Why Scientists Just Realized a Famous Ancient Cave System is Filled Only With Females

On June 24, 2026, a international team of researchers published a study in the journal Cell that upended the scientific understanding of human evolution. For over a decade, South Africa’s famous Rising Star Cave system has been the center of intense debate. The deep, subterranean limestone chambers of the cave had yielded more than 1,500 fossil fragments belonging to Homo naledi—an extinct, small-brained relative of modern humans that lived between 335,000 and 241,000 years ago. Now, using a state-of-the-art molecular technique, scientists have analyzed the ancient proteins preserved within the dental enamel of these fossils. The results were unequivocal: every single one of the tested skeletons, spanning at least 20 individuals, is biologically female.

The discovery has shocked the paleoanthropological community. For years, researchers had assumed that the Rising Star fossils represented a balanced mix of males and females, matching the typical demographic distribution found in modern mammal populations. Some of the larger, more robust skeletons had even been informally nicknamed "Neo" and "DH1" and classified as the premier male representatives of the species. Yet the chemical signatures of their teeth revealed that even these robust individuals were female. The revelation that this famous ancient cave system female skeletons are the only occupants has sent shockwaves through the field, forcing a total re-evaluation of how our ancient relatives lived, died, and treated their dead.

This finding constitutes the first known sex-specific fossil site of any non-human species in the evolutionary record. It raises a profound question: why did an ancient, small-brained species deposit only its females deep inside a highly treacherous, near-inaccessible cave system? The implications of this discovery are currently reverberating through archaeology, genetics, and cognitive science, rewriting the timeline of complex social behaviors and gendered rituals in the deep past.

The Molecular Forensic Feat: Reading Enamel to Trace Deep Time

The reason it took more than a decade to realize the true sex of the Rising Star skeletons lies in the harsh realities of preservation. In warm, humid environments like those of South Africa, ancient DNA degrades rapidly, typically disappearing entirely within a few thousand years. While scientists have successfully sequenced the genomes of Neanderthals and Denisovans from cold northern caves, extracting ancient DNA from Pleistocene fossils in Africa has remained an elusive goal.

To bypass this barrier, the research team—led by South African-born molecular scientist Dr. Palesa Madupe, currently a postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology, and Enrico Cappellini, a professor of paleoproteomics at the Globe Institute of the University of Copenhagen—turned to a different biological repository: tooth enamel.

"Unlike those found in other remains like bone fragments, proteins in tooth enamel are preserved because dental enamel—the hardest tissue in the human body—shields proteins from environmental contamination even for millions of years," Madupe explained upon the study's release. "This makes them ideal carriers of genetic information from deep time".

The team analyzed 23 fossilized teeth representing at least 20 distinct Homo naledi individuals, using a minimally destructive "acid etching" technique. Rather than cutting open or destroying the priceless fossils, the scientists scraped microscopic protein fragments, known as peptides, from the outer enamel. The process left the teeth virtually undamaged, leaving a physical mark "no greater than what a dentist would do to roughen up your teeth to attach braces," according to paleoanthropologist Lee Berger, a National Geographic Explorer in Residence and co-author of the study.

Once the peptides were extracted, the researchers used mass spectrometry to analyze the amino acid sequences of a key protein called amelogenin, which is central to the formation of dental enamel. Amelogenin comes in two distinct sex-linked variants:

Amelogenin-X (AMELX): Coded by the X chromosome and present in both biological males and females.
Amelogenin-Y (AMELY): Coded by the male Y chromosome and present only in biological males.

In a typical mammalian population, a random sample of 20 individuals would show a mix of both AMELX and AMELY signatures. But when Madupe’s team ran the mass spectrometry on the Homo naledi teeth, they found plenty of AMELX proteins, but a complete absence of AMELY. To ensure the findings were not the result of modern contamination, researchers at the University of York’s specialized chemistry facility, led by Dr. Marc Dickinson, analyzed the level of amino acid degradation. The analysis confirmed that the proteins were indeed genuinely ancient, dating back roughly 300,000 years.

"The lack of male markers with the group is truly remarkable," Dickinson stated. Out of the 20 individuals tested, 19 were confidently identified as biological females with a statistical probability of over 95 percent, while the final individual was highly likely to be female.

