It took just one skull, but it allowed anthropologists to drastically simplify our evolutionary family tree. The 1.8-million-year-old specimen was unearthed in the Eurasian country Georgia, and pored over by an international team of scientists led by Georgian paleoanthropologist David Lordkipanidze for eight years. It’s a missing link, of sorts, tying together as a single species other early, diverse fossils that were previously identified as belonging to distinct species. With this new skull, anthropologists think those fossils might, in fact, represent a wide spectrum of traits from a single evolutionary line.

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Experts say this cranium, known as Skull 5, one of the most important ever discovered—it’s completely preserved, adult, and very old. Lordkipanidze and his team compared it with other contemporary fossils. Here’s John Noble Wilford writing for The New York Times:

The discovery of Skull 5 alongside the remains of four other hominids at Dmanisi, a site in Georgia rich in material of the earliest hominid travels into Eurasia, gave the scientists an opportunity to compare and contrast the physical traits of ancestors that apparently lived at the same location and around the same time.

Dr. Lordkipanidze and his colleagues said the differences between these fossils were no more pronounced than those between any given five modern humans or five chimpanzees. The hominids who left the fossils, they noted, were quite different from one another but still members of one species. Skull 5 has a long, apelike face, large teeth and a tiny braincase. If scientists had found these elements as separate fossils across Africa, they would likely label them as coming from different species. But they were found together, with other Dmanisi fossils from the same time.

“Since we see a similar pattern and range of variation in the African fossil record,” Dr. Zollikofer continued, “it is sensible to assume that there was a single Homo species at that time in Africa.” Moreover, he added, “since the Dmanisi hominids are so similar to the African ones, we further assume that they both represent the same species.” They’re calling this unified species “early Homo,” or perhaps a “primitive Homo erectus .” Regardless, this is a landmark discovery in scientists’ search for our deepest roots. There’s still plenty to be sorted out, though, including the chicken-and-egg problem of which came first, a large brain or walking? Some scientists say a large brain forced early humans into an upright posture as they foraged for more nutritious food, while other say the opposite, that walking upright allowed them to find better food which allowed their brains to grow larger. Skull 5 doesn’t definitively answer that, but at least we’re one step closer to a more complete family tree.

Photo Courtesy Georgian National Museum