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.

Neanderthals were shorter than modern humans, and had barrel chests, stocky limbs, and large noses—traits that were well suited to the frigid climes of Europe during the last Ice Age. Ancient DNA retrieved from the bones of two Neanderthals suggests that at least some of them had red hair and pale skin.

Neanderthals have long been depicted as powerfully built brutes that were able hunters, capable of taking down large, dangerous prey, but too primitive to have exhibited modern human behaviors. But starting in the 1950s, scientists began making a series of discoveries that have challenged the stereotype that has long plagued Neanderthals.

In 1957, anthropologists digging in Shanidar cave in northern Iraq discovered the remains of eight adults and two infant Neanderthals that appeared to have been purposefully buried some 60,000 years ago. Fossilized pollen found at the site even suggested that the dead had been buried with flowers. What’s more, some of the adult skeletons showed evidence of injuries that had been tended and healed—which suggests Neanderthals cared for their sick and wounded.

Recent findings also suggest Neanderthals used a diverse set of stone tools, controlled fire, appreciated music, and may have even communicated through song, and that they knew about the medicinal qualities of certain plants. Prehistoric art on Spanish cave walls—consisting of dots and crimson hand stencils—dating back to 41,000 years ago might also have been the work of Neanderthal artists. With so much evidence in favor of their humanity, a growing number of scientists have argued that the Neanderthals’ similarities to modern humans far outweighed any differences, which makes their disappearance all the more baffling.

Neanderthal Vanishing Act

After first venturing out of Africa, Neanderthals thrived in Europe for several hundred thousand years. Some even ventured into Asia, traveling as far east as Siberia. But they mysteriously died out about 30,000 years ago. The last members of the species may have survived in Gibraltar until about 28,000 years ago, but eventually they too disappeared.

Neanderthals vanished around roughly the same time that modern humans arrived in Europe, and many scientists suspect the two events are closely linked. One popular theory is that modern humans contributed to the demise of their close cousins, either by outcompeting them for resources or through open conflict. A recent study of Shanidar 3, one of the Neanderthals discovered in the Iraqi cave in 1959, for example, concluded that he likely died of a rib wound that could have been created by the kind of projectile weapons used by modern humans living at the time.

But in 2006, a discovery was made that challenged this theory. Scientists excavating a site in the Republic of Georgia discovered thousands of bones and teeth belonging to mountain goats that looked as though they had been killed by Neanderthals. The animals were largely prime-age adults, meaning they were large, fast, and hard to capture, so bringing them down would have required considerable skill. Such able hunters, the scientists argued, would not have made easy victims for modern humans.

Another idea is that Neanderthals died out because they couldn’t reproduce fast enough to keep up with modern humans, who eventually outnumbered them. Some studies suggest that even at its maximum, the Neanderthal population only numbered a few thousand. One study of Neanderthal skeletons found that the species grew quickly but reached sexual maturity later than modern humans. Furthermore, Neanderthals may have had a harder time during childbirth due to the head shape of Neanderthal babies.

Yet another theory suggests Neanderthals may have just been the unlucky victims of an environmental catastrophe that mainly affected them but spared modern humans. According to one proposed scenario, several volcanoes in Europe could have erupted in quick succession about 40,000 years ago, before modern humans arrived in Europe. Volcanic ash from the eruptions would have sterilized the land, causing plants—and the mammals that fed on them—to die. Neanderthals, who hunted the animals for food, would have shortly followed.

Did Humans and Neanderthals Interbreed?

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.