for, don’t actually support it.

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That single fact reveals the central problem in great ape cognition research. These are the animals closest to us in evolutionary time. Pan troglodytes and Pan paniscus diverged from our lineage roughly six million years ago. But they are rare, protected, and impossible to study at the population scales that psychology and anthropology usually demand. A typical cognitive experiment might involve six individuals. Results get published. Then another team runs another study at a different institution, with a slightly different method, and finds something that contradicts the first finding, or qualifies it, or fails to replicate it. Over decades, the literature accumulates without quite cohering.

The EVApeCognition Dataset, published this spring in Scientific Data,1 is an attempt to pull that accumulated evidence together. A team led by Alejandro Sánchez-Amaro at the University of Stirling and Daniel Haun at the Max Planck Institute for Evolutionary Anthropology spent years contacting researchers, recovering raw data from archived server folders and old correspondence, standardizing everything into a common format, and releasing it publicly. The result is 262 experimental datasets drawn from 150 publications, spanning 18 years of research at the Wolfgang Köhler Primate Research Center in Leipzig.

The cognitive domains covered are wide. Studies on theory of mind, whether apes can represent what another individual knows or believes, sit alongside experiments on causal reasoning, long-term memory, cooperation, metacognition, tool use, and spatial representation. Social cognition and physical cognition each account for roughly half the dataset. Within social cognition, the largest single category is competition and cooperation, followed by theory of mind and metacognition. Within physical cognition, tools and causality predominate.

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That specificity is what a new study1 from the University of Portsmouth set out to document. The question wasn’t simply whether great apes mirror each other’s facial expressions during play. That much was already known. The question was whether they do it exactly — matching the specific variant their partner used, the way a human spontaneously mirrors a Duchenne smile.

The answer, from a sample of 96 apes across two species and eight social groups, appears to be yes.

Diane Austry, lead author and now based at Durham University, along with a team including Marina Davila-Ross and Guillaume Dezecache, studied 39 Pongo pygmaeus at the Sepilok Orangutan Rehabilitation Centre in Malaysia and 57 Pan troglodytes at the Chimfunshi Wildlife Orphanage in Zambia. The animals were video-recorded during spontaneous social play, and their laugh faces — technically, open-mouth faces, or OMFs — were coded frame by frame at 25 frames per second. Each open-mouth expression was classified as either showing the upper teeth or not. Then the researchers looked at whether the observing animal matched the specific variant its partner had just produced, within three seconds of that production.

That three-second window matters. It spans both the rapid automatic response seen in less than a second, which is measured in facial mimicry research more broadly, and the slower delayed response. The average latency in this study was just over a second for both species: around 1.1 seconds mean, with more than half of all responses landing under one second.

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Two clusters of Pan troglodytes — chimpanzees who had shared the same territory, groomed each other, hunted together, and fathered offspring with the same females — approached one another near the center of their home range. They had done this countless times before. Fission-fusion sociality is the default mode for chimpanzees; groups split and reform constantly across the day. What should have happened was a reunion. Instead, the Western chimpanzees ran. The Central chimpanzees chased them. Then for six weeks, the two groups avoided each other entirely.

That had never been observed before in 20 years of continuous monitoring at Ngogo.

What followed over the next nine years is documented in a paper published this week in Science1 by Aaron Sandel, John Mitani, Kevin Langergraber, David Watts, and colleagues: the largest-ever recorded chimpanzee community fracturing permanently into two rival groups, followed by sustained lethal violence in which the smaller group killed at least seven adult males and 17 infants from the larger one. The team drew on 30 years of demographic records, 24 years of social network data, and a decade of GPS tracking. It is, by any measure, an extraordinary longitudinal dataset.

The Ngogo chimpanzees had been a single community since continuous research began in 1995. By the time the dataset opens, they numbered well over 100 individuals and would eventually peak near 200 — far larger than typical chimpanzee communities. Social network analyses using the Leiden algorithm revealed two loosely defined clusters, Western and Central, throughout this period, but cluster membership was fluid. About 29% of individuals switched clusters from one year to the next. Forty-four percent of infants born between 2004 and 2014 were conceived by parents from different clusters. The whole group shared a single territory and a single reproductive pool. They were, by every meaningful measure, one community.