The statistical reality of this finding is staggering. "The chance of having sampled twenty individuals and they are all from one sex, is quite literally one in a million," said Lee Berger. For the first time, researchers had proof that the ancient cave system female skeletons were the exclusive occupants of the Dinaledi and Lesedi Chambers.

Overturning a Decade of Biased Assumptions

The realization that the Rising Star Cave system contained only female skeletons has dealt a major blow to a long-standing paleontological bias: the assumption that the largest and most robust fossils in any hominin assemblage must be male.

When Homo naledi was first described by Berger and his team in 2015, they noted that the adult fossils recovered from the Dinaledi Chamber looked remarkably uniform. The bones showed almost none of the physical size variation—known as sexual dimorphism—typically expected between males and females of a hominin species. In modern humans, and even more so in species like Australopithecus afarensis (such as the famous "Lucy" fossil), males are on average larger and more muscular than females.

Faced with this lack of variation in Homo naledi, scientists hypothesized that the species simply possessed very low sexual dimorphism. To build their anatomical profile of the species, they selected the largest, most robust specimens—such as the nearly complete skeleton "Neo" from the Lesedi Chamber and the specimen "DH1" from the Dinaledi Chamber—and classified them as males. They assumed that the smaller, more delicate specimens were females.

+---------------------------------------------------------------------------------+
|                         HOMO NALEDI SEX CLASSIFICATION SHIFT                    |
+--------------------------+---------------------------+--------------------------+
| Individual Specimen      | Pre-2026 Assumption       | Post-2026 Protein Result |
+--------------------------+---------------------------+--------------------------+
| "Neo" (Lesedi Chamber)   | Male (Robust, large skull)| Female (AMELX, No AMELY) |
| "DH1" (Dinaledi Chamber) | Male (Robust, holotype)   | Female (AMELX, No AMELY) |
| "DH3" (Dinaledi Chamber) | Female (Small, delicate)  | Female (AMELX, No AMELY) |
+--------------------------+---------------------------+--------------------------+

The new protein analysis has exposed the flaw in this logic. "Neo," the very individual that scientists held up as the prime example of a Homo naledi male, is biologically female. The variation in size among the Rising Star fossils was not a reflection of two sexes with low dimorphism; rather, it was simply the natural variation of a single sex.

"Our study helps resolve the long-standing mystery of why Homo naledi lacked significant variation; it's probably because they could have all belonged to one sex," Madupe noted.

This revelation has forced paleoanthropologists to confront the "default male" template that has historically dominated the interpretation of ancient fossils. By assuming that robust bones automatically pointed to a male identity, researchers had built an entire evolutionary profile of Homo naledi that was fundamentally skewed. The realization that the species' largest known representatives were female opens up a new avenue of research into the physical capabilities, pelvic anatomy, and biological diversity of female hominins.

Who is Affected: The Ripple Effect Across Scientific Disciplines

The impact of the Rising Star discovery extends far beyond the immediate research team, creating a ripple effect that touches multiple scientific communities, academic institutions, and historical narratives.

Hominin Evolutionary Biologists and Anatomists

For scientists who specialize in reconstructing the physical appearance and growth patterns of ancient human relatives, this study completely alters the anatomical reference frame for Homo naledi. Because they now know that all the recovered specimens are female, they must re-evaluate what a male Homo naledi actually looked like. Were males significantly larger, displaying high sexual dimorphism, and thus went unrecognized in other contexts? Or did the species truly have very low dimorphism, meaning males were physically identical to the females but simply absent from this cave? Until a verified male fossil is found and analyzed using paleoproteomics, the physical identity of the Homo naledi male remains an evolutionary blank space.

Cognitive Archaeologists and the Burial Debate

The scientific community has been locked in a fierce, sometimes bitter debate since 2023, when Lee Berger's team first announced that Homo naledi—despite having a brain only a third the size of a modern human's—had intentionally buried their dead, used fire, and carved symbolic markings onto the cave walls. Many critics rejected these claims, arguing that a creature with a brain size of 450 to 600 cubic centimeters (barely larger than a chimpanzee's) was cognitively incapable of such complex, symbolic behavior. They suggested that the bones had washed into the cave naturally over millennia, or had fallen victim to carnivore activity.

