Burial site of all female extinct human relatives baffles scientists

In archeology, the truth lies in the teeth. Our hearty chompers can withstand the test of time, preserving precious genetic data that can help tell the stories of long-deceased animals, and also complicate those narratives. Several teeth discovered in a South African cave system point to a potentially all-female burial site belonging to one of our more mysterious extinct human cousins. More than just fossils, the teeth indicate that these relatives may have had complex cultural practices as well as some odd genetics. The findings are detailed in a study published today in the journal Cell.

Homo naledi is an extinct cousin of our species, Homo sapiens. First discovered in South Africa’s Rising Star Cave system between 2013 and 2015, Homo naledi lived in present-day South Africa between 225,000 and 241,000 years ago. Very little is known about their life on Earth, since scientists have not found many fossils or tools associated with Homo naledi. The limited information that we have primarily comes from their teeth. They show an unusual mixture of primitive and ape-like traits, alongside more human features.

For more than 10 years, scientists have wondered why the adult fossils discovered in the cave’s Dinaledi Chamber looked so identical. The remains showed almost none of the physical variations normally present between males and females. To figure out why, an international team investigated the proteins lurking inside of the skeletal remains. They used a minimally destructive acid etching technique to protect the teeth, while also extracting microscopic fragments from inside of them called peptides. In total, they extracted peptides from 23 teeth representing 20 different individuals.

With the precious peptides in hand, they analyzed the tooth enamel for Amelogenin-Y. This protein is only coded by the male Y chromosome, and indicates that an individual was biologically male.

The team did not find any of this male genetic marker inside of the tooth enamel. To double check this puzzling result, they analyzed the amino acids to prove that the proteins were genuinely ancient and not the result of contamination.

“The lack of male markers with the group is truly fascinating,” Dr. Marc Dickinson, a study co-author and University of York analytical chemist, said in a statement. “It is incredibly exciting to gain a window not only into the biology of our ancestors, but also into how they lived.”

This potentially all-female burial site opens up new questions regarding Homo naledi’s culture and social structure. If the chamber was reserved just for females, it could imply a level of complex and symbolic burial practice that was previously believed to be unique to modern humans.

However, there could be a biological explanation for the burial as well as a cultural one. According to the team, it’s possible that the Homo naledi population was highly isolated, causing the male-specific Amelogenin-Y gene to mutate or be deleted entirely. In this case, males would have been present, but their teeth did not have the typical genetic signature seen in other male teeth.

“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,” added Palesa Madupem, a study co-author and palaeoproteomicist at the University of Copenhagen. “This makes them ideal carriers of genetic information from deep time. Our study helps in the long-standing mystery of why Homo naledi lacked significant variation; it’s probably because they could have all belonged to one sex.”

These ancient females or genetically unique males are shaking up what scientists thought they knew about the dawn of human society and our extinct relatives.

“These findings offer rare insights into a culture that has, until now, been difficult to access directly,” Dickinson said. “Advances in ancient protein analysis are opening the door to a far richer and more nuanced understanding of ancient hominins.”

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