Then the network shifted.

Three independent statistical methods all flagged 2015 as the sharpest structural transition in the 24-year record. Modularity spiked. The two clusters stopped bleeding into each other. A change-point analysis for longitudinal network data identified the transition as statistically unambiguous. What had been a fluid, overlapping social structure crystallized into two distinct blocs. By 2017, the Western and Central chimpanzees occupied largely separate ranges. By 2018, there were no affiliative relationships between members of the two groups, and reproduction between them had ceased entirely. The last cross-group conception had occurred in March 2015 — almost exactly when the behavioral break first appeared.

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Masripithecus moghraensis was recovered from Wadi Moghra, a fossil site in northern Egypt, during fieldwork in 2023 and 2024 by the Sallam Lab team from Mansoura University. The animal lived roughly 17 to 18 million years ago, in the Early Miocene. What the description in Science1 makes clear is that this is the first unambiguous fossil ape from North Africa. That gap in the record had gone mostly unremarked, because nobody had a good way to explain it. Now the explanation looks obvious in retrospect: nobody was looking in the right place.

What the teeth say

The jaw preserves enough to be diagnostic. The canines and premolars are notably large. The molars have rounded, heavily textured chewing surfaces. The whole structure is robust. Working from this anatomy, the research team interprets Masripithecus as a dietary generalist: primarily frugivorous, but built to crack harder foods when fruit was scarce.

Masripithecus reconstruction video. Credit: Professor Hesham Sallam

That dietary profile is not unusual for an ape. What is unusual is the geography and the timing. The Early Miocene was a period of climatic flux. Northern Africa and Arabia were becoming more seasonal. For an ape to have persisted this far north, flexibility would have been a genuine advantage. The anatomy of Masripithecus looks like exactly that kind of solution.

Masripithecus sculpting video, sculpting by Mohammed Hebeish. Credit: Professor Hesham Sallam

The genus name combines Masr, the Arabic word for Egypt, with the Greek píthēkos, meaning ape. The species name honors Wadi Moghra itself. Transparent and practical.

A missing piece hiding in plain sight

Early Miocene ape fossils have historically clustered in East Africa. Sites in Kenya, Uganda, and Tanzania have been the primary source of hominoid material for decades, and the field’s picture of where early ape evolution took place has been shaped accordingly. North African sites of comparable age had turned up cercopithecoid monkeys, but no apes. The prevailing interpretation was that apes simply hadn’t gotten there yet.

Masripithecus complicates that story considerably.

The team placed the new species in phylogenetic context using a Bayesian tip-dating analysis that combined morphological data from living and extinct apes, molecular data from living species, and the geological ages of fossils. The result positions Masripithecus closer to crown Hominoidea than any of the coeval East African fossil apes. Crown Hominoidea is the group containing all living apes and their last common ancestor: gibbons, orangutans, gorillas, chimpanzees, and humans.

That finding has direct implications for where crown Hominoidea originated. Erik Seiffert, a paleontologist at the University of Southern California and co-author of the study, put it plainly: “For my entire career, I considered it probable that the common ancestor of all living apes lived in or around East Africa. But this new discovery, and our new and novel analyses of hominoid phylogeny and biogeography, now strongly challenge that idea.” He has also noted that the strength of the biogeographic scenario does not actually depend on Masripithecus alone, but that the new fossil is very much consistent with it.

The biogeographic analyses point toward northeastern Afro-Arabia as the most probable ancestral homeland. During the Early Miocene, the African and Arabian plates were in their final phase of collision with Asia. Fluctuating sea levels periodically opened land connections across what is now the eastern Mediterranean and Middle East. This region functioned as a corridor. Animals could move through it when conditions allowed.

Masripithecus, positioned where it is phylogenetically, represents a link between the African fossil record and the Eurasian one. Hominoid fossils appear in Europe and Asia from the Middle Miocene onward, and the question of where those animals came from has generated sustained debate. The Egyptian ape does not resolve that debate entirely, but it fills a gap that had been invisible. The corridor was not empty. It had apes in it.

