A Fossil Unearthed in Africa Pushes Back Human Origins
The New York times, July 11, 2002
By JOHN NOBLE WILFORD
French scientists digging in Central Africa have uncovered a skull, virtually complete and almost seven million years old, that belonged to an individual about the size of a chimpanzee. It is, they say, the earliest known member of the human family, by perhaps as much as a million years.
The discovery, described in today's issue of the journal Nature, is being hailed as the most important fossil discovery in decades. Surprised by the age, complexity and geography of the fossils, paleoanthropologists spoke of the find as a critical and perhaps revolutionary turning point in the study of human origins.
The scientists said it was too early to know whether the skull represented a species on a direct ancestral line to humans. In fact, the fossils — a cranium, two lower jaw fragments and several teeth — suggest an evolutionary complexity and diversity in human origins that seem to defy description by the simplified family trees of the past.
What is especially striking, and puzzling, is the skull's mixture of primitive and advanced characteristics. The braincase is apelike, but the face and teeth are more like those of a human. The cranial capacity is similar to that of living chimps.
The skull is of an age, scientists said, that it could be expected to provide telling evidence of life at the time the human and chimpanzee lineages diverged. Some of its characteristics suggest that the skull is closely related to the last common ancestor of humans and chimps and may yield an understanding of what those apelike creatures were like.
In the journal report, the discoverers called the skull "the oldest and most primitive known member of the hominid" family, close to the split of hominids and chimps. As such, they predicted, the find promised "to illuminate the earliest chapter in human evolutionary history."
At any rate, the specimen is sufficiently distinct from apes and other human precursors, or hominids, to be given a new genus and species name by the discovery team, headed by Dr. Michel Brunet of the University of Poitiers in France.
Its formal name is Sahelanthropus tchadensis, recognizing that all the specimens were found in the harsh desert region known as the Sahel in Central Africa south of the Sahara. More commonly, the hominid is being called Toumai, a name often given to children born close to the dry season.
"Toumai is arguably the most important fossil discovery in living memory, rivaling the discovery of the first `ape man' 77 years ago — the find which effectively founded the modern science of paleoanthropology," said Dr. Henry Gee, Nature's paleontology editor.
"This is really an extraordinary find," said Dr. Ian Tattersall, an expert on fossil hominids at the American Museum of Natural History. "It broadens our perspective in two directions — in time and in geography."
The absence of volcanic ash layers at the fossil site prevented the discoverers from dating the specimens in absolute terms and with the usual scientific methods. But a comparison of other fossils found at the site with similar ones from well-dated sites in East Africa yielded an estimate of six million to seven million years for the Chad fossils.
"It's seven million years old, so the divergence between chimp and human must be even older than we thought before," Dr. Brunet said.
Molecular biological studies have indicated that the divergence occurred five million to seven or eight million years ago. Although the research is controversial, Orrorin tugenensis, a specimen reported in Kenya two years ago, had until now claimed the title of earliest hominid, at about six million years. An Ardipithecus ramidus, which lived about 5.8 million years ago in Ethiopia, was a close competitor.
So the Chad discovery opens a window on a fateful period in evolutionary history about which the fossil record has been so sparse that the remains could fit in a shoebox.
Also, none of the other early specimens include almost complete skulls, which are considered more revealing of a fossil species' place in the hominid family. Toumai is about three million years older than the next-oldest hominid skull.
The discovery site, in the Djurab Desert in Chad, is more than 1,500 miles west of the more familiar fossil beds of East Africa, in Ethiopia, Kenya and Tanzania.
As Dr. Daniel E. Lieberman, a Harvard paleontologist, said: "We've been looking exclusively in East Africa and South Africa and basing our evolutionary tree on what we find there. Brunet has reminded us that we must find out what was going on in Central Africa and West Africa, and that's going to be harder to do because of more difficult environmental conditions."
In studying the skull, Dr. Lieberman recognized a third reason, besides the specimen's age and location, for scientists to be excited and challenged by the discovery. That is the skull's mosaic of primitive and advanced characteristics.
"You expect something that age to be very chimplike," Dr. Lieberman said. "But this one's face is the face essentially of a Homo habilis, at two million years ago, and yet this face is almost seven million years old."
This is all the more puzzling because Australopithecus afarensis, the Lucy species that lived 3.2 million years ago, has a decidedly chimp like face. What's happening? Reversing evolutionary patterns and trends, Dr. Lieberman said, is "extremely rare, if not impossible."
Several scientists said the discovery thus seemed to undermine the simplest linear models of hominid evolution. If the earliest hominids like Toumai were directly ancestral to australopithecines like Lucy, Dr. Lieberman pointed out, there would have had to have been two reversals to reach the advanced characteristics of the Homo lineage.
Otherwise, he added, Toumai is ancestral to some other hominids that then gave rise to the Homo species, in which case australopithecines are a side branch outside the human ancestral line.
In an appraisal accompanying the journal report, Dr. Bernard Wood, a paleoanthropologist at George Washington University, favored a "bushy" model of hominid evolution over a simple linear model. The many branches reflect evolutionary diversity in response to new or changed circumstances.
So Dr. Wood said the bushy, or untidy, model "would predict that at six to seven million years ago we are likely to find evidence of creatures with hitherto unknown combinations of hominid, chimp and even more novel features."
Dr. Wood further predicted that Toumai was "just the tip of an iceberg of taxonomic diversity during hominid evolution five to seven million years ago."
One important question is whether the newfound fossil species stood upright and regularly walked on two legs, which has been a defining aspect of hominid behavior. The discovery team has yet to find any skeletal bones associated with Toumai.
Dr. Brunet, the team leader, said the position where the spine entered the head "doesn't prove that he is bipedal, but it shows he could be."