A 1.8 million-year-old skull of a human ancestor, discovered in Georgia, indicates that immigration out of Africa happened much earlier than previously thought.

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The fossil is the most complete pre-human skull uncovered, and gives researchers the earliest evidence of human ancestors moving out of Africa and spreading north to the rest of the world.

The skull and other remains offer a glimpse of a population of pre-humans of various sizes living at the same time - something that scientists had not seen before for such an ancient era.

Movement from Africa

This diversity bolsters one of two competing theories about the way our early ancestors evolved, spreading out more like a tree than a bush.

Nearly all of the previous pre-human discoveries have been fragmented bones, scattered over time and locations.

The findings at Dmanisi are more complete, weaving more of a short story.

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Before the site was found, the movement from Africa was put at about 1 million years ago.

When examined with the earlier Georgian finds, the skull shows that this special immigration out of Africa happened much earlier than previously thought, said study lead author David Lordkipanidze, director of the Georgia National Museum.

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 single species nearly 2 million years ago at the former Soviet republic site.

But they disagree that the same conclusion can be drawn for bones found elsewhere, such as Africa.

However, Mr Lordkipanidze and colleagues point out that the skulls found in Georgia are different sizes but are considered to be the same species.

So, they reason, it is likely the various skulls found in different places and times in Africa may not be different species, but variations in one species.

The adult male skull found wasn't from our species, Homo sapiens.

It was from an ancestral species - in the same genus or class called Homo - that led to modern humans.

Scientists say the Dmanisi population is likely an early part of our long-lived primary ancestral species, Homo erectus.

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