The problem with only looking where the fossils are

The deeper issue this raises is methodological. Paleontology works with what it has, and for decades what it had was concentrated in East Africa. That is partly because East Africa is genuinely rich in fossil hominoids, and partly because sustained fieldwork there has built the infrastructure, expertise, and permit histories that attract more fieldwork. North Africa, by contrast, was less systematically explored for Miocene hominoids. Absence was treated as data when it was likely a sampling artifact.

Sallam’s team spent five years looking for this material. The payoff is not just a new species. It is a reorientation of the question itself. If the common ancestor of living apes lived in northeastern Afro-Arabia, and if that region has been chronically underexplored, then the actual shape of early hominoid diversification may look substantially different from what the current fossil record suggests.

There is more to find. The Wadi Moghra jaw is a reminder that a blank space on a distribution map is not the same thing as an empty landscape.

What followed was not a clean scientific resolution. It was a war.

The term “ape culture wars” was coined by primatologist William McGrew in 2003, but the conflict it names has older roots. In its early form, the question was binary: do non-human great apes have culture or not? Field researchers — biologists and primatologists who spent their careers watching chimpanzees in Gombe, Taï, Mahale, and Bossou — tended to say yes. Psychologists running experiments with captive apes in laboratories tended to be more skeptical, or at least more demanding in what they’d accept as evidence. Two research communities, two methodological traditions, and a fundamental disagreement about what counted as proof.

That version of the war is largely over. The evidence for cross-population behavioral differences in Pan troglodytes — differences that can’t be explained by genetics or ecology alone — has accumulated to the point where most researchers across both camps now accept that great apes have cultures of some kind. The same has been documented in Pongospecies, in bonobos, and to a lesser extent in gorillas. Indirect evidence for social learning, the mechanism typically taken as culture’s precondition, is also documented in capuchins, macaques, cetaceans, birds, and apparently even bumblebees.

The question “do apes have culture” has been replaced by a thornier set of questions: what kind of culture, transmitted through which mechanisms, producing which kinds of products? This is where the current war is being fought, and in a 2026 paper in Evolutionary Human Sciences,1 primatologists Ammie Kalan and Claudio Tennie attempt something unusual — a diagnosis from both sides simultaneously. Kalan works primarily with wild apes; Tennie primarily with captive ones. Their paper is part historiography, part peace treaty, and part metacritique of a field they argue has allowed genuine scientific divisions to calcify into something more tribal.

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But biology is rarely that tidy. New research from a team at the University of Geneva suggests that this neural real estate is less of a private club and more of an ancestral inheritance. When we hear the hoot of a chimpanzee, our temporal voice areas react with a selective intensity that was previously thought to be reserved only for Homo sapiens.

This isn’t just a general reaction to noise. The researchers1 used fMRI to watch the brains of 23 participants as they listened to a variety of primate calls: humans, rhesus macaques, bonobos, and chimpanzees. Even after the team mathematically stripped away the basic acoustic differences like pitch and loudness, a specific subregion in the anterior superior temporal gyrus remained stubbornly active for chimpanzee vocalizations.

It appears that our “human” voice areas are actually tuned to a broader frequency range that includes the sounds of our closest evolutionary neighbors. Interestingly, this neural kinship did not extend to bonobos. Despite being just as closely related to us as chimpanzees, bonobo calls failed to trigger the same specialized response in the human TVA.

The Acoustic Gap

The reason for this distinction likely lies in the anatomy of the throat. Bonobos have a shorter larynx, which gives their vocalizations a much higher fundamental frequency than those of humans or chimpanzees. Chimpanzee voices, by contrast, sit much closer to our own acoustic “sweet spot”.

The researchers hypothesized that the human auditory cortex evolved a tonotopic map tailored to the specific frequencies of human speech. Because chimpanzee calls fall within or near that range, our brains process them using the same specialized machinery we use for our parents, friends, and strangers on the street. Bonobos, through a process of evolutionary neoteny—the retention of juvenile traits into adulthood—seem to have drifted away from that shared acoustic space. Their calls are simply too high-pitched for our voice-processing hardware to recognize them as “voice-like”.

