A fragment of a bone of their offspring, a female and the first-known hybrid child of the distinctly different ancient humans, was recently found in a cave in Siberia. They lived in Eurasia, the denisovans in the East, Neanderthals in the West. Of these few individuals, two are known to either be hybrids or to have had close relatives of a different human species.
What we know about Neanderthals is plenty: They were short, stocky, had large noses (for European winters in the Ice Age), and larger bodies than modern humans.
In each pair of chromosomes, one came from a Neanderthal, the other from a Denisovan.
Still, this scant evidence is enough to show that Denisovans and Neanderthals split from a common ancestor roughly 390,000 years ago, Wei-Haas writes, and to point toward both species' eventual decline around 40,000 years ago.
Nicknamed by Oxford University scientists, Denisova 11 - her official name - was at least 13 when she died, for reasons unknown.
The girl's story has been pieced together from a single fragment of bone found in the Denisova cave by Russian archaeologists several years ago.
"The fragment is part of a long bone, and we can estimate that this individual was at least 13 years old".
This might be because we only discovered the Denisovans in 2010, and have so few fossils to work that we don't even know what they looked like.
"I think they convincingly showed that genetically this individual falls halfway between the Neanderthal and Denisovan fossils found in the same cave", he said. To date, only a handful of bones and teeth have been unearthed from the Denisova Cave in the Altai Mountains of Siberia.
Viviane Slon, a paleogeneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, who did the ancient DNA analysis, says when she saw the results, her first reaction was disbelief.
'Neandertals and Denisovans may not have had many opportunities to meet, ' says Pääbo.
"Now we've got evidence. they would interbreed at a level suggesting that they might probably be considered the same species".
"When you find a needle in a haystack, you have to start wondering if what you're really looking at is a needlestack", John Hawks, an anthropologist at the University of Wisconsin-Madison who was not involved in the study, tells The Atlantic's Zhang.
"It will be good a day when we find the fossil skulls of these Denisovans to work out what hominin they are".
Now, further analysis of one of the bone fragments from the cave has been carried out.
The Denisovans are more mysterious. Thus far they have only been found in Denisova Cave in Sibera as tiny bone fragments.
Today, around two percent of DNA in non-Africans across the globe originates from humans' Neanderthal ancestors. 'But when they did, they must have mated frequently - much more so than we previously thought'.
The DNA of this girl - Denisova 11 - also suggests that there was some quite significant movement of Neanderthal groups between Western Europe and the East. Analysis of her DNA found that rather than being more closely related to a Neanderthal who lived in her home cave sometime prior to her birth, she instead showed more connections to those recovered in Western Europe. For thousands of years, they remained distinct for various possible reasons.