Previously it was believed that there were several different human species living on Earth, including Homo habilis, Homo rudolfensis and Homo erectus.
But scientists who have studied the skull found in Dmanisi, the most well preserved humanoid skull ever uncovered, say it shares characteristics with both H.hamilis and H.erectus.
Writing in the journal Science, the researchers said the skull suggests H.rudolfensis, H.erectus and H.habilis all belong to a single evolving lineage from which modern humans evolved.
The skull was found eight years ago in a site from which researchers have uncovered one of the biggest collections of human remains in the world.
The fossil remains found at the site showed a lot of variation in characteristics and shape that previously confused archaeologists.
But lead author of the study Prof David Lordkipanidze, from the Georgian National Museum in Tbilisi, said it was clear that these features were all from one population.
"When we looked at this variability and compared it with modern humans, you can see this is a normal range of variation," Prof Lordkipanidze told BBC News.
Co-author Christoph Zollikofer from the University of Zurich said: "The five Dmanisi individuals are conspicuously different from each other, but not more different than any five modern human individuals, or five chimpanzee individuals, from a given population.''
But some experts disagree with the conclusions drawn.
Bernard Wood, director of the hominid paleobiology doctoral program at George Washington University, says the skull is from a creature "that we have not seen evidence of before".
"It could be something new and I don't know why they are reluctant to think it might be something new," he said.Do you have any story leads, photos or videos?