In a rocky outcrop on Mount Carmel, in what is now Israel, a group of ancient humans buried their dead about 140,000 years ago. Scientists uncovered the site, called Skhul Cave, in 1928, and about three years later they found the remains of more than a dozen individuals.
The site is one of the oldest examples of burial practices among ancient humans, but researchers were puzzled by the excavated hominins’ anatomy. Some of their skeletal features resembled those of Homo sapiens, while others were more Neanderthal-like, making the species difficult to classify.
The first skeleton discovered at the Skhul burial site belonged to a child between 3 and 5 years old, most likely a girl. Using high-resolution scans of the child’s cranium and jaw, scientists now propose that the individual possessed anatomical traits of both Homo neanderthalensis and Homo sapiens. If that finding is the case, the skull — and other remains at Skhul Cave — represents the earliest known example of interbreeding between Neanderthals and our own species, researchers reported in the July-August issue of the journal L’Anthropologie.
Earlier analysis of DNA in the modern human and Neanderthal genomes suggested that the two species interbred between 50,500 and 43,500 years ago. The new findings could push back this genetic mingling by nearly 100,000 years, said senior study author Dr. Israel Hershkovitz, a professor in the Gray Faculty of Medical and Health Sciences at Tel Aviv University.
They also indicate an extended period of peaceful coexistence between modern humans and Neanderthals in the Levant, a region bordering the eastern Mediterranean Sea, Hershkovitz told CNN.
“What we bring to the story of human evolution is not a short overlap with our relatives, the Neanderthals, but a very long overlap in time and space,” Hershkovitz said. “You would think that those are two Homo groups that are considered to be competing populations. Suddenly, you see that they managed to live together side by side.”
This interpretation of Neanderthal-Homo sapiens hybridization requires caution, however, as anatomical features can be more ambiguous than genetic data, and factors such as an individual’s life history can affect the expression of anatomical traits, said William Harcourt-Smith, a resident research associate at the American Museum of Natural History in New York City and an adjunct professor at the museum’s Richard Gilder Graduate School.
The young age of the individual in the study must also be considered, as childhood growth can affect anatomical variations, added Harcourt-Smith, who was not involved in the new research.
“Most species comparison studies tend to focus on adult individuals only, to minimize this problem,” he said. Scientists therefore need to be careful when using only skeletal data as proof that a fossil represents a hybrid species.
Certain features can also be retained from ancestors and do not necessarily represent hybridization, said Dr. Zeresenay Alemseged, a Donald N. Pritzker Professor in the University of Chicago’s department of organismal biology and anatomy who was also not involved in the new study. Still, this hypothesis that the child’s ancestry included interbreeding “is not farfetched,” Alemseged, who was not involved in the new research, told CNN in an email.
“Previous DNA studies show that the two (species) interbred, and fossil evidence shows that they geographically overlapped in the Levant before 100,000 years ago, when H. sapiens first attempted to leave Africa,” he added. “But the ultimate arbiter is DNA or another biochemical marker.”
Mingling and interbreeding
Modern humans and Neanderthals share an ancestor that originated in Africa, but the two lineages diverged at least 500,000 years ago. The first Neanderthals appeared in Asia and Europe about 400,000 years ago, while H. sapiens evolved in Africa about 300,000 years ago and later migrated to the Asian and European continents.
Outside Africa, populations of Neanderthals and H. sapiens mingled and interbred until Neanderthals went extinct about 40,000 years ago. Today, the genomes of most modern humans whose ancestors migrated to Europe and Asia contain about 1% to 4% of Neanderthal DNA.
When scientists discovered the Skhul fossils nearly a century ago, they suggested that hybridization between the two species could explain the hominins’ unusual anatomy. Tools available at the time were unable to investigate the bones at high resolution, of course.
In the new study, however, researchers from France and Israel used micro-CT scans to capture images of structures of the Skhul child’s skull and jaw in unprecedented detail and then digitally modeled the bones in 3D.
In its overall shape, especially in the curve of the skull vault around the brain, the cranium looked like a H. sapiens skull. But the structure of the bony labyrinth — a rigid area surrounding the inner ear, too small to see except with micro-CT — was a closer match to the anatomy of Neanderthals. The shape of the lower jaw, the inner structure of the teeth and the underdeveloped blood vessel network inside the skull were also more Neanderthal-like.
Skeletons of seven adults and three children who were intentionally buried, as well as isolated bones from 16 other individuals, have been uncovered in the Skhul Cave. Of the 10 burials, each person possessed a different combination of H. sapiens and Neanderthal traits, Hershkovitz said. While the skull of the first child discovered was the only Skhul fossil examined for the study, “all of them manifest what we call ‘mosaic morphology,’ in the sense that they have both Neanderthal and Homo sapiens features.”
The burials at Skhul also call for a reevaluation of the development of culture in early humans, Hershkovitz said. By designating the rocky outcrop as a cemetery, the people who buried their dead there were demonstrating territoriality, a type of social behavior typically associated with the start of agriculture nearly 12,000 years ago.
“And here we see that 140,000 years ago, people were already some kind of territorial group,” Hershkovitz said. “We have to go back and redo our studies of human behavior, not just biology.”
Mindy Weisberger is a science writer and media producer whose work has appeared in Live Science, Scientific American and How It Works magazine. She is the author of “Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control” (Hopkins Press).
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