This finding challenges the idea of human “uniqueness” in brain organization. If our voice-sensitive areas are also chimpanzee-sensitive, it suggests that these regions didn’t appear out of nowhere to support human language. Instead, they likely represent an ancient system for social monitoring that we share with the other great apes. We didn’t build a new room for the human voice; we just redecorated an existing one.

Evolutionary Echoes

This leaves us with a compelling question about the nature of our own perception. When we hear a chimpanzee, are we hearing an “animal,” or is our brain instinctively categorizing that sound as a kind of distorted, familiar “other”?

The data showed that while the human voice still generates the most widespread activity across the temporal cortex, the anterior subregions of the TVA are remarkably sensitive to these cross-species similarities. It is as if the back of the brain handles the complex nuances of human identity and emotion, while the front remains vigilant for the basic, structural signatures of a primate throat.

We often think of our brains as being uniquely adapted to the modern world, but studies like this remind us that we carry deep, prehistoric echoes in our anatomy. Our “human” brain is still, in many ways, a primate brain, reaching out across six million years of separation to recognize the sound of a cousin.

A team led by Christopher M. Smith and Jeffrey T. Laitman recently analyzed1 the inner ears of 14 primate species, including the agile gibbon and the upright Homo sapiens. They used 3D imaging to map the delicate landmarks of the bony labyrinth, the hard shell that houses our sensory organs. Their goal was to see if the balance half of the ear is actually a single unit or if it is made of independent parts.

The team found that the vestibular system is not a monolith. It is modular.

A Third Room in the Labyrinth

In the traditional view, the semicircular canals and the otolithic organs are grouped together as one peripheral vestibular system. The canals are the gyroscopes that detect rotation, while the otolithic organs, the utricle and saccule, detect linear movement and the constant pull of gravity. We have often assumed that these parts evolve in lockstep. If an animal develops larger canals to handle acrobatic movement, we assumed the rest of the vestibular system grew with them.

Smith and his colleagues found that this integration is a myth. Their statistical analysis of species like Pan troglodytesand Gorilla gorilla showed that the canals and the otoliths are semi-independent modules. They follow different evolutionary trajectories and respond to different selective pressures. The researchers propose a tripartite model instead: a cochlear system for hearing, a canalicular system for rotation, and an otolithic system for gravity.

This shift in perspective is more than an academic reclassification. It reveals that our balance system has a hidden flexibility. The different parts of the ear are evolvable on their own. This modularity allows a species to fine-tune its sense of gravity without necessarily changing how it senses a head turn.

The Independent Evolution of Gravity

The evidence for this decoupling is clearest when looking at our own lineage. Across the great apes, the otolithic system has undergone a large-scale increase in relative size. In the past, researchers noticed that catarrhine primates had enlarged vestibules, but they often dismissed this as a byproduct of semicircular canal enlargement.

This study suggests the opposite. The expansion of the vestibule in hominoids is an independent evolutionary event. It may be linked to the complex ways apes move through three-dimensional space, where sensing the orientation of the body relative to gravity is just as critical as sensing the speed of a swing.

There is also a deep history here that reaches back to the water. The otolithic organs in mammals still carry a shadow of their original function in fish as vibration detectors. Modern mammals can actually hear certain bone-conducted or airborne sounds through their otoliths. By treating these organs as a distinct system, we begin to see the ear not just as a balance sensor, but as an ancient, multi-purpose tool for navigating a physical world.

This tripartite model changes how we read fossils. When we find the petrous bone of an ancient primate, we can no longer look at one part of the vestibular system and assume it tells the whole story of how that animal moved. The canals might tell us about its agility, but the otoliths tell a separate story about its relationship with gravity. We are looking at two different evolutionary clocks ticking inside the same small bone.

A new comparative analysis published in Evolution Letters1 challenges that framing at its root. Working across 100 primate species including Homo sapiens, researchers from the University of Lincoln found that how frequently a species engages in everyday, non-lethal aggression tells you remarkably little about whether it kills conspecifics. Mild aggression and lethal violence appear to have followed different evolutionary trajectories. They are not the same trait in different intensities. They are, in a meaningful phylogenetic sense, different things.

