In 1856, Johann Carl Fuhlrott, a schoolteacher and natural historian, was puzzled by some bones that quarrymen working in the Neander Valley in Germany brought to him.
They didn't look like the bones of homo sapiens that had been found in the area as well. In fact, he could see at a glance that they were almost certainly much older. He deduced they were the bones of a "primitive human."
Fuhlrott sent the remains to Hermann Schaaffhausen, an anatomy professor at the University of Bonn, for a professional medical analysis. Together, they presented the findings in 1857, arguing that the bones belonged to a race of humans that existed before the "modern" era.
In 1864, William King, an Anglo-Irish geologist, was the one who actually proposed the scientific name Homo neanderthalensis. He noticed the skull's unique features (like the heavy brow ridge) were distinct enough from modern humans to warrant a separate species classification.
Once carbon dating was used to date the Neanderthal bones, paleontologists realized that modern Homo sapiens and Homo neanderthalensis occupied the same general areas at the same time. Could there have been interbreeding of the two species?
The answer to that question awaited the modern science of genome mapping. The theory was confirmed in 2010 when analysis of the first draft of the Neanderthal genome showed that interbreeding likely occurred in the Middle East shortly after modern humans migrated out of Africa, around 50,000 to 60,000 years ago.
Another breakthrough occurred in 2014 when scientists sequenced the DNA of a 40,000-year-old human jawbone found in Romania. They discovered he had a Neanderthal ancestor just four to six generations back (essentially a Neanderthal great-great-great-grandparent).
New studies have pushed the timeline of these interactions even further back. A new genetic analysis published in the journal Science suggests that humans and Neanderthals were mating as early as 250,000 years ago.
Why do some modern humans show more recent Neanderthal DNA than others? In some cases, far more recent.
Another genetic analysis published in 2026 revealed the remarkable evidence that it was human females mating more often with Neanderthal males. "Did human women venture into Neanderthal populations, or were the Neanderthal males drawn to larger human enclaves?" asks Associated Press reporter Adithi Ramakrishnan." Were these interactions peaceful, confusing, secretive or even violent?"
“I don’t know if we’ll ever get a definitive answer to how this happened, since we can’t travel back in time,” said population genetics expert Xinjun Zhang with the University of Michigan, commenting on the new analysis.
But the study, published Thursday in the journal Science, shows “that whenever Neanderthals and modern humans have mated, there has been a preference for male Neanderthals and female modern humans, as opposed to the other way around,” said author Alexander Platt, who studies genetics at the University of Pennsylvania.
Scientists know that Neanderthals and humans mated because there is a small but important percentage of Neanderthal DNA in most modern humans outside of sub-Saharan Africa — including genes that can help us fight some diseases and make us more susceptible to others.
But they have also known that the Neanderthal DNA is not distributed evenly throughout the human genome.
Scientists discovered, through gene mapping, a lack of Neanderthal DNA in the human "X" chromosome, which determines sex. Perhaps the Neanderthal DNA in the "X" chromosome wasn't beneficial and was overwritten in modern humans.
Or could it be a clue to how the two species intermingled?
To try to solve the riddle, Platt and colleagues looked instead at the Neanderthal genome and the human DNA that got interspersed during a “mating event” 250,000 years ago.
When comparing these genes, they found more of a human fingerprint on the Neanderthal X chromosome — the same chromosome that, in humans, has less Neanderthal DNA than would be expected.
The most likely explanation for this mirror image pattern is mating behavior. That’s because of the way sex chromosomes are passed from parents to children, explained Platt. Because genetic females have two X chromosomes and genetic males have one X and one Y chromosomes, two out of every three X chromosomes in a population, on average, are inherited from people’s mothers.
Until or unless we can figure out why human females mated more often with Neanderthal men, the mystery of the nature of human-Neanderthal interactions will remain. We know that there was a time toward the end of the Neanderthals (30,000 years ago) when both species were competing for the same resources. Some paleoanthropologists believe that modern humans, with better weapons and more potent germs, were able to eliminate the competition through violent confrontations.
That may have been part of the answer. There was also a changing climate that caused the megafauna to become extinct. Once their game disappeared, the Neanderthals soon followed.
For whatever reason, Neanderthal populations dwindled until inbreeding and low population density eventually caused their extinction. Or did it?
We carry from 1% to 4% of Neanderthal DNA in our bodies. In that sense, they didn't disappear. We simply absorbed them.
