Part of our weekly "In Focus" series—stepping back, looking closer.
The Neanderthals are both the most familiar and the least understood of all our fossil kin.
For decades after the initial discovery of their bones in a cave in Germany in 1856 Homo neanderthalensis was viewed as a hairy brute who stumbled around Ice Age Eurasia on bent knees, eventually to be replaced by elegant, upright Cro-Magnon, the true ancestor of modern Europeans.
Science has long since killed off the notion of that witless caveman, but Neanderthals have still been regarded as quintessential losers—a large-brained, well-adapted species of human that went extinct nevertheless, yielding the Eurasian continent to anatomically modern humans, who began to migrate out of Africa some 60,000 years ago.
Lately, the relationship between Neanderthals and modern humans has gotten spicier.
According to a new study that analyzed traces of Neanderthal DNA in present-day humans, Neanderthals may have been interbreeding with some of the ancestors of modern Eurasians as recently as 37,000 years ago. And another recent study found that Asian and South American people possess an even greater percentage of Neanderthal genes.
"These are complexities in the out-of-Africa story that certainly I would not have anticipated two or three years ago," said Chris Stringer, a paleoanthropologist at the Natural History Museum in London and author of Lone Survivors: How We Came to Be the Only Humans on Earth.
(See pictures of a reconstructed Neanderthal and take a Neanderthals quiz.)
Blurring the Line
In their original incarnation, Neanderthals were viewed as the primitive, backward cave dwellers of Eurasia, far less complex than the sophisticated Homo sapiens who used language and developed sophisticated art as they migrated out of Africa and conquered the world.
But new studies are making it much harder to draw a clean line between us and them.
"It's increasingly difficult to point to any one thing that Neanderthals did and Homo sapiens didn't do and vice versa," said John Shea, an archaeologist at Stony Brook University in New York.
"These Ice Age people, both Neanderthals and Homo sapiens, survived, thrived, and increased their numbers under conditions that would probably kill people nowadays, even ones that are equipped with modern survival technology."
No Hanky-Panky Necessary?
The draft sequence of the Neanderthal genome, published in the journal Science in 2010, provided the first compelling genetic evidence that Neanderthals and H. sapiens had more in common than just an ancestor in Africa hundreds of thousands of years ago.
The researchers, under the direction of Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology, found that 2.5 percent of the genome of an average human living outside Africa today is made up of Neanderthal DNA. The average modern African has none.
This suggested that some interbreeding had taken place between the two kinds of human, probably in the Middle East, where the early modern humans migrating out of Africa would have encountered Neanderthals already living there.
The even larger percentage of Neanderthal DNA found in Asians and South Americans, announced in Science in August, could indicate additional interbreeding in Asia long ago, or could mean that the percentage of Neanderthal DNA in Europeans was diluted by later encounters.
Not everyone is convinced that interbreeding was responsible for similarities in the Neanderthal and H. sapiens genomes. "The similarities they're seeing may be ancient," Shea noted.
Another recent study, published in the journal Proceedings of the National Academy of Sciences in August, calculated that the shared DNA could have come from an earlier, common ancestor of Neanderthals and H. sapiens—no hanky-panky necessary.
A new study by Pääbo's team, published last week in PLOS Genetics, also considered the possibility that the presence of Neanderthal DNA in people living outside Africa today could be traced far back, to the common ancestor of both Neanderthals and modern humans in Africa.
Perhaps the early modern humans who left Africa 60,000 years ago were already genetically more similar to the Neanderthals—who had left hundreds of thousands of years before—than were the modern human populations that stayed behind in Africa. In that case, no interbreeding would have needed to occur to account for the trace of Neanderthal DNA in non-Africans today.
To test the two hypotheses, Pääbo's group analyzed the lengths of segments of Neanderthal DNA in modern Europeans to determine when Neanderthal genes may have mixed with those of modern humans. The date they came up with for the gene flow was 37,000 to 86,000 years ago, and most likely 47,000 to 65,000 years ago.
This date strongly suggests there was indeed interbreeding between "us and them," when H. sapiens was moving into the Middle East from Africa and would have encountered populations of Neanderthals already settled there.
"This [interbreeding] could have been a really powerful mechanism for humans to adapt as they moved into Eurasia," said Sriram Sankararaman, a statistical geneticist at Harvard Medical School and the lead author of the PLOS Genetics study.
Another group, publishing last year in Science, for example, determined that modern humans gained from Neanderthals a family of genes that helps the immune system fight off viruses. Breeding with the locals could have unwittingly given H. sapiens a survival advantage in a new land.
"[Neanderthals] are not just some extinct group of related hominids," Pääbo said. "They are partially ancestors to people who live today."
Take any two unrelated humans today, Pääbo noted, and they'll differ in millions of places in their genetic code. But the Neanderthal genome varies on average from that of H. sapiens in only about a hundred thousand positions. Pääbo and his colleagues are now trying to figure out the consequences of those differences.
(Related: "Sex With Humans Made Neanderthals Extinct?")
Act Like a Man?
Regardless of the similarities to our DNA, how "human" were Neanderthals in their sensibilities?
Last month a study led by the Gibraltar Museum and published in PLOS ONE documented a multitude of fossil remains of bird wings, particularly from big black raptors, at Neanderthal sites in southern Europe. The team suggested that Neanderthals could have been plucking feathers from the wings for personal use or even for ritual ornaments.
"We have other evidence for Neanderthals preferring mineral pigments that are dark, blackish color," Stony Brook's Shea said. "There may be something for them with the color black just as there seems to be something for us with the color red."
(Related: "Neanderthals' Last Stand Was in Gibraltar, Study Suggests.")
Sophisticated art, however, still appears to remain in the realm of H. sapiens.
The ancestors of modern humans left behind images of animals and other objects in caves around the world, most famously at Lascaux cave and Chauvet Cave (pictures) in southern France. Paintings in the latter cave could be as ancient as 37,000 years old. (See a prehistoric time line.)
Images found in a cave called El Castillo on the Spanish coast were recently dated at more than 40,800 years old: a time before Neanderthals disappeared, raising the tantalizing possibility that they were indeed the artists. However, "it hasn't been demonstrated that Neanderthals produced any of that cave art," the Natural History Museum's Stringer said.
The simpler answer is that H. sapiens, who had also reached Europe by that time and are known to have produced later but similar art, were responsible.
Neanderthals, though, have proven advanced in other ways.
They used pigments and may have made jewelry; some made complex tools. "We know they buried their dead," Stringer said. In 2010, researchers from the Smithsonian Institution even found evidence that the Neanderthal diet included a diverse mixture of plants, and that they cooked some of the grains. (Related: "Neanderthals Ate Their Veggies, Tooth Study Shows.")
"Cooking something like oatmeal is not what we would have imagined," said John Hawks, paleoanthropologist at the University of Wisconsin-Madison. With no pots, Neanderthals may have cooked inside leaves, Hawks suggested. "That starts to sound like cuisine."
"Neanderthals have gone from being different from us to being like us," Hawks noted. "They're looking like [Homo sapiens] hunter-gatherers look."
But while modern humans continued to develop cultural complexity and spread across the globe, the Neanderthals vanished. Why remains a mystery.