The Chadian skull is twice as old as "Lucy," the fossilized hominid found in 1974 in Ethiopia that was once considered the mother of all humanity. It's also older than the 5.8-million-year-old controversial remains known as "Millennium Man" that were found in Kenya in 2000 and older than the 5.8 million-year-old teeth and bones found in Ethiopia last July.
"The reason the fossils and their contexts are important is that it is only through these and their proper analysis that we can come to truly know our past," explains Tim White, an anthropologist at the University of California at Berkeley who led the excavation of the 5.8-million-year-old hominid teeth and bones in Ethiopia.
The 'Wrong' Side of the Valley
The wind-seared location of the Toumaï find was also unusual and challenges the theory that human ancestors evolved on the east side of the African Rift Valley while apes evolved on the west.
The valley formed eight million years ago when a geologic fault collapsed into the earth. On the east side, the climate became arid and forests turned to dry grasslands. Scientists had assumed that humans evolved here since there were no trees to offer high vantage points. In order to see over the grass, early humans would need to walk upright.
Brunet's 40-member team unearthed Toumaï from the western side of the valley that had remained lush with trees and lakes millions of years ago.
They also found signs of ancient crocodiles, snakes and turtles in the region. Brunet dated plant and animal remains in the soil around the fossils to settle on the approximate age of the hominid. While other scientists appear to accept Toumaï's estimated age of six to -seven million years, they remain surprised by the ancient hominid's looks.
Strange Mix of Features
"Sahelanthropus tchadensis shows a mix of primitive and evolved characteristics never seen before," says Chris Stringer, an anthropologist at the Natural History Museum in London.
Some argue, for example, that Toumaï's small face and teeth appear more modern than Lucy, the 3-million-year-old hominid skeleton. While his face may appear modern, Toumaï's skull is long and narrow and once held a small, chimpanzee-sized brain.
Perhaps most striking is the skull's brow line, which is remarkably large. Brunet explains the large brow line is what suggests Toumaï was male since it's a common male trait that's thought to help attract females.
"My guess is the competition between these early hominids was intense and natural selection favored males with large brow ridges," says David Pilbeam, a Harvard University anthropologist and a co-author of the Nature study.
Brunet's team did not find any leg bones of Toumaï but the point where his spinal cord entered his skull suggests Toumaï likely walked upright. Spinal cords in humans connect at the bottom of the skull since they carry themselves and their heads in a vertical line.
He may have walked on two feet but researchers say it's difficult, if not impossible, to know if this ancient hominid was a direct ancestral link to humans or possibly a false start within the apparently complex "bush" of life.
"There are lots of new questions," says Brunet. "This is just the beginning of our knowledge of the human lineage."