By sequencing the genome of an ancient human with unparalleled accuracy, scientists have for the first time revealed the relationship between her species and our own.
In 2010, Svante Pääbo of the Max PlanckInstitute and his colleagues sequenced DNA from a finger bone fragment discovered in the Denisova Cave in southern Siberia. They found that it belonged to a young girl of a previously unknown group of archaic humans, dubbed Denisovans.
By splitting the DNA double helix and using each of its two strands, the team was able to sequence every position in the Denisovan genome about 30 times over - making the analysis as accurate as one for a present-day human.
"This is an extinct genome sequence of unprecedented accuracy," says Matthias Meyer, the lead author of the study. "For most of the genome we can even determine the differences between the two sets of chromosomes that the Deniosovan girl inherited from her mother and father."
The analysis shows that there was little genetic variation amongst Denisovans, suggesting that despite spreading throughout large parts of Asia, their population was never large for long periods of time.
It also shows the genetic changes - some associated with brain function or nervous system development - that distinguish modern humans from their ancient relatives.
For their latest study, the team compared the Denisovan genome with those of the Neanderthals and eleven modern humans from around the world.
Their findings confirm previous work showing that modern populations from the islands of southeastern Asia share genes with the Denisovans.
In addition, the genomes of people from East Asia and South America include slightly more genes from Neanderthals than those of people in Europe.
The analysis reveals that there was less genetic variation amongst the Denisovans than in present-day humans, probably because an initially small Denisovan population grew quickly while spreading over a wide geographic range.
"If future research of the Neanderthal genome shows that their population size changed over time in similar ways, it may well be that a single population expanding out of Africa gave rise to both the Denisovans and the Neanderthals," says Pääbo.
"This research will help determining how it was that modern human populations came to expand dramatically in size as well as cultural complexity, while archaic humans eventually dwindled in numbers and became physically extinct."