The discovery of the ancient cave system female skeletons changes the playing field for cognitive archaeologists. A natural accumulation or accidental trap cannot explain why only females—ranging from newborn infants to elderly adults—were deposited in the cave, while every single male was excluded. The gender-segregated nature of the assemblage strongly points to an active, cultural selection process, providing powerful support to the theory of intentional, ritualized mortuary practices.

The "Underground Astronauts"

In a poetic turn of scientific history, the discovery has deeply affected the very team of cavers who first brought the fossils to light. In 2013, when Lee Berger needed to excavate the Dinaledi Chamber, he had to post an ad on Facebook looking for experienced cavers who could fit through a claustrophobic, 100-foot-long chute that narrowed to just seven inches wide. The six scientists selected for the dangerous, high-risk mission were all women, quickly dubbed the "underground astronauts".

Now, thirteen years later, these women have learned that the ancient individuals they carefully extracted from the darkness of the Dinaledi Chamber were, like them, an entirely female group. The parallel has added a deeply personal, symbolic layer to a project that has consistently pushed the boundaries of traditional fieldwork.

What Changes: Dismantling the "Natural Accumulation" Hypothesis

For over a decade, the primary alternative explanation to the "intentional burial" hypothesis was that the Rising Star fossils had ended up in the cave through natural geologic or taphonomic processes. Detractors argued that the hominins might have been caught in a sudden flash flood, swept down a sinkhole, or dragged into the cave by ancient predators.

However, the confirmation of an all-female assemblage makes these natural scenarios highly improbable, if not statistically impossible. In nature, physical forces such as gravity, water currents, and geologic collapses do not discriminate based on biological sex. If a natural disaster had wiped out a group of Homo naledi, or if a flood had washed their remains into the cave, the resulting fossil assemblage would reflect a natural demographic cross-section of the population: roughly equal numbers of males and females.

+-----------------------------------------------------------------------------------+
|               NATURAL ACCUMULATION VS. INTENTIONAL DEPOSITION                     |
+------------------------+--------------------------+-------------------------------+
| Feature                | Natural/Accidental Trap  | Intentional Mortuary Practice |
+------------------------+--------------------------+-------------------------------+
| Sex Ratio              | 50/50 Male-Female        | 100% Female (Sex-segregated)  |
| Age Distribution       | Random/Catastrophic      | All ages (Infants to Elderly) |
| Location Accessibility | Open sinkhole/Waterway   | Deep, dark, 7-inch chute      |
| Bone Preservation      | Highly fragmented/Worn   | Unusually complete/Intact     |
+------------------------+--------------------------+-------------------------------+

The demographic profile of the Rising Star fossils further refutes the accidental trap theory. The assemblage is not a collection of young, active foragers who might have climbed into the cave and gotten lost. It includes a wide spectrum of ages: a two-year-old infant, young children, toddlers, adolescents, young adults, and elderly individuals whose teeth were worn down to the roots.

If these individuals had simply crawled into the cave to seek shelter, or had been chased inside by a predator, it is highly implausible that only the females and their offspring would enter, leaving no males behind. Furthermore, if children were present, one would expect to find at least some male infant or juvenile bones. Yet even the youngest children in the sample completely lacked the Amelogenin-Y protein.

By proving that the ancient cave system female skeletons were the exclusive occupants of the chamber, the Cell study has effectively dismantled the natural accumulation argument. The presence of an all-female group across all age brackets can only be explained by a deliberate, active choice made by Homo naledi. They were selectively transporting the bodies of their deceased females—and potentially their dependent offspring—deep into the earth.

Short-Term Consequences: A Resurrected Academic Firestorm

In the immediate wake of the June 2026 announcement, the scientific community has split into two competing camps, igniting a fresh academic firestorm over how to interpret the all-female data. While many researchers view the study as definitive proof of sex-specific mortuary practices, others are racing to find alternative biological explanations that do not require attributing complex, symbolic culture to a small-brained hominin.

