Our ancestors likely had sex with Neandertals, but when and where did these encounters take place? The discovery of a 55,000-year-old partial skull of a modern human in an Israeli cave, the first sighting of Homo sapiens in this time and place, offers skeletal evidence to support the idea that Neandertals and moderns mated in the Middle East between 50,000 and 60,000 years ago. What’s more, the skull could belong to an ancestor of the modern humans who later swept across Europe and Asia and replaced the Neandertals.
The find supports a raft of recent genetic studies. A 2010 analysis, for example, found that up to 2% of the genomes of today’s Europeans and Asians consist of Neandertal DNA, a clear sign of at least limited interbreeding in the past. Two years later, scientists compared ancient DNA extracted from Neandertal fossils to that of contemporary modern human populations around the world, concluding that this interbreeding took place in the Middle East, most likely between 47,000 and 65,000 years ago. And last year, a 45,000-year-old modern human found in Siberia, the oldest modern to have its genome sequenced, was revealed to have harbored a little more than 2% Neandertal DNA, allowing researchers to refine the interbreeding event to roughly 50,000 to 60,000 years ago.
From the Neandertal side, this time and place make sense. That’s because numerous skeletons dated to that time period have been found in caves in Israel and other parts of the Middle East over the years, and Neandertals were still living in the region as late as 49,000 years ago. Yet the other side of this mating partnership has been conspicuously absent from the fossil record of the Middle East: Although modern humans from Africa appear to have ventured into some of these same caves as early as 120,000 years ago, none have been found after about 80,000 years.
The new find from Israel’s Manot Cave, reported online today in Nature, provides the missing fossil evidence that modern humans were indeed in the right place at the right time to mate with Neandertals. The cave, located just 40 kilometers north of numerous other sites where both Neandertal and earlier modern human skeletons have been found, was discovered in 2008 during construction work in the area, when a bulldozer shaved off part of its roof. When researchers from the Hebrew University of Jerusalem entered the 80-meter-long cave to have a look, they quickly discovered the partial human skull sitting on a ledge in one of its side chambers. Beginning in 2010, a team led by Tel Aviv University anthropologist Israel Hershkovitz and other scientists has been excavating in the cave, uncovering stone tools and charcoal from several different prehistoric time periods.
The skull, however, was not associated directly with any of these artifacts, and the researchers are not sure how it got to its perch on the cave ledge. (The cave had been sealed for between 15,000 and 30,000 years before the bulldozer opened it.) So the team had to try to date it using more direct techniques. Fortunately, unlike most other caves in Israel, Manot was apparently continuously wet during the time that early humans occupied it, and so the skull was covered with a thin layer of calcite (calcium carbonate, which is also a major component of stalactites and stalagmites). The team took 11 calcite samples from the outer and inner surfaces of the skull and dated them using the uranium-thorium method, which uses the natural decay of radioactive uranium into thorium as a clock and is often used to date caves. The average age for the samples came out at 55,000 years, putting the fossil smack in the middle of the window of opportunity for mating with Neandertals.
Nor was there much doubt that the skull belonged to a modern human, the team concluded after analyzing its features. For example, the widest part of the skull is high up on the head, and the parietal bones—those that make up much of the side of a skull—were parallel to each other and oriented vertically, features that distinguish the skulls of modern humans from the more curvy skulls of Neandertals. Thus, although the face, jaws, and teeth of the Manot skull are missing, the team was still able to conclude that it most closely resembles that of the modern humans who began colonizing Europe about 45,000 years ago and might even have been their direct ancestors.
“Manot is the first and only modern human securely dated to 50,000 to 60,000 years ago outside the African continent,” Hershkovitz says. That means that the Middle East “was the most likely place for the love affair” between modern humans and Neandertals. Other experts agree. “This is an exciting find, and it certainly falls in a crucial temporal and geographic gap in the human fossil record,” says Katerina Harvati, a paleoanthropologist at the University of Tübingen in Germany. “This is something we did not have [fossil] evidence for earlier.”
Chris Stringer, a paleoanthropologist at the Natural History Museum in London, adds that the partial skull “clearly represents a modern human and closely resembles the skulls of European Cro-Magnons,” the modern humans who took over Western Europe from the Neandertals and possibly helped drive them extinct by about 35,000 years ago.
As for the all-important dating of the skull, Stringer and others think the team probably got it right. “They did a good job,” says Jean-Jacques Hublin, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Hublin is not sure that the Manot fossil represents an ancestor of all the moderns who colonized Europe and Asia, however, because it is not clearly associated with stone artifacts in the cave and thus can’t be clearly assigned to those later, more widespread cultures. He suggests that Manot might represent an earlier “first wave” of colonization, which was “later followed by a second, more successful wave.”
The best way of answering such questions, these researchers say, would be to get ancient DNA out of the Manot skull so it could be compared both Neandertal and modern human genomes. Hershkovitz agrees, although he is not optimistic, because DNA is seldom preserved in the hot climate of the Middle East. “Nevertheless,” he says, “we are planning to give it a try.”