An entire ancient human species, Homo naledi, is known from specimens found at just one site in South Africa, back in 2013.
At first, 15 individuals were found inside the Rising Star cave system. Now, archaeologists have specimens of at least 20 of these ancient humans, who lived between 236,000 and 335,000 years ago.
A new examination of the ancient proteins preserved in their teeth suggests that the site is surprisingly lacking in males and may even be an all-female site.
If that's the case, it could add weight to the controversial theory that Rising Star may be the earliest evidence of deliberate human burial rituals.
Researchers investigated the sex of these H. naledi individuals based on the presence of peptides unique to a protein encoded by the human Y chromosome, preserved in the dental enamel of fossilized teeth.
Amelogenin X is present in both biological sexes, since both have X chromosomes (females have two, while males have one). But amelogenin Y is linked to the Y chromosome and associated with male biological sex.
And in all the dental enamel samples examined, not a single one contained the signature for amelogenin Y.
As astronomer Carl Sagan famously said, "absence of evidence is not evidence of absence".
There's no way to know, based on this analysis, that the individuals discovered at Rising Star all had the female chromosome pattern XX – only that peptides unique to amelogenin Y are missing from the samples.
As the research team points out, the absence of amelogenin Y can be explained in a few ways: the group could, indeed, be all-female; or perhaps the males in this H. naledi population were missing the gene for some reason.
"Either scenario would have very interesting implications for the biology and evolution of H. naledi," the team writes in their published paper.
Statistical analysis by the researchers suggests it's highly likely that most of the individuals found here were female.
This still doesn't count as a positive confirmation. But for ancient DNA that has survived in warmer climates across vast stretches of time, those peptide signatures of amelogenin X and Y are the only biomolecular clues currently available for determining the sex of ancient human 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 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."
If all the specimens we've discovered for H. naledi are female, then where are all the males? As far as we know, human reproduction depends on one gamete from each sex.
Perhaps H. naledi culture buried their dead in separate locations, based on sex (though the question of whether they were intentionally buried at all is still debated by experts).
If this is a burial site, then perhaps Rising Star was reserved for females. Could there be another burial cave, hidden somewhere underground, filled with the bodies of their male counterparts?
Some scientists argue H. naledi, with their relatively small brains, could not have been capable of such complex cultural practices.
Others, including the team describing the finds at Rising Cave, say it's not the size of the brain that matters – though paleontologists not involved in the H. naledi studies have questioned the completeness of the evidence, which they say isn't convincing.
An earlier paper by paleoanthropologist Lee Berger, who was also involved in this new study, claimed that H. naledi did indeed practice deliberate burial, but was ripped to shreds by other researchers in the field.
This new paper doesn't make such bold claims, stating that further evidence is needed to verify how this all factors into the burial theory.
So, it's worth exploring another possible explanation for the absence of Y-linked peptides in these samples.
The protein sequences sampled among these individuals suggest the population had very low genetic diversity, and were perhaps affected by isolation and/or inbreeding.
This could have resulted in genetic mutation or deletion of the amelogenin Y gene, a mutation that would spread easily through a genetic bottleneck.
If that were the case, some of the Rising Star individuals may very well be male: they just don't bear the features archaeologists currently use to classify them.
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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.