"It shows that our past is like that of any other mammal," said Meave Leakey, head of the paleontology department of the National Museums of Kenya, in an interview with National Geographic Today. "[We have] a very complicated, diverse past with lots of different species; many of which became extinct."

According to Leakey and the authors of the report, which appears in the March 22 issue of Nature, the fossil find points to diet-driven adaptation, in which species of hominin evolved to take advantage of different food sources.

Two Distinct Species

The skull of Kenyanthropus platyops was discovered in 1999 by Justus Erus, a team member. The expedition was searching the 3.2 to 3.5 million-year-old sediments of the Lomekwi River for contemporaries of Lucy.

Since the skull discovery, team member Christopher Kiarie has been conducting meticulous work to establish the genus and species of the skull, which is characterized by small molar teeth, and a tall flat face.

"Platyops means 'flat face,'" said Leakey.

This physical description is vastly different from that of Australopithicus afarensis, said Leakey. Lucy and her kin had protruding faces and large teeth.

"The shape of the face clearly differentiates it from Australopithicus afarensis and species that lived later," said Leakey.

According to the scientists, both tooth size and shape are indicators of diet and the way in which a species chews its food.

"The architecture of the face really depends on the diet and how its jaws are working," said Leakey. "This is significant because it means these ancestors were inhabiting a different ecological niche."

Both Kenyanthropus platyops and Australopithicus afarensis could have existed side by side without direct competition for food resources, says Leakey.

Before the discovery of Kenyanthropus platyops, most scientists believed that hominins did not develop a flat face until approximately two million years ago.

Changing Picture of Evolution

Though the discovery seems to blur the lines of humankind's family tree, according to Daniel Lieberman, an anthropologist at George Washington University in Washington, D.C., "This is part of a trend. We're getting to know our ancestors better."

Since the 1970s, anthropologists have been uncovering evidence that modern humans did not follow a linear evolution in which one species evolved from the previous species. The finds suggest that instead, numerous species existed, and each adapted to different environments.

Multiple species of hominin had been discovered from time periods earlier and later than Lucy and her kin, but the fossil record showed only one ancestor for the middle Pleiocene, approximately 3.5 million years ago, says Lieberman.

"[Finding Kenyanthropus platyops] was what I expected," said Leakey. "If you look at the evolution of any other mammal, there's usually a radiation of species and just a few species survive. It didn't seem right that there was only one line of evolution [for this time period]. There should have been other species around."

Kenyanthropus platyops helps to fill in the picture of this period of evolution, said Lieberman. It indicates that multiple, different-looking species coexisted and filled different ecological niches.

"We had a long and complex past," said Leakey. "And we are the single surviving species. Our existence is not secure. Just like everything else, we can be extinct too."