A study published in the journal Cell on June 24 analyzed proteins extracted from the tooth enamel of multiple Homo naledi skeletons found in the Rising Star cave system in South Africa. The analysis detected no male-specific genetic markers, leading researchers to propose two main hypotheses: that the remains represent an all-female burial site, or that the species may possess a genetic anomaly that masks male sex.
The Study and Its Findings
Researchers analyzed dental enamel proteins from 20 Homo naledi individuals, represented by 23 teeth. The remains were recovered from the Dinaledi Chamber of South Africa's Rising Star cave system. Homo naledi is a hominin species whose remains have been dated to between approximately 335,000 and 241,000 years ago.
The team used a minimally destructive acid etching technique to extract peptides from the tooth enamel. The analysis focused on the amelogenin gene (AMEL) variants: AMELX, which is found in both sexes, and AMELY, which is typically present in biological males and codes for the Amelogenin-Y protein. Only the AMELX variant was detected in all samples.
Amino acid analysis confirmed that the extracted proteins were ancient and not modern contamination. The study represents the largest extinct hominin population to undergo this type of protein analysis.
Proposed Explanations
Researchers have presented two leading hypotheses to explain the absence of the male-specific genetic marker:
1. Sex-Selective Burial: Project leader Lee Berger suggested the all-female composition may be a result of cultural burial practices that separated individuals by sex or gender. John Hawks noted that if this is the case, the site would represent the oldest known sex-selective burial ground.
2. Genetic Deletion: The possibility of a systematic deletion of the AMELY gene in Homo naledi cannot be excluded. While such deletions are rare in modern humans, they have been observed in at least one Neanderthal specimen. If the AMELY gene were absent in the species, biological males would appear as females in this analysis.
“Either scenario—the absence of males or the presence of a gene deletion—would have significant implications for understanding the species.” — Researcher Enrico Cappellini
Paleoanthropologist Elizabeth Sawchuk cautioned that the failure to detect AMELY does not prove the absence of males, only that none were detected in the sample.
Other Related Findings
The study also found that Homo naledi shares a variant of the collagen gene with Paranthropus robustus, a hominin that lived in the same region one to two million years ago. The significance of this shared variant is unclear; it may indicate common ancestry or environmental overlap. Researchers noted that further sampling of other African hominins, such as Australopithecus africanus and Homo erectus, is needed for a clearer understanding.
Potential Implications
If the sex determination is correct, it would mean that all current anatomical and behavioral knowledge of Homo naledi is based on female specimens. Berger stated that descriptions of the species may need to be revised if male skeletons are found in the future.
The study demonstrates that proteomic analysis can be applied to Pleistocene fossils with minimal damage, potentially enabling future sex determination for other extinct hominin groups.