The Protein That Remembers Sex
For more than a decade, no one could say with certainty whether the Homo naledi skeletons pulled from South Africa's Rising Star Cave were male or female, because this species had erased the usual clues. When researchers at Wits University first discovered and excavated the remains in 2013, the adult fossils showed almost no variation in size, shape, or other physical traits, according to the study published in the journal Cell. The sexual dimorphism that paleontologists rely on to sex fossils, the difference in stature between males and females seen in nearly every other hominin, simply wasn't there. The bones were silent on the question that matters most when you're trying to understand who lived, who died, and who ended up in a cave chamber 30 meters underground.
Then a single protein, coded only on the Y chromosome and preserved in tooth enamel for millions of years, gave 23 teeth from at least 20 individuals a binary test. All came back negative. An international team of researchers searched for amelogenin-Y within the enamel of Homo naledi fossils from the Cradle of Humankind and found a complete absence of the male marker in every specimen examined, according to the study. Not one tooth carried the protein fragment that signals biological maleness. The findings suggest that all individuals examined from the Rising Star Cave system may have been biologically female.
Here is the part that is strange. The probability of randomly sampling 20 individuals that are all from one sex is, according to the study, quite literally one in a million.
When Bones Lie, Enamel Speaks
Tooth enamel is the hardest tissue in the human body and shields proteins from environmental contamination for millions of years, per the study. While DNA degrades in warm, wet conditions like those in South African caves, and while bone morphology grows ambiguous in species that lack the usual size differences between sexes, enamel holds its molecular cargo across geological time. The researchers used a minimally destructive acid etching technique to extract ancient protein fragments called peptides from the remains, then analyzed the samples with a mass spectrometer to identify which proteins were present, according to the study.
Dr. Fazeelah Munir, working with Dr. Marc Dickinson and Professor Kirsty Penkman at the NEaar facility in York's Department of Chemistry, analyzed amino acids in the tooth enamel and confirmed that the proteins were authentic and preserved from the ancient samples, the study noted. The protein analysis represented the first time anyone had successfully extracted and identified ancient proteins from Homo naledi fossils. It also marked the largest extinct hominin population to ever be examined using ancient protein analysis, involving collaborating institutions that included the University of Copenhagen, the University of the Witwatersrand, the National Geographic Society's Rising Star project, and 13 other global institutions.
Amelogenin-Y is uniquely coded onto the male Y chromosome. Its presence in enamel is binary: either the protein appears in the mass spectrometer reading, signaling a male individual, or it does not. In 23 teeth representing at least 20 Homo naledi individuals, the amelogenin-Y peak never appeared. Each tooth answered the same question the same way: no.
The Uniformity No One Could Explain
The morphological puzzle had been visible from the beginning. Adult Homo naledi fossils from the Dinaledi Chamber were all very similar when first excavated, showing variation so minimal that it defied the pattern seen in nearly every other hominin assemblage, according to the study. Typically, sexual dimorphism is expected among hominins, males and females differ in skeletal robustness, pelvic architecture, skull size, or some combination of traits that allows researchers to sort remains by sex even without DNA. Neanderthals show it. Homo erectus shows it. Early Homo sapiens shows it.
Homo naledi did not. The uniformity was noted in initial publications but remained unexplained, a footnote in a larger story about a species with a bizarre mix of primitive and derived traits, a small brain, and a presence in a cave system so difficult to access that it required specially sized excavators to squeeze through passages no wider than a human torso. The lack of dimorphism could have meant the sample was skewed, or it could have meant Homo naledi simply evolved less size difference between sexes than other hominins. Without a way to sex the skeletons, the question hung open.
Palesa Madupe, the lead study author and a molecular scientist currently at the Max Planck Institute for Evolutionary Anthropology in Germany, led the protein analysis that closed the question. The amelogenin-Y absence does not just confirm that the examined individuals were female. It transforms the uniformity from an unexplained observation into a pattern that demands a mechanism.
Follow the Loop
One-in-a-million odds mean the sample is either astronomically unlucky or the cave preserves something structured about Homo naledi social behavior. If the absence of males is not random, then Rising Star Cave records something about who entered, who died there, or who was placed there. The protein has become a witness to a pattern the bones could not articulate: a sex-specific use of space, a gendered death practice, or a differential preservation that erased males from the record while leaving females intact.
Lee Berger, a National Geographic Explorer in Residence, corresponding author of the study, and an Honorary Professor at Wits University, has previously argued that the distribution and condition of Homo naledi remains suggest intentional body disposal rather than accidental death or predation. The all-female protein signature adds a new dimension to that hypothesis. If bodies were being placed in the cave, the placement was not random across sexes. If individuals were dying in the cave, the deaths were not happening to a mixed-sex population. If the cave was used for some repeated behavior, that behavior involved females and not males, or involved both but preserved only one.
The protein does not answer which mechanism is correct. It only confirms that a mechanism exists, that the pattern is real, and that the Rising Star assemblage is unlike any other hominin fossil collection in a way that goes beyond anatomy and into the realm of social structure. The amelogenin-Y that never appears is now as informative as any protein that does.
The Hardest Tissue, the Longest Memory
Twenty teeth, each no larger than a thumbnail, each holding a protein fragment that survived longer than the species itself. The enamel that once grew in a living mouth, layer by microscopic layer, is now the only tissue that remembers sex. The mass spectrometer reads the peptides, plots the peaks, and renders a verdict that bone morphology could not. Where the amelogenin-Y peak should rise, the line stays flat. The male that was never there. The absence that is now a presence of pattern, written in the language of proteins that outlast everything else we are made of.
