The ancient human species Homo naledi is primarily recognized from fossils discovered at a single location in South Africa in 2013. Initially, 15 individuals were found in the Rising Star cave system, but archaeologists have since identified remains from at least 20 individuals dating back 236,000 to 335,000 years.
Recent analysis of proteins preserved in their teeth indicates a notable absence of males, suggesting the site might have been exclusively female. This finding could lend support to the debated theory that Rising Star represents early evidence of intentional human burial practices.
Researchers determined the sex of these H. naledi individuals by identifying peptides specific to a protein encoded by the human Y chromosome, which were preserved in the dental enamel of the fossilized teeth.
The protein amelogenin X is found in both sexes, as both have X chromosomes, but amelogenin Y is linked to the Y chromosome, indicating male biological sex. Notably, none of the examined dental enamel samples contained amelogenin Y.
Carl Sagan famously stated, “absence of evidence is not evidence of absence.” This analysis does not confirm that all individuals at Rising Star had the female XX pattern; it merely shows a lack of amelogenin Y.
The research team explains that the absence of amelogenin Y could mean the group was all female, or that the males lacked the gene for some reason. “Either scenario would have very interesting implications for the biology and evolution of H. naledi,” the team notes.
Statistical analysis suggests it is highly likely that most individuals found were female.
Yet, this does not serve as definitive proof. In warmer climates, ancient DNA often survives poorly, making amelogenin X and Y peptides the only available biomolecular indicators for determining sex in ancient fossils.
“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,” says evolutionary anthropologist Palesa Madupe from the University of Copenhagen. This preservation makes them valuable carriers of genetic information from the past. The study offers insights into why Homo naledi might have lacked significant variation, possibly due to being of one sex.
If all known H. naledi specimens are indeed female, it raises questions about the whereabouts of the males, as human reproduction requires input from both sexes. It’s possible that H. naledi culture separated burials by sex, though the idea of intentional burial is still debated.
If the Rising Star site was designated for females, there might be another undiscovered burial site for males. While some scientists argue that H. naledi, with their smaller brains, could not have engaged in such complex cultural practices, others, including the team studying the Rising Star finds, emphasize that it’s the brain’s wiring, not size, that matters. Some paleontologists outside the H. naledi studies have expressed skepticism about the evidence’s completeness.
An earlier paper by paleoanthropologist Lee Berger, who also participated in this study, suggested that H. naledi practiced deliberate burial, but this was highly criticized by other researchers in the field. The recent paper refrains from making such claims, indicating the need for further evidence to support burial theories.
Another explanation for the absence of Y-linked peptides might involve low genetic diversity within the population, possibly due to isolation or inbreeding, leading to genetic mutations or the loss of the amelogenin Y gene. This could mean some Rising Star individuals were male but did not display the typical markers used by archaeologists for classification.
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“The lack of male markers with the group is truly fascinating. It is incredibly exciting to gain a window not only into the biology of our ancestors, but also into how they lived,” says fossil chemist Marc Dickinson from the University of York in the UK. “These findings offer rare insights into a culture that has, until now, been difficult to access directly. Advances in ancient protein analysis are opening the door to a far richer and more nuanced understanding of ancient hominins.”
The research was published in Cell.
