Scientists in Germany have discovered a new human species that lived in Siberia around 30,000 to 50,000 years ago.
The evidence changes the picture of pre-human habitation on Earth.
The species lived at the same time as modern humans and Neanderthals and shared a common ancestor with them.
Scientists found the fossilised remnants of a tiny finger bone that probably belonged to a small child.
Dr Johannes Krause of the Max Planck Institute for Evolutionary Anthropology in Germany is the author of a paper on the finding which has been published in Nature.
He says the fossil was like nothing he had seen before.
"I did several analyses to make sure that this DNA is authentic, that this DNA is really old and that this DNA is what had seemed to be a new hominine lineage," he says.
"A new discovery, different from human DNA and different from Neanderthal DNA, for example."
His colleague and co-author, Professor Svante Paavo, was not initially convinced.
"It was of course absolutely amazing and I first really didn't believe him. I thought he was pulling my leg," he says.
The discovery crucially suggests the species were living at the same time and in the same place as modern humans and Neanderthals.
The deputy director of the INST at the University of Adelaide, Dr Jeremy Austin, says it is a very significant find.
"They've found this hominine lineage. It's not a modern human. It's not a Neanderthal. It's not a chimpanzee. It's something sort of halfway in between, at a time period when no-one thought that these sort of ancestral human forms were still alive," says Austin.
Paavo says the new species shared a common ancestor with Neanderthals and modern humans. The common ancestor left Africa about 1 million years ago.
"Maybe it is oversimplification to think about particular migrations out of Africa," he says.
"There might have been more or less continuous flow, and the picture that's going to emerge in the next years might be a much more complex one."
It was only a few years ago that the 'Hobbit of Flores' was discovered and proclaimed a new species.
Austin says with the discovery of this new human in Siberia, scientists are adjusting to the possibility that four species were living at the same time when previously it was thought there were only two.
"If all this paper does is say there's a whole pile of diversity out there, essentially there's more human species we don't know about, then that's a really important finding."