The team, led by Bonaventura Majolo and Samantha Wakes, assembled data on five distinct types of aggression: between-group mild aggression, within-group mild aggression, between-group adulticide, within-group adulticide, and infanticide. The data covered wild, non-provisioned, group-living primates only — captive populations excluded — and drew on published literature spanning 1950 to mid-2022, supplemented by unpublished data gathered through a survey of primatologists. Thirty-nine researchers contributed data on 28 species. The final dataset represents 43 genera across 11 primate families.

The analytical framework was Bayesian phylogenetic modeling, which allowed the team to test the strength of evolutionary correlations between each pair of aggression types while controlling for shared ancestry and five socio-ecological variables: sexual dimorphism, within-group sex ratio, group size, dietary folivory, and degree of territoriality.

What phylogeny explains, and what it doesn’t

The first notable result is structural. Phylogenetic relatedness explains more variance in lethal aggression than in mild aggression. Infanticide shows the strongest phylogenetic signal among the five types; within-group mild aggression shows the weakest. This means the distribution of lethal behaviors across primate lineages maps more predictably onto evolutionary history than the distribution of everyday squabbling does. Species that kill tend to come from lineages where killing has occurred before. Species that bicker don’t show the same taxonomic clustering.

When the team looked at correlations between aggression types, a clear pattern emerged. The three lethal types — between-group adulticide, within-group adulticide, and infanticide — were positively and moderately correlated with one another. Species where adults kill members of rival groups are also more likely to kill members of their own group, and more likely to engage in infanticide. The correlation between within- and between-group adulticide was particularly robust, with around 97–98% of the posterior distribution pointing in the same direction across both the null and full models.

Mild aggression told a different story. Neither type of mild aggression showed a meaningful evolutionary relationship with any of the lethal forms. The credible intervals for those pairwise correlations straddled zero. In the full model controlling for socio-ecological variables, the relationship between between-group mild aggression and infanticide, for instance, dropped to near nothing. The team ran five different versions of the dataset — varying how mild aggression was calculated — and obtained consistent results throughout.

The sex-specific analyses added another layer. When lethal aggression was split by the sex of the attacker, the evolutionary correlations among male lethal behaviors were strong: male between-group adulticide, male within-group adulticide, and male infanticide were tightly linked. In females, the picture was murkier. Female lethal aggression types were weakly correlated in the null model, with only between- and within-group female adulticide showing a stronger relationship when socio-ecological controls were added. Male infanticide had the highest phylogenetic signal of any category tested, with phylogeny accounting for roughly 59% of variance.

What escalation models get wrong

Animal contest theory generally predicts that mild aggression can escalate to lethal violence under the right conditions: high-value resources, closely matched opponents, or situations where the cost of continued conflict falls below the cost of backing down. This is a reasonable prediction. But it implies that the two types of aggression share an underlying logic, that mild conflict and lethal conflict sit on the same decision tree.

The Lincoln study’s findings suggest the decision tree model may be misleading, at least at the evolutionary level. Primates that engage in frequent mild aggression are not simply primates running a higher baseline risk of eventual killing. The socio-ecological conditions that would need to align for escalation to happen are, in mildly aggressive species, rarely present. Alternatively, those species may have developed conflict management behaviors — reconciliation, third-party intervention, social constraints — that function as effective circuit breakers. The result is evolutionary decoupling: lineages that argue a lot did not produce lineages that kill a lot.

This has practical implications for how socio-ecological models are built. Classical frameworks for understanding primate social structure incorporate predictions about competitive intensity but say little about when, or whether, that competition tips into lethality. The new findings suggest those transitions operate under different logic than the mild aggression preceding them.

