Charles Darwin was not sold on this definition, say some historians, though it took decades for his looser concept of species to catch on. Today, biologists appreciate that the separation of species is a process, said University of Pennsylvania biologist Paul Schmidt. Groups can diverge and cross back all the time, sometimes producing a mix of fertile and infertile offspring.
"A biologist would say, 'Yeah, this is not a big deal - it happens all the time,' " he said. But such messy relationships between species may be a little harder to accept when it comes to human beings.
In the past, even those who accepted that we were not designed by a deity could revel in our image as the end point of a powerful evolutionary branch, a winner that prevailed in the game of natural selection.
Turns out we tangled with our neighboring branches, just as other animals have.
The evidence that Neanderthals mixed into human ancestry comes from a comparison between modern human DNA and DNA extracted from 38,000-year-old Neanderthal bones, part of an ongoing "Neanderthal Genome Project."
The researchers said they were expecting a confirmation of the prevailing story of humanity as a young species that arose in Africa around 100,000 years ago and started expanding outward, replacing various archaic "species" of humans in Europe, Asia and the Middle East.
The most famous of the purported losers was the Neanderthal, whose ancestors split from ours about 400,000 years ago. Over the intervening time, evolution shaped them to be stockier, heavier and stronger than modern humans. The word Neanderthal has become synonymous with brutishness and stupidity, and yet their brains were slightly larger than ours.
Then, about 50,000 to 80,000 years ago, members of the group known as anatomically modern humans began to move out of Africa and into Neanderthal territory. Neanderthals appeared to have gone extinct around 24,000 years ago.
This latest genetic analysis indicates they didn't die out completely but mixed with our ancestors, thereby leaving all non-Africans a bit of Neanderthal DNA.
The researchers who carried out the analysis shied away from talking about what this meant for us as a species. "It's hard to answer whether we're a different species or a sub-species," said geneticist Richard Green of the University of California, Santa Cruz. "It gets to the point really where we have to ask ourselves whether these labels are useful."
To Alan Templeton, a Washington University biologist, this latest DNA analysis confirms what he has long argued - that Neanderthals are part of our species and, more generally, that we need to re-think the idea of species as it applies to us.
For years, Templeton has been comparing the DNA of people from around the world, looking for patterns that reveal aspects of our evolution. From that, he said, evidence was already mounting against the "total replacement" story of our past. But it was hard for many to let go.
"The idea of modern humans coming out and conquering the whole world seemed to have a very strong emotional appeal to people," he said.
Beyond that, he said, our messy past with Neanderthals shakes up our old-fashioned notions about purity. "The idea that there are pure groups or pure races is total nonsense," he said, but it is still ingrained in our culture. The reality is that living things tend to cross when given the chance.
Templeton said that when it comes to distinguishing species, scientists apply different criteria to other animals than they do to us. For example, different groups of common chimps are much more genetically distinct from each other than modern humans are from Neanderthals, and yet we have no trouble lumping all the common chimps into one species.
We have a history of applying the notion of species arbitrarily, he said, often using it to make science conform with some other agenda. "During the age of exploration, Europeans went out and saw pygmies and Australian aborigines and other groups and thought of them as different species," he said.
Princeton University anthropologist Alan Mann said he considered our attitude toward Neanderthals an extension of 18th- and 19th-century racism.
"Neanderthals have been getting really poor press," he said. "As society moved away from thinking of non-Europeans as being inferior - or not being attractive - Neanderthals have taken their place."
He considers Neanderthals as human as we are. New evidence shows they used pigments, made jewelry, and probably talked. An earlier genetic analysis showed that they share with us a version of a gene, called FOXP2, thought crucial for the development of language and lacking in all non-human animals.
If Neanderthals were human, who else should we include in the club?
Traditionally, anthropologists granted humanity not just to our species but to our broader taxonomic category. Our genus, Homo, is Latin for human.
The other members of our genus were upright walkers, some of whom used fire, hunted, gathered in bands, and possibly to some extent talked.
But categorizing living things by genus is even more arbitrary and artificial than classifying them by species, said Owen Lovejoy, professor of anthropology at Kent State University. These categories are made up to help people organize the world, he said, but they are not real.
"Taxonomy is the enemy of wisdom as far as I'm concerned," he said. For him, the question of who is human is outside of science. "What does the word human mean anyway?"
What science is telling us is we're not the pinnacle of evolution but a snapshot in time, a chapter in an ongoing story. Washington University's Templeton said this Neanderthal mixing was just one twist in a complicated past, full of separations and reunions, divergence and cross-breeding.
Why, then, did Neanderthals contribute just a little to our genetic heritage?
It may be that there were fewer of them and their genes were swamped by this bigger group of humans who left Africa later, Princeton's Mann said.
Neanderthals hung on for thousands of years in some frigid and harsh parts of Eurasia. "They really were an amazingly adaptive and successful group," he said. "It's a form of racism that we have played down their abilities to enhance our own."
Contact staff writer Faye Flam at 215-854-4977 or firstname.lastname@example.org.