Throwing the origins of humankind into question, researchers in Kenya have unearthed a battered but almost complete skull of a new species of human with a surprisingly delicate face dating from 3.5 million years ago.
The new species is nestled in the roots of the human family tree during a period when scientists thought only one ancestral species existed, leaving it unclear just which was the direct forebear of modern humankind.
Using their new specimen to rework humanity's pedigree, paleoanthropologist Meave Leakey and her colleagues at the National Museums of Kenya in Nairobi argue in research being made public today that the small-brained creature is so unique it belongs not just to a new species but to an entirely new genus. They formally christened it Kenyanthropus platyops the flat-faced man of Kenya.
Its tiny teeth, distinctive jaw structure and relatively modern face set it apart from the only other early human species known to have been alive at that period, the researchers report in Nature. That species, called Australopithecus afarensis, is best known by the 3.2-million-year-old skeleton called Lucy. Until now, it was thought to be humanity's direct forerunner.
"The differences (between the two species) are so obvious and significant," Leakey said. "They are the opposite of what you would expect."
The find is further evidence that humanity emerged from an evolutionary maze of false starts, dead-ends and competing adaptations to its African homeland, several experts said.
So far, few people outside Leakey's team have been able to examine the new fossils directly. Based on details of the fossils published today in the research report, however, many experts in the study of human origins hailed the find as provocative albeit unsettling evidence of early human diversity.
"This is both a very welcome and at the same time extraordinarily intriguing fossil find," said Donald Johanson at the Institute of Human Origins at Arizona State University, who discovered the Lucy fossils in 1974.
"It is certainly a new species," Johanson said. "And the unique combination of anatomical characteristics in this specimen probably do justify a new genus." Even so, Johanson still believes afarensis is a likely candidate for the direct ancestor of modern humans.
All told, the new specimen is the sixth new prehuman species to be discovered in the past decade, encompassing up to 4 million years of human evolution.
Taken together, these growing collections of age-stained fossil teeth, broken femurs, shards of bone and shattered skulls have turned the scholarly procession of human descent into a parade of primitive hominids vying for a prominent place in the pecking order of evolutionary history.
The ensuing scientific debates pit passionately held theories about human origins against the reality of a remarkably sparse fossil record. The newest discovery sheds light on an especially important period of human development.
"Until this skull was found, there was only one kind of early hominid (in this period) that you could possibly make ancestral to humanity," said University of Utah geologist Frank Brown, who helped determine the age of the rocks in which the fossils were preserved.
"This makes people rethink these relationships," he said. "It says there was more than one kind of creature running around then related to us."
Already, several scientists have suggested that the newly discovered species might supplant afarensis as the most likely line of direct human ancestry. Leakey herself, however, stopped short of making that claim, saying that she would not be surprised if even more early human species turned up in the same period of evolutionary history, any one of which might be a better candidate.
"I don't have any scientific grounds to say that this is directly ancestral," she said Wednesday in an interview in Los Angeles. "It certainly is a branch of the human family tree, but it may be a twig that became extinct."