The Genetic Mutation Hypothesis

The primary alternative explanation being put forward by critics is the "Y-chromosome deletion" hypothesis. In modern humans, there are rare genetic conditions where the AMELY gene on the Y chromosome is mutated or deleted entirely. Biologically male individuals with this mutation are fully functional males, but their teeth do not produce the Amelogenin-Y protein. Because they lack the AMELY gene, their enamel-protein signature would appear identical to that of a biological female (showing only AMELX).

This phenomenon is not entirely theoretical; a similar AMELY deletion has been documented in ancient DNA from at least one Neanderthal male. Critics argue that if the Homo naledi population using the Rising Star Cave was highly isolated, inbred, and small, this genetic mutation could have easily spread through the group, eventually becoming fixed. Under this scenario, the cave could have contained both males and females, but the males simply carried a mutated Y chromosome that made them invisible to the paleoproteomic test.

"Either scenario would have very interesting implications for the biology and evolution of H. naledi," the research team acknowledged in their paper.

If the genetic mutation theory is correct, it would reveal that Homo naledi was an extremely isolated, highly inbred population on the brink of genetic collapse. If the cultural burial theory is correct, it would prove that symbolic mortuary behavior predates modern humans by hundreds of thousands of years.

To resolve this debate, geneticists and molecular biologists are already scrambling to develop new paleoproteomic assays. They are searching for other, non-amelogenin proteins coded by the Y chromosome that might still be preserved in the Rising Star teeth. If they can find even a single, non-mutated male marker, they can definitively rule out the Y-deletion hypothesis.

The Rush to Test Global Fossil Collections

A more immediate, highly practical consequence of the study is the widespread adoption of the minimally destructive paleoproteomic method. For decades, museums and antiquities authorities have been extremely reluctant to allow genetic testing on rare, highly prized hominin fossils, because traditional DNA extraction requires drilling into the bone or destroying a portion of the specimen.

But because the acid-etching technique used on the Homo naledi teeth is virtually non-destructive, institutions around the world are suddenly greenlighting protein testing on their collections. Within the next few years, we are likely to see a massive wave of sex-attribution studies on famous, disputed fossils from Europe, Asia, and Africa. This could resolve long-standing mysteries regarding the sex of famous Neanderthal, Denisovan, and Homo erectus specimens, completely rewriting our understanding of demographic distribution and physical variation in ancient human relatives.

Long-Term Consequences: Redefining the Dawn of Human Ritual

If the "cultural burial" hypothesis holds up against the genetic mutation theory, the long-term consequences will fundamentally alter our understanding of the evolution of human society, gender roles, and the origins of religious and symbolic thought.

The Evolution of Gendered Social Roles

For decades, mainstream anthropology has operated under the assumption that gender-segregated social roles and symbolic rituals were relatively late developments in human history, arising only with the emergence of anatomically modern Homo sapiens or Neanderthals. The earliest widely accepted evidence of a sex-specific burial site was a Neolithic cave in Portugal, which contained a high majority of female skeletons associated with specific agricultural mortuary practices—dating back only a few thousand years.

+---------------------------------------------------------------------------------+
|                       TIMELINE OF SEX-SPECIFIC BURIAL PRACTICES                 |
+------------------------+-----------------------+--------------------------------+
| Site                   | Associated Species    | Estimated Age                  |
+------------------------+-----------------------+--------------------------------+
| Rising Star Cave, SA   | Homo naledi           | ~300,000 Years Ago             |
| Neolithic Cave, Portugal| Homo sapiens          | ~5,000 Years Ago               |
+------------------------+-----------------------+--------------------------------+

Finding a sex-segregated burial site that is 300,000 years old—and created by a species with a brain the size of an orange—shatters this timeline. It suggests that gendered social structures and symbolic division of space are not the product of a massive, complex brain, but are instead deeply rooted, ancient behaviors that predate the evolution of our own species.

If Homo naledi had a specific, ritualized practice of depositing only their deceased females in the Dinaledi Chamber, it implies they possessed a complex understanding of social identity, biological sex, and perhaps even gender roles. It suggests that the division of labor, social bonding, or spiritual beliefs in their society were highly structured around sex.

