Two Des Moines University Medicine and Health Sciences researchers are helping unravel one of the most intriguing mysteries in human evolution.
New findings from the ongoing Homo naledi project suggest that all 20 fossil individuals from a cave in South Africa were female — a discovery that is prompting scientists to reconsider assumptions about this ancient human relative’s behavior, social structure and treatment of the dead.
Heather Garvin, PhD, a forensic anthropologist and interim chair of DMU’s department of anatomy, and Caroline VanSickle, PhD, a paleoanthropologist and associate professor of anatomy, have been involved in the Homo naledi project since its inception. In 2014, they joined an international team gathered to study a large assemblage of fossil remains discovered deep within South Africa’s Rising Star Cave system, determining that they represented a previously unknown species of our genus.
The Science Behind Homo Naledi
Since its discovery, Homo naledi has challenged researchers’ understanding of human evolution. The species combines a mix of ancestral and modern anatomy, including a small brain, the ability to walk on two legs and curved fingers that suggest climbing ability.
Garvin’s work on this project has focused on cranial remains, including reconstructing the skull, estimating brain size and analyzing body size to characterize sex differences. VanSickle has focused on the anatomy of the pelvis, which tends to differ between sexes in human relatives who walked on two legs due to adaptations for childbirth, though it remains unclear if that was the case for Homo naledi.
For years, researchers believed the cave assemblage included both males and females. Based on that assumption, teams, including Garvin and VanSickle, published studies exploring body-size variation and skeletal differences due to childbirth to understand the differences between the sexes.
That interpretation was recently challenged when a research team led by Palesa Madupe, a researcher with the Max Planck Institute for Evolutionary Anthropology, examined ancient proteins preserved in tooth enamel of Homo naledi. The team looked for particular amelogenin proteins that are produced by a gene on the Y chromosome. The presence of these proteins would indicate that the individuals were male. Yet none of the 20 Homo naledi individuals tested showed evidence of the protein, suggesting all were female.
New Findings Create New Questions
While not directly involved in the recent publication that identified all individuals as female, Garvin and VanSickle continue to work on other research questions about this enigmatic human ancestor.
“The finding that they’re all females changes your perception,” Garvin says. “We have a lot of biases about how we expect our human ancestors to behave, and this finding challenges those assumptions.”
The discovery raises new questions about why the remains of female adults and children were in the cave.
“If these were humans and we found a cave filled with female adults and children, we would assume there was some cultural meaning behind it,” says VanSickle. “Now we’re asking whether we should be thinking about Homo naledi in similar ways.”
The species has presented mysteries from the beginning. Based on its anatomy, researchers initially expected Homo naledi to be much older. Instead, the fossils were dated to approximately 250,000 to 300,000 years ago, placing them in the same general time period as early Homo sapiens. The overlap suggests that multiple hominin species lived at the same time, making the story of human evolution more complex than previously understood.
Researchers have also proposed that Homo naledi may have intentionally placed its dead deep within the cave system. If true, such behavior could indicate forms of mortuary practice and possibly the controlled use of fire, since navigating the cave’s dark passages would likely have required a light source. These behaviors have traditionally been associated with species considered more cognitively advanced.
“I love how many questions there are with Homo naledi,” says Garvin. “We don’t know all the answers, and that’s part of what makes it so exciting.”
What’s Next
The realization that the cave remains may all be female could provide new insights into those questions. Researchers are preparing to reanalyze fossils studied years ago. VanSickle is hoping to digitally reconstruct an adult and juvenile pelvis, now that she knows there are no sex differences to contend with. She hopes the results will improve our understanding of growth, development and childbirth in ancient human relatives.
Meanwhile, research teams continue exploring nearby cave systems in search of additional fossils, including potential males that could help clarify the population represented at the site.
For Garvin, the discovery of the fossil remains all being female is a reminder of how science advances.
“You have a hypothesis based on the evidence available at the time, but new evidence may arise that changes that understanding,” she says. “That’s the scientific process.”
Beyond what it reveals about Homo naledi, the finding contributes to a larger effort to understand human evolution and why modern humans are the way they are.
“‘Nothing in biology makes sense except in the light of evolution,’” says VanSickle, quoting the geneticist Theodosius Dobzhansky. “Understanding our evolutionary history helps explain why the human body is structured the way it is today, and Homo naledi challenges what we thought was possible in the past.”