A tiny bone fragment found in a Siberian Cave belonged to the first-known hybrid child born of different ancient humans.

Key points: DNA reveals first-generation Neanderthal/Denisovan hybrid child

DNA reveals first-generation Neanderthal/Denisovan hybrid child The discovery suggests the two groups mingled and mated a lot more frequently

The discovery suggests the two groups mingled and mated a lot more frequently But questions still remain as to who the Denisovans were

A DNA analysis shows the young girl's mother was Neanderthal and her father was another mysterious ancient human known as a Denisovan.

While we've known for a long time the two groups intermingled from traces of their DNA in modern humans, this is the first direct evidence of their dalliances.

"We knew from previous studies that Neanderthals and Denisovans must have occasionally had children together," the study's lead author Viviane Slon from the Max Planck Institute for Evolutionary Anthropology, said in a statement

"I never thought we'd be so lucky as to find an actual offspring from the two groups."

Dr Slon and her colleagues analysed DNA extracted from a fossil known as Denisova 11, which was unearthed in the Denisova Cave in 2012.

The bone — which was probably a leg bone — appears to be from a girl aged around 13 years old who was born at least 50,000 years ago.

DNA tests on the bone fragments show they belong to the child of a Neanderthal mother and a Denisovan father ( T. Higham, University of Oxford )

The DNA analysis reveals the girl's mother was more closely related to Neanderthals from western Europe than Neanderthals that occupied the Denisova cave sometime earlier.

And her Denisovan father also had Neanderthal in his DNA.

The researchers said the findings, published on Thursday in Nature, indicates Neanderthals from the east and west migrated and bred with the Denisovans.

And they appear to have hooked up a lot more often than we thought, said David Lambert from the Australian Research Centre for Human Evolution at Griffith University, who was not involved in the research.

"There's always been a kind of implicit understanding without a whole lot of evidence that these hybridisation events were rare," said Professor Lambert.

"But this makes you start to wonder if they did interbreed whenever they came into contact."

Who were the Denisovans anyway?

Neanderthals and Denisovans separated from each other around 400,000 years ago.

But very little is known about the Denisovans, and when they overlapped or co-existed with Neanderthals, Professor Lambert said.

The only known fossils are a finger bone, three teeth and the long bone fragment from five different individuals excavated from a single cave in the Altai Mountains.

"Denisovans are so rare, so we don't know whether Denisovans and Neanderthals had an extensive overlap over long periods of time," he said.

"We've only got the excavations in the Denisova cave."

Fossils of five Denisovans and one Neanderthal have been found in Denisova cave ( Wikimedia: )

Dr Lambert said the new discovery also raised questions about who the Denisovans were.

"There's always been some groups ... who think that Denisovan people were just the eastern branch of Neanderthals, that they weren't a distinct species," he said.

"Now we've got evidence ... they would interbreed at a level suggesting that they might probably be considered the same species."

They may not even be Neanderthals or Denisovans, said Michael Westaway, also from Griffith University.

"It makes you wonder if the Denisovans' genomes aren't simply Homo erectus [another earlier species of human, also known as Java man]," said Dr Westaway, who was not involved in the research.

"It will be good a day when we find the fossil skulls of these Denisovans to work out what hominin they are."

Neanderthal and Denisovan legacy lives on

What we do know is that both Neanderthals and Denisovans disappeared around 40,000 years ago.

But their dalliances with modern humans left their mark.

Today, a small percentage of Denisovan DNA is found in Australian Aboriginals and people from Melanesia and Papua New Guinea.

"The Denisovans were absorbed into Neanderthals and modern humans," Dr Lambert said.

"The three groups are starting to look like groups that might have interchanged huge amounts of DNA over long periods of time."

He said the new findings may change the way researchers look at genetic lineages.

"Rather than wondering whether [genetic sequences in modern humans] came from Neanderthal or Denisovan, maybe they came from hybrid individuals."