A 3.5-million-year-old, flat-faced early human skull, which paleontologists found poking from the crumbling Kenyan earth, could push "Lucy" out of our ancestral family tree.
For 20 years, most scientists have agreed that Australopithecus afarensis, represented by the famous Lucy skeleton found in Ethiopia, is the direct ancestor of the many branches of hominids — upright walking human-like primates — including modern man.
But Meave Leakey and colleagues report in this week's journal Nature that the new species, named Kenyanthropus platyops, Greek for "flat-faced man of Kenya," lived at the same time as Lucy's species. And, they argue, modern humans can't be directly related to both. The finding casts sudden confusion over scientists' understanding of early humans and possibly adds a whole new main trunk to the human family tree.
"Kenyanthropus shows persuasively that at least two lineages [of early humans] existed as far back as 3.5 million years ago," says Leakey who is the daughter-in-law of Louis and Mary Leakey and wife of Richard Leakey, famous paleontologists who made several significant hominid finds in the last 50 years. "The early stages of human evolution are more complex than we previously thought."
Scientists say Lucy and the flat-faced man are too different physically to be closely related. To accommodate the new, unusual find, Leakey created a new genus to include the specimen.
"The shock that this find will give the human evolution community is on the same scale as the Lucy find," says Fred Spoor, professor of evolutionary anatomy at University College London and an author of the article describing the new species. "What people immediately want to know is, can you tell whether we came from one or the other? And the answer right now is no."
Sharp Eyes Spy Tooth
Patrick Gathogo, a Kenyan currently studying geology at the University of Utah, remembers walking behind Leakey, leader of the expedition, during the summer of 1999 when the skull was found near Lake Turkana. The region has been one of the most fertile areas in the world for hominid fossil finds.
A Kenyan research assistant, Justus Erus, spotted a tooth poking from the brown dirt. Gathogo recalls that everyone stopped and leaned down for a closer look. Everyone was silent.
"Most of the time, when you find a fossil, you know immediately what it is," says Gathogo, who says all members of the team learn to identify the ancient animal fossils they routinely find in the field.
"It's when no one says anything that you know you've found a hominid," he says.
A More Human Face
When the soil was carefully brushed away, Leakey and her team had uncovered the oldest, most complete cranium of an early human. Geologists have dated the skull to between 3.2 million and 3.5 million years ago based on the known sequence of volcanic ash in the area.
The skull has a bewildering mixture of physical features — a flat, human-looking face, small brain and small teeth.
"Flat faces [in early hominids] are associated with a specialization of chewing," says Spoor. "It always went with really huge teeth. We always thought these characteristics went together in a neat package, until we found this skull."
Leakey and colleagues believe this shows that Kenyanthropus had a much different diet than Lucy's species. The different physical features show both were likely adapted to different environments, which would have prevented competition for food, says Spoor. The two could even have lived side by side, he says.
Of all other known hominids, Kenyanthropus bears a striking resemblance to another unusual skull found by Richard Leakey, Meave's husband, in the 1970s. That hominid, named Homo rudolfensis, dates to a much later period of about 2 million years ago.
Rudolfensis has long puzzled scientists and has been moved between two different hominid groups — Homo and Australopithecus — because researchers were never really sure where to put it, says Spoor. The Nature article proposes that rudolfensis could be descended from Kenyanthropus.
The topic likely to occupy evolutionary biologists for some time is Leakey's assignment of a new genus Kenyanthropus, which effectively creates two main trunks to the human family tree.
But Daniel Lieberman, an evolutionary anthropologist at George Washington University, says the new genus, for the time being, is the most comfortable classification of the new skull.
"None of the other possible solutions seem feasible," he said in a Nature article commenting on the Leakey find. "My guess is that it will be quite a while before we can confidently determine the position of Kenyanthropus platyops in the human evolutionary tree."