In 1856, laborers working in a limestone quarry near Düsseldorf, Germany, unearthed bones that scientists initially thought belonged to a deformed human.
The skull was oval shaped, with a low, receding forehead, distinct brow ridges, and bones that were unusually thick.
Subsequent study revealed that the remains belonged to a previously unknown species of hominid, or early human ancestor, that was similar to our own species, Homo sapiens.
In 1864, the specimen was dubbed Homo neanderthalensis, after the Neander Valley where the skull was discovered.
Neanderthals were our closest evolutionary relatives. Their ancestors left Africa before modern humans, venturing into Europe as far back as 500,000 years ago, and were still there when our ancestors embarked on the same journey about 70,000 years ago.
Neanderthals and modern humans actually lived alongside each other in Europe for several thousand years before Neanderthals vanished some 30,000 years ago.
Their disappearance is one of the most enduring mysteries in all of human evolution.
But perhaps the most controversial theory for why there are no more Neanderthals is that they interbred with modern humans and the two lineages merged into one.
According to this idea, most of modern humanity—with the possible exception of some Africans who are descended from humans who never left Africa—is part Neanderthal.
Evidence for interbreeding comes largely from the study of fossils that, according to some scientists, show hybrid traits from both species.
For example, anthropologist Erik Trinkhaus believes that a 29,000-year-old skull discovered in Romania belonging to a modern human has an unusually long and flat forehead and unusually large molars.
There is some genetic evidence to support the interbreeding theory as well. In 2010, a team of scientists comparing a rough draft of the Neanderthal genome with that of modern humans concluded that most humans have 1 to 3 percent Neanderthal DNA.
The team suggested that the first opportunity for Neanderthal-human interbreeding probably occurred about 60,000 years ago, after modern humans had left Africa but before they had made significant inroads into Europe.
However, recent computer models suggest the genetic similarities shared between Neanderthals and modern humans could also be due to the two species sharing a recent common ancestor rather than hybridization.