The discovery, along with the remains of four other individuals who lived at the same time, in the same place, has generated intense excitement among palaeontologists who believe the finds could re-write the early history of human evolution.
The skull and its lower jawbone were found at a palaeontology site near the medieval town of Dmansi in the foothills of the Caucuses in Georgia, which has become one of the most important centres for understanding human origins outside Africa.
A team of researchers has spent the past eight years studying the fossil skull, which was first excavated in 2005, and its jawbone, discovered in 2000 but only now re-united with its "owner".
The first scientific description of "skull 5", published in the journal Science, indicates that the adult male had a large, long face, heavy features, large jaw and teeth but an exceptionally small brain case, less than half of the size of a typical human today and not much bigger than a gorilla's brain.
The four other skulls are thought to have belonged to an elderly, toothless male, another adult male, a young female and an adolescent of unknown sex. The scientists do not know whether they were part of the same family group or had lived at precisely the same time.
Dating technology based on argon isotopes found that they lived between 1.77m and 1.85m years ago. The site was next to a river where big game and fierce predators made frequent encounters - bones of large saber-toothed cats and an extinct giant cheetah have also been found alongside those of large herbivores.
All five skulls were unearthed from what were probably underground dens where the large carnivores had dragged their prey. These had collapsed soon afterwards, which had helped to preserve the animal and human remains in good condition.
"Dmansi is a unique snapshot of time - maybe a time capsule that preserves things from 1.8 million years ago," said Professor David Lordkipanidze of the Georgia National Museum in Tblisi, the lead author of the study.
"This was a place where was big competition between carnivores and hominins [ancient humans]. It seems that they were fighting for the carcasses, and unfortunately for the hominins, but fortunately for us, they were not always successful," Professor Lordkipanidze said.
What has surprised the scientists is the range of physical variation between the five individuals. This has led them to suggest that several other early human species living at about the same period in Africa may in fact all belong to the same species.
"Had the braincase and the face of skull 5 been found as separate fossils at different sites in Africa, they might have been attributed to different species," said Christoph Zollikofer, a neurobiologist at the Anthropological Institute and Museum in Zurich.
"The [five skulls from Dmanisi] look quite different from one another, so it's tempting to publish them as different species. Yet we know that these individuals came from the same location and the same geological time, so they could, in principle, represent a single population of a single species," Professor Zollikofer said.
Despite their anatomical differences, 3-D modelling of the five skulls shows that they would only have been as physically varied as any five individuals chosen at random from a population of either modern humans or chimpanzees.
"Thanks to the relatively large Dmanisi sample, we see a lot of variation. But the amount of variation does not exceed that found in modern populations of our own species, nor in chimps and bonobos," Professor Zollikofer said.
"Furthermore, since we see a similar pattern and range of variation in the African fossil record, it is sensible to assume that there was a single Homo species at that time in Africa," he said.
"And since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species," he added.
The scientists suggest that the Dmanisi individuals probably belonged to Homo erectus, the first human species to emerge from Africa, and that certain African species such as Homo habilis, which had lived earlier than H. erectus, may actually be the same species as Dmanisi man.
Ian Tattersall of the American Museum of Natural History in New York told Science that the latest, fifth skull from Dmanisi is "undoubtedly one of the most important ever discovered", while Tim White of the University of California, Berkeley, described it simply as "an iconic fossil".
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