A recently discovered 2 million-year old flat-faced skull from East Africa has suggested that at least three early human species coexisted there.

A team led by Meave Leakey, the daughter-in-law of renowned scientist Louis Leakey, discovered facial bones and jawbones in the African continent that do not fit into the human family tree and, therefore, paleoanthropologists conclude that some pre-human species existed 2 million years ago.

According to 70-year-old Leakey, the new findings are clearly different from two other known fossils of the same period. All the fossils date between 1.78 million and 1.95 million years.

While we are the direct descendants of Homo erectus, researchers have found that another more primitive hominid species called Paranthropus bosei also lived in the region at the time. However, during the course of evolution the ape-like creatures became extinct.

"Our past was a diverse past," Leakey told BBC News, "our species was evolving in the same way that other species of animals evolved. There was nothing unique about us until we began to make sophisticated stone tools."

The new-found skulls have "a really distinct profile" and are "something very different," noted Meave Leakey.

"Human evolution is clearly not the straight line that it once was," study co-author Susan Anton, an anthropologist at New York University said.

The new discovery, however, generated a host of controversies among researchers.  

San Francisco Gate reported that in an e-mail message, UC Berkeley anthropologist Tim D. White wrote: "We still don't understand Homo habilis because there are still too few fossils." He further adds that "This is a field that continues to be plagued by practitioners unfamiliar with the degree to which individuals within species can vary."

The research has been published in the journal Nature.