Fossil Find Offers New Glimpse Into Ancient Human Ancestry
Skull bones of the earliest known ancestor of humankind have emerged from the swirling sand dunes of Chad--fossils more than 6 million years old that will revolutionize the scientific understanding of human beginnings, researchers announced Wednesday.
The nearly complete cranium, fractured jaw bones and teeth-- 3 million years older than any other hominid skull discovered to date--offer experts the first glimpse of a mysterious period of evolution between 6 million and 7 million years ago when the earliest ancestors of humans first branched off from chimpanzees, gorillas and other great apes.
As the first and only tangible evidence of hominids from such an important period, the new fossils illuminate that crucial epoch like a single candle burning in the dark basement of time.
“Unquestionably, this is one of the most important fossil discoveries of the last 100 years,” Harvard University anthropologist Daniel Lieberman said. “It is the oldest skull by far of a human ancestor. This will have the scientific impact of a small nuclear bomb.”
Details of the find were announced Wednesday in Chad and published today in the British research journal Nature.
The discovery suggests that human roots go deeper than previously thought, said experts in England, Japan and the United States who have examined the fossils. The first pre-humans may have branched off from the ancestors of chimps and other apes a million years earlier than genetic evidence of molecular DNA has hinted.
“This is the closest we have gotten to the last common ancestor” between humans and their closest modern relative, chimpanzees, said UC Berkeley anthropologist Tim D. White.
The teeth, brain case and facial features also appear “incredibly modern,” suggesting that this diminutive lakeshore dweller easily could be a direct ancestor of contemporary humanity. It is evidence that the earliest human ancestors bore little resemblance to any modern ape.
And because the fossils were found so far from the East African Rift Valley, long considered to be the cradle of humanity, scientists conclude that these primitive hominids, as the early ancestors of humanity are called, ranged more widely than researchers expected.
“It is of tremendous significance for its age, its completeness, the location where it was found and its fascinating combination of features,” said Harvard’s David Pilbeam, an authority on human beginnings and a member of the research team. “It is a stunning find.”
The fossil fragments from the newly named species were unearthed last July in the scorching Djurab desert of Chad in Central Africa by the Mission Paleoanthropologique Franco-Tchadienne, a team of 40 researchers from 10 countries led by French paleontologist Michel Brunet at the University of Poitiers, who has explored the region for 30 years.
“It’s a lot of emotion to have in my hand the beginning of the human lineage,” Brunet said in a statement in Chad. “I have been looking for this for so long.”
The cranium was spotted protruding from the crumbling sandstone by Ahounta Djimdoumalbaye, a student at the University of N’Djamena in the Chad capital.
The remains belonged to small-brained creatures not much taller than a chimp that was neither ape nor completely human. They lived long before the first tools were made or fire was mastered.
The newly discovered species “shows a mix of primitive and evolved characteristics never seen before and that are surprising to see 6 million years ago,” said paleontologist Chris Stringer at the Natural History Museum in London. “It had an apelike brain size and skull shape, combined with a more human-like face and teeth. Its discovery shows how much evidence has been missing up to now.”
Taken together, the combination of features appears so unusual that the French researchers assigned the creatures to a new genus and species called Sahelanthropus tchadensis. Brunet and his colleagues dubbed the hominid find “Toumai,” after a word in the local Goran language meaning “hope of life” that traditionally is given as a nickname to children born in the dry season.
The bits and pieces discovered so far represent the remains of five individuals, the researchers said.
Nothing of the torso or limbs has been found, so researchers have no idea how they walked, used their arms or how erectly they stood. Brunet concluded that Toumai did walk upright because the hole for the spinal cord at the skull’s base resembles those found in hominids known to be bipedal.
The strongest evidence for the link to humanity are the teeth, experts said. Only three have been discovered to date. They reveal the blunt canine incisors and thickened dental enamel associated with more highly evolved hominid species that did not appear until millions of years later.
The cranium’s heavy brow ridges--the token of a species in which males compete fiercely for the attention of females--are a feature not seen in hominids except in the genus Homo, to which humans belong, said Harvard’s Pilbeam, who has examined the fossils. The cranium, therefore, may have belonged to a male.
All in all, “it plays havoc with the tidy model of human origins,” said Bernard Woods of George Washington University, who is an expert on human origins.
The striking combination of features could mean the creatures represent a small sample of a huge and hitherto unsuspected diversity of ancient hominids, Woods said. The popular image of a neat line to humans from a common ancestor with chimps may be misleading.
The excavation site is an ancient lake bed a thousand miles from the East African areas where almost all other early hominid discoveries have been made.
There, layers of time have been stripped by the wind to reveal a broad but shallow band of fossil-bearing rock that appears to date from 6 million to 7 million years ago. All of the hominid bone fragments found so far turned up in a single sandy patch of earth measuring about 600 square yards.
The flat hardscrabble desert in which Brunet and his colleagues work today, raked by the hot breath of the Sahara and the Sahel, is a stark contrast to the lush landscape of marshes, rivulets and wooded meadows where Toumai lived and died so many millions of years ago.
At that time, Lake Chad was an inland sea almost the size of France that alternately rose and fell with cycles of climate change. Its shores were home to a rich variety of grazing animals, and its waters teemed with prehistoric hippos and crocodiles.
Based on the evidence of fossil animal bones found with the Toumai remains, Patrick Vignaud at the University of Poitiers in France and his colleagues estimated the age of the Toumai find as 6 million to 7 million years. They readily acknowledged that using the evidence of biology to date the site is not completely reliable. More absolute dating requires techniques that use radioactive decay to gauge the passage of time, and the rocks of the region do not contain the proper isotopes.
The dating of the Toumai find is important because, relying on genetic evidence, scientists have assumed that the ancestors of chimpanzees and humans split into separate evolution branches around 5 million years ago. If Vignaud’s work holds up, scientists will have to push that divide farther back in time.
Several independent experts said they did not doubt the antiquity of the find but expected the “loose” dating estimates will be revised as better data become available. “So only time will tell where in the circa six- to seven-million-year time period the Chad remains actually fit,” said anthropologist Gen Suwa at the University of Tokyo.
So far, the fossils pose far more questions than they can answer.
No one is sure just where they belong in the line of human evolution. Do these bones belong to the sole pre-human species alive in this early era or just one of many? Are the fossils an evolutionary dead-end or the forefather of us all? How completely will the first chapters of the human story have to be rewritten?
Researchers will literally have to dig harder for answers in the hope that Africa will yield more fossils.
The Chad site has “enormous potential” for more important finds, UC Berkeley’s White said. “Whether that potential is realized depends on the movement of the dunes, the glint of the sun and the skill of the people working the site.”