The data on infanticide versus adulticide also bear consideration. Across the full dataset, infanticide was present in roughly 65% of species, adulticide in about 20%. The gap is large. Part of the explanation is methodological: infanticide has been studied more systematically in primates, so the literature is more likely to capture its presence. But risk asymmetry also matters. Killing an infant carries a much lower cost to the attacker than killing a conspecific adult, where both parties face genuine injury risk. Adulticide, when it occurs, tends to require numerical advantage, often attacking with multiple individuals against a single target. The team notes that planning-intensive adulticide, involving coordination, advance positioning, and target selection, may require levels of proactive aggression that are rare outside Pan and Homo sapiens. For most non-human primates, lethal attacks on adults are more likely stochastic: an isolated individual caught in the wrong encounter at the wrong time.

There is a broader implication here for how scholars interpret violence statistics, including the kind of cross-cultural historical comparisons that underpin arguments about whether modernity has made humans less violent. Treating adulticide, infanticide, and inter-group killing as a single undifferentiated measure of violence may obscure more than it reveals. The three types are correlated, but moderately, and the correlation is weaker in females than in males. Rolling them together into one “violence score” risks producing conclusions that reflect the amalgamation rather than any underlying biological reality.

The study cannot address rates, only presence or absence of lethal aggression. Whether a species kills once across decades of observation, or kills regularly, might matter enormously for understanding adaptive value and evolutionary trajectory. That data simply does not exist for most species at the scale required. The number of primate species with continuous long-term research across multiple populations is small, and adulticide is precisely the kind of rare event most likely to be missed or underreported. The figures for how many species show adulticide should be read as conservative.

What the study does establish is that the question “are humans violent?” is poorly formed if it treats violence as a single trait. Whether Homo sapiens has an evolved tendency toward mild aggression is a separate question from whether we have an evolved tendency toward lethal aggression, which is a separate question from whether we kill adults versus infants, within our social groups or outside them. Those distinctions are not just semantic. They have different phylogenetic histories, different socio-ecological triggers, and apparently different evolutionary dynamics. A species can be argumentative without being murderous. A species can be murderous without being especially argumentative. The relationship between the two is not a gradient. It is, as the data suggest, close to zero.

Further Reading

• Gómez, J.M., Verdú, M., González-Megías, A., & Méndez, M. (2016). The phylogenetic roots of human lethal violence. Nature, 538(7624), 233.

• Gómez, J.M., Verdú, M., & González-Megías, A. (2021). Killing conspecific adults in mammals. Proceedings of the Royal Society B, 288(1955), 20211080.

• Lukas, D., & Huchard, E. (2014). The evolution of infanticide by males in mammalian societies. Science, 346(6211), 841–844.

• Lukas, D., & Huchard, E. (2019). The evolution of infanticide by females in mammals. Philosophical Transactions of the Royal Society B, 374(1780), 20180075.

• Wrangham, R.W. (1999). Evolution of coalitionary killing. American Journal of Physical Anthropology, 110(S29), 1–30.

• Wrangham, R.W. (2018). Two types of aggression in human evolution. Proceedings of the National Academy of Sciences, 115(2), 245–253.

• Wilson, M.L., et al. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414–417.

• Pinker, S. (2011). The Better Angels of Our Nature: The Decline of Violence in History and Its Causes. Penguin.

• Dwyer, P., & Micale, M. (2021). The Darker Angels of Our Nature: Refuting the Pinker Theory of History & Violence. Bloomsbury Publishing.

Two of those primates have now given up a little more of their story. Researchers led by Siobhán B. Cooke of Johns Hopkins University have described the first known lower jaws of Stirtonia victoriae, an ancient relative of modern howler monkeys. The specimens were recovered from La Repartidora, a fossil locality in the La Victoria Formation, by Andrés Vanegas of the Museo de Historia Natural La Tatacoa. They sat in sediments dated between 13.3 and 13.6 million years old. The study, published in PaleoAnthropology1 in 2026, uses three-dimensional geometric morphometrics to analyze both the shape of the teeth and the overall form of the mandible, and the results are clear: S. victoriae was eating a substantial quantity of leaves, making it the earliest confirmed folivore in the platyrrhine record.

That’s a specific, bounded claim. But the implications spread outward in ways that matter for understanding how primate diversity in the New World actually developed.

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