A 1.5-million-year-old skull and an equally old jaw found in Kenya are helping rewrite the history of early man, eliminating one reputed ancestor from the human lineage and suggesting that another was much more primitive than previously believed, researchers said Wednesday.
Analysis of the jawbone shows that Homo habilis, once thought to be a direct ancestor of Homo erectus and thus of humans, lived side by side with H. erectus, making them sister species rather than mother and daughter.
"They coexisted at the same time and in the same place for half a million years," said anthropologist Fred Spoor of University College London, a coauthor of the paper appearing in the journal Nature. "How likely is it that one would give rise to the other?"
Coauthor Maeve G. Leakey of Stony Brook University in New York added, "The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition."
The situation is similar to modern Homo sapiens and Neanderthals living side by side in Europe 50,000 years ago, said anthropologist William Kimbel of the Institute of Human Origins at Arizona State University, who was not involved in the research.
Researchers once thought that Neanderthals were a predecessor of modern humans, but it eventually became clear that they were an evolutionary dead end. Now it seems the same is true of H. habilis, Kimbel said.
The finds "are consistent with a growing consensus" that the evolutionary tree of humans is highly branched rather than a single linear trunk, he said. The diversity, he said, tells us that "there is very little in the events of the early Pleistocene that can be seen as foretelling human adaptations."
Homo habilis -- "handy man" -- is the oldest representative of the genus Homo, dating from about 2.5 million years ago. The species was defined by Mary and Louis Leakey based on fossils found in Tanzania between 1962 and 1964. Short and with disproportionately long arms, it was the least similar to humans.
Homo erectus -- "upright man" -- dates from about 1.8 million years ago. It was originally described in the 1890s, and specimens have been found throughout Africa, Europe and Asia. Specimens bear a striking resemblance to modern humans, but the brain is about one-quarter smaller.
Modern humans -- Homo sapiens, or "knowing man" -- originated in Africa about 200,000 years ago and are characterized primarily by a larger brain capacity.
The two new fossils were found in 2000 east of Lake Turkana by a team headed by Spoor, Leakey and her daughter, anthropologist Louise N. Leakey, also of Stony Brook. The jawbone was identified as H. habilis because of the distinctive pattern of its teeth. Analysis of the rock around it by Frank Brown and Patrick Gathogo of the University of Utah dated the jawbone to 1.44 million years ago, making it by far the youngest H. habilis found. The most recent previous specimen dated from about 1.62 million years ago.
The skull, which was remarkably well preserved because it was almost fully embedded in sandstone, was found nearby and dated to 1.55 million years ago. The skull was so well preserved that researchers could tell that its growth plates had fused, indicating that it was that of an adult, Spoor said.
Its physical characteristics indicate that it is H. erectus, but it is the smallest such skull found, a feature that suggests the species was more primitive than thought.
"The direct importance of these is that they both come from the same site," Kimbel said. Researchers had previously found similar overlapping specimens at different locations, but it was possible to argue that they were geographical variants. Finding them at the same site eliminates that argument, he said.
Spoor thinks the two species coexisted by occupying separate ecological niches. The smaller teeth and jaws of H. erectus, for example, indicate that H. habilis had a tougher diet.
The evidence suggests that both species descended from a common ancestor somewhere between 2 million and 3 million years ago. Few fossils from that period have been found.
The small H. erectus skull indicates that variability in skull size in that species -- called sexual dimorphism -- "was substantially greater than in modern humans and chimpanzees, and closer to gorillas," Spoor said. Greater variability is considered a more primitive feature.
"We know that species who are very variable in size and show a major difference between genders are organized with one dominant male and several females in a harem, with other males on the periphery," he said.
That H. erectus may have still been very dimorphic suggests the possibility of a reproductive strategy that mostly was polygamous
New jaw fossils might suggest a direct line of descent between two species of early humans, including the one to which "Lucy" belongs.
The 3.2 million-year-old Lucy, the earliest known adult hominid, was found in Ethiopia in 1974 by U.S. paleontologists Donald Johanson and Tom Gray. Lucy and her kind, Australopithecus afarensis, stood upright and walked on two feet, though they might also have been agile tree-climbers.
Anthropologists have suspected an ancestor-descendant relationship between the Lucy species and a predecessor--Australopithecus anamensis--based on their similarities but lacked fossils from an intervening period.
Now, Australopithecus fossils found in the Woranso-Mille area of the Afar Region, Ethiopia, fill the date gap between A. anamensis (4.2 to 3.9 million years ago)-and the Lucy species (3.0 to 3.6 million years ago). The species identifications for all the bones remain uncertain, though it appears that some are A. afarensis.
Yohannes Haile-Selassie, a physical anthropologist at the Cleveland Museum of Natural History, says his team's 2007 field season in the Woranso-Mille region uncovered the key evidence.
"We recovered fossil hominids that date to between 3.5 and 3.8 million years ago," Haile-Selassie said in a prepared statement. "These specimens sample the right time to look into the relationship between Australopithecus anamensis and Australopithecus afarensis and will play a major role in testing the ancestor-descendant hypothesis."
The team had found teeth from this time frame at the site over the past few years, but the new material includes more complete jaws that will enable better comparisons, he said.
"We have about 35 specimens, mostly isolated teeth, but including one partial skeleton which we believe will give us a lot of information on the post-cranial morphology of early human ancestors," Haile-Selassie told LiveScience.
At least 40 hominid specimens have been recovered from the site so far, including the complete jaws and the partial skeleton. The latter was found in 2005. The team started work in the Woranso-Mille region in 2004 and quickly started finding fossils of early hominids as old as 3.7 million years old, Haile-Selassie said.
"We were sure of their importance," he said, "because we knew right from the beginning how old these bones were. We used what is called biostratigraphy to estimate the age of the fossils. Now, we have radiometric dates from volcanic rocks that we sampled from the study area, and now we have a minimum age of 3.5 million years and a maximum of 3.8 million years."
Ethiopia, Kenya and Tanzania have yielded many of the earliest hominid and ape fossils that have allowed anthropologists to piece together the history of human evolution.