Remains of what may be humanity's earliest direct ancestor, a chimp-sized creature that walked the cool, wooded highlands of East Africa more than 5 million years ago, have been discovered in Ethiopia, an international team of researchers announced Wednesday.
"This is the first evidence that we have for the existence of hominids between 5.2 million and 5.8 million years ago," said Yohannes Haile-Selassie, an Ethiopian graduate student at UC Berkeley who discovered the fossils. "That is what makes this a big discovery."
In all, Haile-Selassie found bones belonging to five individuals at sites located miles apart and separated by several hundred thousand years in time. During three years of grueling field work, Haile-Selassie culled 11 teeth, a jaw and other bone fragments from volcanic cobbles at five sites along the goat trails and arroyos of Ethiopia's arid Awash region about 140 miles from Addis Ababa.
One of the fossils, a 5.2-million-year-old toe joint, is strong evidence that the creature walked upright. Its sheer antiquity bolsters the theory that the defining characteristic that sets these early hominids apart from the rest of creation is the ability to walk upright, rather than brain size, speech or the ability to use tools, the scientists said.
The fossils are documented in two research reports published today in the journal Nature.
The findings come from the time when the common ancestors of monkeys and humans first diverged into separate species, between 5 million and 10 million years ago. Today, the Awash is desert, but when these creatures were alive more than 5 million years ago, it was a region of wooded glades. That is additional evidence that the earliest ancestors of humanity arose among the safety of the trees and not in the open grasslands, as has often been suggested.
The discovery calls into question a widely held theory that environmental change spurred human evolution. According to that theory, the changing environment dried up the forests millions of years ago. Early hominids were forced onto the open savanna, where upright walking was a survival advantage for hunters and hunted alike.
All the fossils of the most primitive pre-humans discovered so far, however, have been found in ancient forests.
"The discovery that the earliest hominids were confined to wooded habitats for the first nearly 2 million years of their known existence should give pause to savanna-based theories of hominid origins and early evolution," said UC Berkeley anthropologist Tim White, who co-directs the 45 scientists from 12 countries involved in the research project.
Owen Lovejoy, an anthropologist at Kent State University in Ohio who has examined the fossils, put the matter simply. The discovery is "pretty revolutionary," he said.
Although the area was lush and well-watered 5 million years ago, the hominid's homeland was hardly a pastoral Eden. Violent volcanic eruptions were common. Showers of hot volcanic ash periodically smothered trees and grasses, said the team's geologist, Giday WoldeGabriel at Los Alamos National Laboratory. "It is hard to imagine that life would go on normally in such hostile environment conditions," he said. The animals inhabiting the area "were real survivors."
The fossils offer hints of a creature only marginally more than an ape that gave rise to a surprisingly complex family tree encompassing more than a dozen primitive pre-human species and, eventually, modern humankind.
"We are now finally beginning to get a look at the base of the hominid lineage, very close to the last common ancestor," said White. "We know almost nothing about that ancestor except that it existed," he added. "So this is our first peek."
In key anatomical details, the fossils closely resemble a pre-human species called Ardipithecus ramidus, discovered by White and his colleagues in 1994. That creature lived in the same area more than 4 million years ago, but the new fossils are at least a million years older than the earliest known specimens of the breed.
Accordingly, Haile-Selassie decided to assign the new bones to a new subspecies, calling it Ardipithecus ramidus kadabba.
Other experts in human origins, however, were more cautious.
"This is going to be controversial," said Donald Johanson at Arizona State University's Institute of Human Origins. The new fossils were found about 50 miles north of the site where Johanson in 1974 unearthed the famous 3-million-year-old fossil known as Lucy.
Too little is known about any of the primordial primates that lived at this time to judge whether this newly discovered, previously unknown creature is ape, chimp or human ancestor, said paleoanthropologist Bernard Wood at George Washington University. "You need more evidence than they have been able to find in order to say this is a hominid,' Wood said. Even so, he added, "this is really great stuff."
The rugged volcanic terrainmade fossil prospecting unusually difficult.
"You might call it almost a miracle that Haile-Selassie came up with his discoveries," said Gen Suwa, an expert in human origins at the University of Tokyo. "Anthropologists could well have worked in the general area for decades without finding these hominids."
The remains are the third major--and controversial--human fossil discovery in Africa this year.
In January, researchers led by Martin Pickford of the College de France announced that they had discovered 12 fossils in Kenya belonging to a previously unknown 6-million-year hominid. The creature represented an entirely new genus and species, they contended, which they named Orrorin tugenensis.
Many experts in human origins argued that the fossils belonged to an ancestor of chimpanzees instead. Haile-Selassie, Johanson and other scientists say that until more fossils of Orrorin tugenensis are unearthed, no one can tell whether it is the earliest hominid, the last common relative of humans and chimpanzees, the earliest chimpanzee or an ape species that became extinct.
In March, paleontologist Meave Leakey reported the discovery in Kenya of a 3.5-million-year-old skull and other fossils belonging to yet another new genus and species, Kenyanthropus platyops. Her find challenged the popular notion that the hominid species to which Lucy belongs is humanity's direct forerunner.
But White at Berkeley and several researchers are not yet convinced that what Leakey found is even a separate species, let alone a genus in its own right.
As word of Haile-Selassie's discovery spread this week, the fossils also became bones of contention.
Several scholars were not sure the fossils should be assigned to a new subspecies, suggesting that there simply is too little fossil material to be sure of its place in the scientific scheme of things.
Questions also arose concerning the creature's proper place in the human line of descent: Is this the earliest direct ancestor of modern humanity, as its discoverers believe, or is it perhaps an evolutionary dead end?
"The evidence isn't good enough to say a great deal yet," Wood cautioned.