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When examined with the earlier Georgian finds, the skull “shows that this special immigration out of Africa happened much earlier than we thought and a much more primitive group did it,” said David Lordkipanidze, the study’s lead author and director of the Georgia National Museum. “This is important to understanding human evolution.”

For years, some scientists have said humans evolved from only one or two species, much like a tree branches out from a trunk, while others say the process was more like a bush with several offshoots that went nowhere.

Even bush-favouring scientists say these findings show one species nearly two million years ago at the Georgian site. But they disagree the same conclusion can be made for bones found elsewhere, such as Africa.

Mr. Lordkipanidze and colleagues point out the skulls found in Georgia are different sizes but are considered to be the same species. So, they reason, skulls found in different places and times in Africa may not be different species, but variations in one species.

To see how a species can vary, just look in the mirror, they said.

The adult male skull found was not from our species, Homo sapiens. It was from an ancestral species of the Homo genus that led to modern humans. Scientists say the Dmanisi population is likely an early part of our long-lived primary ancestral species, H. erectus.

Fred Spoor at the Max Planck Institute in Germany, a competitor and proponent of a busy family tree with many species disagreed with the study’s overall conclusion, but lauded the Georgia skull discovery as critical and even beautiful.

“It really shows the process of evolution in action,” he said.

The Associated Press