By Brian Switek, National Geographic
PUBLISHED October 19, 2013

Researchers, led by Georgian National Museum anthropologist David Lordkipanidze, first found the complete lower jaw of a fossil human in 2000. The cranium turned up five years later, at the fossil-rich Dmanisi site 96 miles southwest of Tbilisi, and is now being reported in the journal Science.

«It was discovered on August 5, 2005—in fact, on my birthday,» Lordkipanidze says. He adds that the fossil’s importance was clear as soon as the team saw it, but required eight years of preparatory analysis.

That is because Skull 5 is what paleoanthropologists often refer to as a «mosaic,» or mixture of features seen in earlier and later humans. The skull’s face, large teeth, and small brain size resemble those of earlier fossil humans, but the detailed anatomy of its braincase—which gives clues to the wiring of the brain—is similar to that of a more recent early human species called Homo erectus. This combination of features has fueled a long-running discussion over whether the Dmanisi humans were an early form of Homo erectus, a distinct species called Homo georgicus, or something else.

Dmanisi Fossil Trove

The newly described skull isn’t the only one that has been found at Dmanisi. At least five relatively complete skulls have been found there in the last two decades. Those individuals may not have actually lived alongside each other, but apparently occupied this same place within a window of a few thousand years more than 1.75 million years ago.

Lordkipanidze and his coauthors suggest that, taken together, these skulls demonstrate how the Dmanisi humans varied in appearance from one individual to the next. «Together, our analyses suggest that Skull 5 and the other four early Homo [human] individuals from Dmanisi represent the full range of variation within a single species,» study senior author Christoph Zollikofer of Switzerland’s University of Zurich, said at a briefing on the new skull discovery.

Using morphometrics to gauge skull shape for each fossil skull, Lordkipanidze and colleagues found that the Dmanisi humans varied from each other in facial features and brain size, for example, about as much as modern humans do from each other. In other words, despite minor differences, they all belonged to the same species.