"If this interpretation is correct, it would be the oldest known sex-selective burial site."

Ancient Proteins Reveal All-Female Homo naledi Burial Site

A study published in Cell on June 23, 2021, analyzed ancient proteins from the tooth enamel of 23 Homo naledi fossils, representing at least 20 individuals. The analysis found that none of the samples contained the Amelogenin-Y protein, a genetic marker typically associated with the male Y chromosome. Researchers concluded that based on this evidence, all tested individuals are female.

The fossils were excavated from the Dinaledi Chamber in the Rising Star cave system in South Africa, part of the Cradle of Humankind World Heritage Site. The species Homo naledi was first discovered in 2013 and is dated to have lived between approximately 335,000 and 241,000 years ago. It is characterized by a mix of primitive and human-like traits, including a notably small brain.

Methodology

Scientists from institutions in South Africa, Denmark, the United States, and the United Kingdom conducted the analysis using paleoproteomic techniques. They applied a minimally destructive acid etching method to extract peptides from the fossilized tooth enamel. By sequencing the amelogenin gene (AMEL), the team looked for the AMELY variant, which is male-specific, and the AMELX variant, which is found in both sexes. Only the AMELX variant was detected. Amino acid analysis confirmed that the proteins were ancient and not modern contamination. This is the largest extinct hominin population to undergo protein sex-determination analysis.

Interpretations and Conflicting Viewpoints

The absence of the male-specific genetic marker has led to two primary scientific hypotheses:

• Sex-Selective Burial: Project leader Lee Berger suggested the all-female composition may be due to cultural burial practices that separated individuals by sex or gender. Researcher John Hawks noted that if this interpretation is correct, it would be the oldest known sex-selective burial site.

• Genetic Feature of the Population: Some researchers note that a systematic deletion or mutation of the AMELY gene in Homo naledi cannot be excluded. Such deletions are rare in modern humans but have been observed in at least one Neanderthal. If present in Homo naledi, biological males would appear as female in this analysis.

Researcher Enrico Cappellini stated that either scenario would have significant implications for understanding the species.

Elizabeth Sawchuk cautioned that the failure to detect AMELY does not prove the absence of males, only that they were not detected. Michael Petraglia urged caution, noting that high female-to-male sex ratios are observed in non-human primates and could explain the finding without implying burial rituals. Researchers also noted that sampling 20 individuals of one sex is statistically unlikely but not impossible.

Related Findings

The study also found that Homo naledi shares a collagen gene variant with Paranthropus robustus, a hominin that lived in the same region 1–2 million years ago. The significance of this shared variant is unclear; it may indicate common ancestry or overlap. Further sampling of other African hominins like Australopithecus africanus and Homo erectus is needed.

Implications

If the sex determination is accurate, all existing knowledge of Homo naledi anatomy and behavior is based on female specimens. Berger stated that descriptions of the species may need to be revised once male skeletons are found. The study demonstrates that proteomic analysis can be applied to Pleistocene fossils with minimal damage, potentially enabling future sex determination in other extinct hominin groups.