Where Are the Males? The Hunt for the "Male Chamber"

The confirmation of an all-female site immediately raises a thrilling, yet perplexing question: if the Rising Star Cave system was reserved exclusively for females, then where are the males?

"Intriguingly, these results raise profound new questions, chief among them: if the H. naledi individuals in the Rising Star cave system are all females, then where are the males?" asked Enrico Cappellini. "We need to deploy a new generation of palaeoproteomic tools to address this and other fascinating questions".

This question is set to drive exploration efforts in South Africa's Cradle of Humankind for the next decade. Paleoanthropologists and cavers are already planning targeted surveys of the surrounding limestone karst landscape, searching for undiscovered chambers, hidden chutes, or adjacent cave systems.

                                  RISING STAR CAVE SYSTEM
                                 (Hypothetical Layout)
                                 
                                     [Surface Entrance]
                                             |
                                             |  (Narrow 100ft Chute)
                                             v
                             +-------------------------------+
                             |    DINALEDI & LESEDI CHAMBERS |
                             |                               |
                             |   [ALL-FEMALE SKELETONS]      |
                             +-------------------------------+
                                             ?
                                             ? (Is there a second, undiscovered chute?)
                                             v
                             +-------------------------------+
                             |      UNDISCOVERED CHAMBER     |
                             |                               |
                             |      [ALL-MALE SKELETONS?]    |
                             +-------------------------------+

If Homo naledi practiced strict sex-segregated burial, it is highly likely that there is a second, as-yet-undiscovered chamber hidden deep within the Rising Star system, or in a nearby cave, containing exclusively male skeletons. Finding such a "male chamber" would provide absolute, irrefutable proof of the cultural burial hypothesis, settling the debate once and for all and offering a historic look into the social organization of an extinct hominin.

Redefining Hominin Cognition

The long-term impact on how we view cognitive evolution cannot be overstated. For generations, the dominant narrative of human evolution has been brain-centric: as our ancestors' brains grew larger, they became capable of language, tool use, fire, art, and ritual. This "encephalization" model tied complex behavior directly to cranial capacity.

Homo naledi has consistently defied this model. With their ape-like shoulders, curved fingers adapted for climbing, and tiny brains, they physically resembled primitive australopithecines like Lucy, who lived millions of years ago. Yet, their hands, feet, and legs were remarkably human-like, and they apparently engaged in highly advanced, symbolic behaviors.

By adding sex-specific burial rites to their behavioral repertoire, Homo naledi forces us to decouple complex culture from massive brain size. It suggests that social complexity, ritual, and symbolic thought can emerge in species with small brains, raising the possibility that we have vastly underestimated the mental lives of other extinct, small-brained hominins, such as Homo floresiensis (the "Hobbits" of Indonesia) or early Homo erectus.

Looking Ahead: The Next Frontier in Deep-Time Proteomics

As the scientific community digests the reality of the Rising Star all-female discovery, several key milestones and research directions will define the next few years of paleoanthropology.

The immediate priority for researchers is to resolve the genetic-mutation-versus-cultural-burial debate. Within the next 12 to 18 months, scientists expect to publish follow-up studies utilizing newly developed proteomic assays. By target-testing non-amelogenin proteins that are linked to the Y chromosome, they hope to either find hidden male markers (confirming the Y-deletion mutation) or definitively prove the absolute absence of biological males.

Simultaneously, the physical exploration of the Rising Star Cave system is entering a highly focused phase. Cavers and "underground astronauts" are deploying advanced 3D-mapping technology, ground-penetrating radar, and thermal imaging to scan the limestone walls for hidden passages. The search for a corresponding "male chamber" or other gender-segregated deposition sites will likely dominate field seasons for years to come.

Finally, the success of the tooth enamel protein analysis on the Homo naledi fossils has opened up a new era of molecular paleontology. We are likely to see this non-destructive technique applied to other mysterious fossil assemblages across the globe. Whether these future studies reveal more sex-segregated ancient sites or dismantle other long-held assumptions about hominin biology, one thing is certain: our understanding of the deep human past will never be the same. The silent, ancient cave system female skeletons of Rising Star have finally spoken, and their message has rewritten the history of human culture.