Now they have got a face.
Using genetic data, scientists have now produced reconstructions of these long-lost relatives.
"We provide the first reconstruction of the skeletal anatomy of Denisovans," said study author Liran Carmel of the Hebrew University of Jerusalem in Israel.
"In many ways, Denisovans resembled Neanderthals, but in some traits, they resembled us, and in others they were unique," Carmel said.
Overall, the researchers identified 56 anatomical features in which Denisovans differed from modern humans and/or Neanderthals, 34 of them in the skull, according to a report published in the journal Cell.
For example, the Denisovan's skull was probably wider than that of modern humans or Neanderthals. They likely also had a longer dental arch.
Rather than relying on DNA sequences, the researchers extracted anatomical information from gene activity patterns.
Those gene activity patterns were inferred based on genome-wide DNA methylation or epigenetic patterns.
To test the method the researchers developed, they first applied it to two species whose anatomy is known: the Neanderthal and the chimpanzee.
They found that roughly 85 per cent of the trait reconstructions were accurate in predicting which traits diverged and in which direction they diverged.
By focusing on consensus predictions and the direction of the change, rather than trying to predict precise measurements, they were able to produce the first reconstructed anatomical profile of the little-understood Denisovan.
The evidence suggests that Denisovans likely shared Neanderthal traits such as an elongated face and a wide pelvis.
It also highlighted Denisovan-specific differences, such as an increased dental arch and lateral cranial expansion, the researchers said.
"Studying Denisovan anatomy can teach us about human adaptation, evolutionary constraints, development, gene-environment interactions, and disease dynamics," Carmel said.
( With inputs from IANS )