The discovery eight years ago of a 1.8-million-year-old skull of a human ancestor buried under a medieval Georgian village indicates our family tree may have fewer branches than some believe, scientists say.
The skull, along with other partial remains previously found at the rural site, offer a glimpse of a population of pre-humans of various sizes living at the same time - something that scientists had not seen before in such an ancient era.
This diversity bolsters one of two competing theories about the way our early ancestors evolved, spreading out more like a tree than a bush, according to a study published Thursday in the journal Science.
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, director of the Georgia National Museum and the study's lead author.
"This is important to understanding human evolution," he said.
For years, some scientists have said humans evolved from only one or two species, much like a tree branches out from a trunk. Others say the process was more like a bush, with several offshoots that went nowhere.
Even bush-favoring scientists say these findings show a single species nearly 2 million years ago at the site in the former Soviet republic. But they disagree that the same conclusion can be made about for bones found elsewhere, such as in Africa.
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's probable that various skulls found in different places and from different periods 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 say.
"Danny DeVito, Michael Jordan and Shaquille O'Neal are the same species," Lordkipanidze said.
The adult male skull found in Georgia wasn't from our species, Homo sapiens; it was from an ancestral species in the genus Homo. Scientists say the Dmanisi population is probably an early part of our long-lived primary ancestral species, Homo erectus.
Tim White of the University of California at Berkeley wasn't part of the study but praised it as "the first good evidence of what these expanding hominids looked like and what they were doing."
Fred Spoor of the Max Planck Institute in Germany, a proponent of a bushy family tree with many species, disagreed with the study's overall conclusion, but he lauded the Georgia discovery as critical and even beautiful.
"It really shows the process of evolution in action," he said.
Spoor said it seems to have captured a crucial point in the evolutionary process where our ancestors transitioned from Homo habilis to Homo erectus - although the study authors said that depiction is going a bit too far.
The researchers found the first part of the fossil, a large jaw, below a medieval fortress in 2000. Five years later - on Lordkipanidze's 42nd birthday - they unearthed the well-preserved skull, gingerly extracted it, put it into a cloth-lined case and popped champagne. It matched the jaw perfectly. They were probably separated when the individual lost a fight with a hungry carnivore, which pulled apart his skull and jaw, Lordkipanidze said.
The skull was from an adult male just shy of 5 feet with a massive jaw and big teeth but a small brain, implying limited thinking capability, study co-author Marcia Ponce de Leon, of the University of Zurich, said. It also seems to be from the evolutionary point where legs were getting longer, for walking upright, and hips smaller, she said.
"This is a strange combination of features that we didn't know before in early Homo," Ponce de Leon said ( via washingtonpost.com ).